The suitability of a target weed for classical biological control should be considered early in the process of plant invasion. Concerns have been raised about the recent arrival of Mikania micrantha Kunth in south Florida and its potential to spread and invade natural and managed ecosystems. This weed is native to the neotropics, and has been introduced into many Asian countries and, more recently, into Australia. In Asia, M. micrantha is particularly problematic in plantation crops, but also threatens natural areas and disturbed ecosystems. Several aspects of the biology and ecology of M. micrantha are discussed in this review to evaluate its suitability as a target of biological control in Florida. Based on an ecological niche model, the climate in southern and central Florida is highly favorable for the invasion of this weed. Previous biological control programs provide valuable information for Florida scientists about the availability of natural enemies and potential areas for future foreign surveys. Genetic comparison of the Florida population and native range populations, in addition to climate matching, should be considered when selecting areas for foreign exploration. Finally, the authors used a scoring system and risk-benefit–cost analysis to evaluate M. micrantha as a potential target for biological control in Florida. We suggest that similar feasibility evaluations should be conducted not only for well-established problematic weeds but also for new invaders with a known history of invasiveness.

Nomenclature: Mile-a-minute, Mikania micrantha Kunth

Diversified grasslands that contain native plant species are being recognized as important elements of agricultural landscapes and for production of biofuel feedstocks as well as a variety of other ecosystem services. Unfortunately, establishment of such grasslands is often difficult, unpredictable, and highly vulnerable to interference and invasion by weeds. Evidence suggests that soil-microbial “legacies” of invasive perennial species can inhibit growth of native grassland species. However, previous assessments of legacy effects of soil occupancy by invasive species that invade grasslands have focused on single invasive species and on responses to invasive soil occupancy in only a few species. In this study, we tested the hypothesis that legacy effects of invasive species differ qualitatively from those of native grassland species. In a glasshouse, three invasive and three native grassland perennials and a native perennial mixture were grown separately through three cycles of growth and soil conditioning in soils with and without arbuscular mycorrhizal fungi (AMF), after which we assessed seedling growth in these soils. Native species differed categorically from invasives in their response to soil conditioning by native or invasive species, but these differences depended on the presence of AMF. When AMF were present, native species largely had facilitative effects on invasive species, relative to effects of invasives on other invasives. Invasive species did not facilitate native growth; neutral effects were predominant, but strong soil-mediated inhibitory effects on certain native species occurred. Our results support the hypothesis that successful plant invaders create biological legacies in soil that inhibit native growth, but suggest also this mechanism of invasion will have nuanced effects on community dynamics, as some natives may be unaffected by such legacies. Such native species may be valuable as nurse plants that provide cost-effective restoration of soil conditions needed for efficient establishment of diversified grasslands.

Interpretive Summary: In the Midwestern United States, regulatory, market, and policy pressures could convert large areas from annual agriculture to seminatural grassland agroecosystems, e.g., as part of a national effort to produce energy crops. Native grassland perennials could be used in these grasslands to reduce production costs, conserve soil quality, conserve native biodiversity, and enhance carbon sequestration in grassland agroecosystems. However, producer interest in seminatural grassland systems is reduced by current difficulties in reliable and cost-effective establishment of these species, and weed management during establishment is a major concern. Many lines of evidence suggest that weedy exotic species can alter soils physically, microbially, or both, creating a “legacy: that persists after control or removal of these species. This legacy effect may contribute significantly to the risk of additional weed invasion and poor performance of desirable species during grassland establishment. We examined such legacy effects of smooth brome, crested wheatgrass, and leafy spurge, three exotic perennials that are highly invasive in grasslands. These species had strong inhibitory legacy effects on certain native species, but other native species were unaffected. We found that native species did not have inhibitory legacy effects, suggesting that managers should expect that successful plant invaders may leave soil legacies that will inhibit native growth in the establishment phase. We found that some natives were unaffected by such legacies, and these particular species may be valuable as “nurse plants” or cover crops that provide cost-effective conditioning of soils, thereby restoring soil conditions needed for efficient establishment of desirable native species.

In Florida, air-potato is an invasive weed with high management priority, which may soon be targeted using classical biological control. This yam was introduced during the early 20th century by the United States Department of Agriculture (USDA) from areas throughout its extensive range. Our objectives were to characterize the genetic diversity of the invasive population in Florida and to identify the source regions of introduction. Authorities have often asserted the African provenance of the species in Florida, but our analyses, conducted using chloroplast markers, indicate that Florida air-potato is more similar to specimens examined from China than to those from Africa. Low intraspecific genetic diversity in Florida indicates that the invasive population was the result of at least two introductions becoming established in Florida.

Nomenclature: Air-potato, Dioscorea bulbifera L

Interpretive Summary: Two distinct, but genetically similar, chloroplast haplotypes were identified in the Florida population of the invasive yam vine, air-potato. Assuming no contribution of somatic mutation in the invasive population, these data are consistent with a minimum of two introductions into Florida. This study contributes baseline information regarding the diversity of air-potato before implementation of biological control in Florida. In the course of examining plant material to locate a putative source population, we characterized the genetic diversity of African air-potato, sampling six haplotypes on that continent. Three of the most prevalent African haplotypes were also geographically widespread, being found in both East Africa and West Africa. These three are also the most genetically distant from Florida and Chinese haplotypes, relative to other African haplotypes, as determined by parsimonious network analysis. A neighbor-joining analysis supported the relationships inferred in a statistical parsimony network. Our study did not find evidence that haplotypes were geographically structured in Africa. Despite a small number of microsatellite markers employed, this methodology confirmed the similarity of Chinese and Florida samples. Future work using a greater number of markers is expected to reveal greater intraspecific diversity, whereas further sampling in Asia and Oceania may localize Florida's source population to a regional, rather than a continental, level. Integrative taxonomic approaches are needed to synthesize morphological treatments of this species with more recent molecular studies, including the present work.

Cutleaf teasel (Dipsacus laciniatus) is an exotic, invasive plant that infests roadsides and other minimally disturbed areas. Plants in established stands appear to be a mixture of rapidly growing rosettes and rosettes with developing reproductive structures. Research that is focused on seed characteristics and their contribution to the spread of plants may be a key to precluding spread of cutleaf teasel in the field. Field studies were conducted to determine the viability and germinability of seeds after flowering, seedling emergence patterns, and seed persistence. Flowering (60% of anthesis) was observed under natural conditions on July 24, 2004, and July 16, 2005. Seeds harvested 12 d after flowering exhibited 43% viability and 2.5% germination. Seed weight and viability were greatest 30 d after flowering, but germination was < 32%. Seedling emergence was monitored over a 12-mo period with the greatest emergence in April and October with 33% of seeds germinating. Seed persistence was evaluated over a 3-yr period under field conditions. Up to 84% of the germinated seeds had germinated during the first year, with 6% of seeds remaining viable after 3 yr. Although seed persistence was relatively short, the rapid development of seeds following flowering as well as seedling emergence in both fall and spring suggests management practices are needed throughout the year to restrict reestablishment spread of cutleaf teasel.

Nomenclature: Cutleaf teasel, Dipsacus laciniatus L

Interpretive Summary: Cutleaf teasel is an exotic biennial plant in the northeastern United States. Reproduction is only by seed. Studies of seed viability after flowering, germination, and persistence showed that cutleaf teasel produced viable seeds 12 d after flowering, and completed maturation in 30 d. Emergence was concentrated in 2 mo, April and October, and 6.1% of the seeds remained viable in the seed bank after 3 yr. Results suggest that herbicides should be applied after the two peaks of emergence and repeated for at least 3 yr. Mowing cutleaf teasel plants must be done before flowering to prevent dispersal of viable seeds. Monitoring seed production and seed bank depletion will help to prevent or reduce cutleaf teasel infestations.

The Eurasian perennial mustard, hoary cress, Cardaria draba (Brassicaceae), is an invasive weed in western North America that can displace native plants and other desirable forage species in pastures and rangelands. This study investigated the competitive ability of 11 grasses representing nine species in the genera Festuca and Poa from either North America (NA) or Europe (EU) against hoary cress. Hoary cress was grown alone and with four different grass densities under controlled conditions with ample water and nutrient supply, typical for riparian and disturbed habitats, in which hoary cress infestations are commonly found. Of the five Festuca grasses tested, only Schedonorus phoenix ( =  F. arundinacea) decreased hoary cress biomass. Four of the six Poa grasses tested decreased hoary cress biomass and were ranked by decreasing competitive effects as follows: P. annua > P. compressa (EU) > P. pratensis (EU) > P. compressa (NA). The most competitive grass species also experienced higher intraspecific competition. Within both grass genera, Eurasian species were more competitive against hoary cress than their North American congeners, which suggests that hoary cress establishment may be facilitated by lower biotic resistance in the invaded range. Regardless of origin, sod-forming Poa species were competitive at low densities, while Festuca species only had significant effects at high densities if at all. Based on our results we recommend the use of Poa species for restoration of riparian and disturbed sites following the control of hoary cress infestations to restrict recolonization.

Nomenclature: Annual bluegrass, Poa annua L. POAAN; Canada bluegrass, Poa compressa L. POACO; hoary cress, Cardaria draba (L.) Desv. [ =  Lepidium draba L.] CADDR; Idaho fescue, Festuca idahoensis Elmer; Kentucky bluegrass, Poa pratensis L. POAPR; red fescue, Festuca rubra L. FESRU; rough fescue, Festuca campestris Rydb.; Sandberg bluegrass, Poa secunda J. Presl.; sheep fescue, Festuca ovina L. FESOV; tall fescue, Schedonorus phoenix (Scop.) Holub [ =  Festuca arundinacea Schreb.] FESAR

Interpretive Summary: Hoary cress is an exotic clonal perennial mustard that invades disturbed rangeland and riparian areas in the western United States This study compared the competitive ability of 11 native and exotic grass species with regard to hoary cress in a greenhouse experiment that mimicked environmental conditions typically found at habitats highly suitable to hoary cress invasion (abundant nutrients and moisture). We used some of the grass species most commonly used in revegetation and found that three out of six Poa species tested decreased hoary cress plant biomass during its initial establishment phase, whereas only one of five Festuca species tested affected hoary cress growth. European grasses generally were more competitive than grasses of North American origin. The lower competitive ability of North American species suggests that there is a lower level of biotic resistance to hoary cress invasion in our native communities. We recommend planting of Poa species for restoration of nutrient-rich, riparian habitats, following chemical or mechanical control of hoary cress to limit recolonization o

We investigated the use of sheep for controlling the spread of purple loosestrife in a wet meadow in upstate New York from June to August 2008. Changes in the purple loosestrife population and vascular plant community structure were monitored as a function of the grazing of two ewes, “rotated” through four “experimental” paddocks at 2- to 3-d intervals. Comparative data were collected in “reference” paddocks from which sheep were excluded. Purple loosestrife was heavily grazed and most plants did not flower in experimental paddocks. Purple loosestrife cover declined by 40.7% in the experimental paddocks but did not change significantly in the reference paddocks. After grazing, species richness was 20% higher in experimental than reference paddocks.

Nomenclature: Purple loosestrife, Lythrum salicaria L

Interpretive Summary: Although livestock is widely used to manage nuisance plant species on rangelands, little is known about the effectiveness of this approach, called “targeted grazing,” in wetlands, and controlled, systematic studies on the effectiveness of targeted grazing are scarce. We investigated the efficacy of using Romney sheep to control the spread of purple loosestrife in a wet meadow in upstate New York. We were interested in the impacts of the sheep on both the invasive population and on the larger plant community. The sheep were rotated through a system of small (200-m²) paddocks using a protocol, intensive rotational grazing (IRG), that employs high stocking densities, i.e., two to four times more livestock biomass than conventional grazing, and high frequency rotations (2 to 3 d per paddock). As such, IRG mimics the distribution, in time and space, of large, wild, herbivorous herding mammals on a landscape. The sheep fed on purple loosestrife (and other invasives), preventing flowering and reducing purple loosestrife cover by about 40%, relative to reference paddocks from which sheep were excluded. After the grazing phase of the study, species richness was 20% higher in the grazed paddocks than in the reference paddocks. Although a great deal remains to be learned about the impacts of targeted grazing and its underlying mechanisms, it appears that the technique can be used to help manage purple loosestrife, and perhaps other invasive plant species, in emergent wetlands and wet meadows.

Tamarisk, a shrub or small tree native to Eurasia, was introduced to North America in the early 1800s and is now naturalized throughout many riparian areas of the southwestern United States, where extensive research has been conducted. It is a more recent invader to the northern Great Plains, and fewer studies have been conducted on tamarisk ecology and management in this area. The objectives of this research were to investigate the overwintering potential of tamarisk seeds in Montana and the relationship between hydrologic conditions and historic tamarisk establishment. Emergence of seedlings from seeds stored for different time periods at a range of temperatures was evaluated in a greenhouse study. Emergence rates declined after a 7-d storage period, but storage time had no effect on subsequent emergence rates, and seeds stored at −14 C and 5 C had greater emergence rates than those stored at 20 C and 35 C. Patterns in tamarisk establishment were assessed through age and hydrologic data collected from a reservoir (Fort Peck), a regulated river (Bighorn), and an unregulated river (Yellowstone) in Montana. These data indicated that tamarisk establishment at the reservoir was closely related to historic water levels, whereas establishment on rivers was not related to flow. However, data from the rivers indicated that recruitment differed between regulated and unregulated rivers, with the regulated river having less recruitment after the period of initial colonization than the unregulated river. Our results show that tamarisk seeds have the ability to overwinter in Montana and can establish under a range of flow conditions, indicating potential recolonization of sites after tamarisk removal.

Nomenclature: Tamarisk, species in the genus Tamarix L., primarily Chinese tamarisk, Tamarix chinensis Lour., saltcedar, Tamarix ramosissima Ledeb., and their hybrids

Interpretive Summary: Tamarisk is a shrub or small tree that has invaded riparian areas of the southwestern United States, and is becoming more of a concern in the northern Great Plains. Although considerable ecological research has been conducted on tamarisk in the Southwest, much less information exists to guide its management in its northern range. This research investigated tamarisk seed overwintering potential in Montana and the effects of reservoir (Fort Peck) level and river (Bighorn and Yellowstone) flows on tamarisk establishment. Results showed that tamarisk seeds have the ability to survive cold and cool temperatures (−14 and 5 C) for at least 6 mo, but that warm and hot temperatures (20 and 35 C) led to declines in seedling emergence, with no seeds surviving more than 90 d at 35 C. Results from rivers showed that tamarisk trees can establish under various flows, including very high and very low peak flows. Together, these results suggest that sites where tamarisk has been removed could be subjected to recolonization from a short lived seedbank or from other seed sources, regardless of flow conditions. Therefore, managers should implement a monitoring program to detect new colonization after treatment. Results from the reservoir showed that tamarisk established as the water level receded, suggesting that managers should monitor around the shoreline as water levels decline to enhance early detection of new tamarisk populations. Finally, tamarisk populations decline subsequent to submergence by rising water, indicating that they can be controlled by raising the water level in reservoirs.

Perennial pepperweed (Lepidium latifolium) is a potential threat to biodiversity and ecosystem function in the communities that it invades. The mechanism for its successful invasion of riparian and wetland environments includes reproduction via seed and root propagules and its ability to withstand long duration flooding and saline conditions once established. Controlling this species presents a number of challenges for land managers, including difficult property access, limited herbicide choice, and the varied success of weed control measures. In this study, we tested the efficacy of a nonchemical-modified tarp treatment, and compared posttreatment stem counts to herbicide treatments with Mow–glyphosate and chlorsulfuron in a wildland setting. We found that tarping applied in combination with a mow and till treatment was effective at controlling perennial pepperweed at levels similar to herbicide treatments with Mow–glyphosate and with Mow–chlorsulfuron. However, Mow–Till–Tarp treatment is extremely time consuming and has the potential to limit native plant community recovery.

Nomenclature: Chlorsulfuron; glyphosate; perennial pepperweed, Lepidium latifolium L. LEPLA

Interpretive Summary: Perennial pepperweed (Lepidium latifolium) threatens wildland and natural areas where maintaining existing biodiversity and restoring degraded sites is a key concern to land managers. Herbicides are an effective control technique for perennial pepperweed, but are not always feasible in natural areas. In this study we tested the implementation of two tarping techniques as nonchemical alternatives to glyphosate and chlorsulfuron herbicide use for controlling perennial pepperweed in riparian areas.

We compared the efficacy of Mow–Tarp and Mow–Till–Tarp techniques to Mow–glyphosate and Mow–chlorsulfuron techniques for controlling perennial pepperweed. This comparison showed that a tarp treatment, where an area is mowed, tilled, and then tarped, is effective at controlling perennial pepperweed infestations. This method can control up to 100% of perennial pepperweed under a tarped area and is comparable to Mow–glyphosate and Mow–chlorsulfuron treatments. In addition, we found that a Mow–glyphosate treatment is just as effective at controlling perennial pepperweed as a Mow–chlorsulfuron treatment 1 yr after treatment. The Mow–glyphosate method can be applied to emergent vegetation and can be less detrimental to the native vegetation than a Mow–Till–Tarp technique, which leaves behind exposed soil and can require subsequent restoration.

Bushkiller, an aggressive perennial vine native to Southeast Asia, has invaded several sites in Alabama, North Carolina, Texas, Louisiana, and Mississippi. Bushkiller has only recently been discovered in North Carolina. The potential economic and environmental consequences associated with established exotic invasive perennial vines and the lack of published control measures for bushkiller prompted research to be conducted at North Carolina State University that may be used in an early-detection rapid-response program. Field and greenhouse studies were conducted to determine bushkiller response to selected foliar-applied herbicides. Field study 1 evaluated efficacy of glyphosate, triclopyr, triclopyr plus 2,4-D, triclopyr plus aminopyralid, and triclopyr plus glyphosate applied postemergence to bushkiller. No control was evident from any treatment at 10 mo after application. In a separate experiment, aminocyclopyrachlor, imazapyr, metsulfuron, sulfometuron, and sulfometuron plus metsulfuron were applied postemergence to bushkiller. Control with aminocyclopyrachlor, imazapyr, sulfometuron, and sulfometuron plus metsulfuron was 88 to 99% at 10 mo after application. Each treatment was also applied to bushkiller in a greenhouse trial. Aminocyclopyrachlor and triclopyr-containing treatments generally resulted in the greatest control, lowest dry weights, and shortest vine lengths among the treatments. These results indicate that several herbicides may be employed initially in an early-detection, rapid-response program for bushkiller. Additional research is needed to determine how effective these herbicides would be in multiple-season treatments that may be required at well established bushkiller infestation sites.

Nomenclature: 2,4-D; aminocyclopyrachlor, 6-amino-5-chloro-2-cyclopropyl-4-pyrimidinecarboxylic acid; aminopyralid; glyphosate; imazapyr; metsulfuron; sulfometuron; triclopyr; bushkiller, Cayratia japonica (Thunb.) Gagnep

Interpretive summary: Bushkiller is an exotic invasive perennial vine in the Vitaceae family that has been documented in Texas, Louisiana, Mississippi, Alabama, and North Carolina. In North Carolina, bushkiller has recently been listed as a Class B Noxious Weed by the North Carolina Department of Agriculture. Bushkiller aggressively overtops neighboring vegetation, making it problematic ecologically and economically. An extensive literature review provided no guidelines for control of bushkiller. Therefore, selected herbicides were evaluated to initiate development of bushkiller control guidelines. Further studies are needed to document the effects of multiple-season applications of these herbicides. Our results from one season of treatments indicated that multiple-season applications of a selected herbicide will be necessary for eradication of existing bushkiller infestations.

Vigorous stands of perennial grasses can effectively provide long-term control of many invasive plants on rangelands. However, in degraded conditions, successful reestablishment of perennial grasses can be compromised by invasive annual grasses, such as downy brome. Propoxycarbazone-sodium is a selective herbicide currently labeled for downy brome control in small grains, but its potential use on rangelands is unknown. Studies were conducted from 2004 through 2008 at three rangeland sites in Colorado and Nebraska to evaluate downy brome control and perennial grass injury with propoxycarbazone-sodium and imazapic. Propoxycarbazone-sodium provided satisfactory downy brome control with grass injury equal to or less than imazapic when rainfall followed the fall application. A second set of studies was conducted from 2007 to 2008 at Lingle, WY, and Scottsbluff, NE, to determine the plant-back interval and postemergence application response of seven perennial grass species to propoxycarbazone-sodium and imazapic. Grass tolerance to both herbicides was good when applied 90 and 120 d before planting (DBP). However, grass injury increased as plant-back interval decreased. The greatest impact on plant biomass was observed from herbicide applied at planting or after planting. Crested and intermediate wheatgrass (Agropyron cristatum and Thinopyrum intermedium) biomass production was not affected when herbicides were applied 90 or 120 DBP. Western wheatgrass (Pascopyrum smithii) and Russian wildrye (Psathyrostachys juncea) showed tolerance to imazapic applied before planting. Smooth brome (Bromus inermis), sheep fescue (Festuca ovina), and orchardgrass (Dactylis glomerata) showed the least amount of tolerance to propoxycarbazone-sodium and imazapic.

Nomenclature: Propoxycarbazone-sodium; imazapic; downy brome, Bromus tectorum L.; crested wheatgrass, Agropyron cristatum (L.) Gaertn.; intermediate wheatgrass, Thinopyrum intermedium (Host) Barkworth & D.R. Dewey; orchardgrass, Dactylis glomerata L.; Russian wildrye, Psathyrostachys juncea (Fisch.) Nevski; sheep fescue, Festuca ovina L.; smooth brome, Bromus inermis Leyss.; western wheatgrass, Pascopyrum smithii (Rydb.) Á. Löve

Interpretative Summary: The establishment of perennial grasses that can provide long-term control of invasive plants can be jeopardized by infestations of downy brome. Propoxycarbazone-sodium is a selective herbicide for downy brome control in small grains, but its performance on established perennial grass stands in rangelands was unknown. A study was conducted in Colorado and Nebraska and indicated satisfactory downy brome control with propoxycarbazone-sodium at 45 g ha⁻¹ (0.04 lb ac⁻¹) when applied in fall. Downy brome control was consistent when fall applications were followed by seasonal rainfall. An additional finding of the study was that grass injury was equal to or less than that caused by imazapic. In a second study, the response of seven perennial grasses to propoxycarbazone-sodium at 60 g ha⁻¹ and imazapic at 105 g ha⁻¹ applied 120, 90, and 30 d before planting (DBP), at planting, and 30 d after planting (DAP) was examined. Grass tolerance to both herbicides was good for applications 90 and 120 DBP. Grass injury increased as herbicide applications were made closer to planting. Crested and intermediate wheatgrass biomass production was not affected when herbicides were applied 90 or 120 DBP. Western wheatgrass and Russian wildrye showed tolerance to imazapic applied before planting. When planting orchardgrass, smooth brome, or sheep fes

Orange hawkweed is a perennial European plant that has colonized roadsides and grasslands in south-central and southeast Alaska. This plant is forming near-monotypic stands, reducing plant diversity, and decreasing pasture productivity. A replicated greenhouse study was conducted in 2006 and repeated in 2007 to determine the efficacy of six herbicides (aminopyralid, clopyralid, picloram, picloram chlorsulfuron, picloram metsulfuron, and triclopyr) for orange hawkweed control. Based on results of the greenhouse trials, replicated field studies were conducted at two sites each year in 2007 and 2008 with three rates each of aminopyralid and clopyralid to determine efficacy of orange hawkweed control and impacts on nontarget native vegetation. In the field, only aminopyralid at 105 g ae ha⁻¹ (0.1 lb ae ac⁻¹) and clopyralid at 420 g ae ha⁻¹ controlled orange hawkweed consistently, with peak injury observed 1 yr after treatment. Control with clopyralid was slightly less than that provided by aminopyralid at all observation times, except at Homer, AK, in 2007, where there was a near-monotypic stand of orange hawkweed, and clopyralid did not remove all orange hawkweed plants. Aminopyralid controlled clover (Trifolium spp.), seacoast angelica (Angelica lucida), arctic daisy (Chrysanthemum arcticum), common hempnettle (Galeopsis tetrahit), and willow (Salix spp.) in the treated areas. Other plant species, such as grasses and some annual forbs, recovered or increased following control of the hawkweed. Clopyralid had less impact on nontarget species with most recovering the year after treatment. In a pasture system, where grasses are preferred to forbs and shrubs, aminopyralid has an advantage because it controls a broader array of forbs compared with clopyralid. In natural areas, where the desire to retain biodiversity and the aesthetics of multiple forb species mixed with grasses and willows is preferred, clopyralid will leave greater species diversity than aminopyralid.

Nomenclature: Aminopyralid; chlorsulfuron; clopyralid; metsulfuron; picloram; triclopyr; Orange hawkweed, Hieracium aurantiacum L. HIEAU; arctic daisy, Chrysanthemum arcticum L. CHYAR; clover, Trifolium spp.; common hempnettle, Galeopsis tetrahit L. GAETE; seacoast angelica, Angelica lucida L. ANLU; willow, Salix spp

Interpretive Summary: Land managers are often faced with the task of selecting a weed control treatment from among a wide variety of options. Frequently there are people with a wide variety of concerns, opinions, and ideas that will want to question or even legally challenge the manger's decision. Orange hawkweed is a nonindigenous, invasive plant species that is spreading rapidly in Alaska, and there are few options for its control. Chemical control methods for orange hawkweed have not been studied in Alaska. In addition, there are concerns about impacts on native plant species and hopes that reduced rates of herbicides will result in acceptable control with minimal impacts on other plant species. The results of this research indicate that higher rates of both aminopyralid and clopyralid are needed to effectively control orange hawkweed. In a pasture system, where grasses are preferred to forbs and shrubs, aminopyralid has an advantage because it will control many other species compared with clopyralid, and there will be an increase in grass productivity. In a field, where the aesthetics of multiple forb species mixed with grass and willows is preferred, clopyralid will have a reduced effect on many more of these species than will aminopyralid.

The objective of this study was to use fecal near-infrared reflectance spectroscopy (NIRS) to estimate the percent spotted knapweed in the diet of grazing ewes to fine tune grazing prescriptions. In trial 1, five ewes were randomly selected from a band of 900 ewes to estimate weekly variation in percent spotted knapweed in the diet. Fecal samples from these ewes were collected weekly from June 22 through August 17 (nine collections per ewe for a total of 45 observations). In trial 2, fecal samples were collected from 89 ewes in the band to estimate variability in percent spotted knapweed in the diet among sheep at two sampling dates corresponding to the bud (July 13) and postflowering (August 15) stages of spotted knapweed phenology. In trial 1, the percent spotted knapweed in the diet was similar (P > 0.05) on June 22, June 29, July 13, and July 20 but was greater (P < 0.05) on July 6 than June 22 and 29. Percent spotted knapweed in the diet was also greater (P < 0.05) from July 27 through August 17 than from June 22 through July 20. On July 13 in trial 2, 55% of ewes had 0 to 5% spotted knapweed in their diets, whereas 44% of ewes had 5 to 20% spotted knapweed in their diets. On August 15, only 1% of ewes had < 10% spotted knapweed in their diets, whereas 44% of ewes had 20 to 25% spotted knapweed in their diets. The estimated percent spotted knapweed in the diet of the 89 ewes in trial 2 was similar to that of the five ewes during the same time period in sampling trial 1 (5.3% compared with 5.0% for sampling trials 1 and 2, respectively, on July 13 and 22.0% compared with 20.7% on August 15 and 17 for trials 1 and 2, respectively). This is the first study to estimate percent spotted knapweed in the diet of individual grazing ewes. These data suggest that the best time to graze spotted knapweed–infested pastures would be in late July or early August, when spotted knapweed is flowering, but before viable seed production.

Nomenclature: Spotted knapweed, Centaurea stoebe L. ssp. micranthos (Gugler) Hayek

Interpretive Summary: Spotted knapweed is an aggressive invasive species that has replaced native perennial grasses on many foothill range and pasture lands in the United States and Canada. Herbicides, insects, pathogens, and fire have not effectively contained the spread of this weed. In recent years, there has been a renewed interest in using small ruminant grazers in integrated weed management programs. This has led to the need to characterize differences in diet preferences more specifically throughout the grazing period and between individual animals under typical grazing conditions. Ours is the first study to estimate percent spotted knapweed in the diets of individual sheep throughout the grazing season. Percent spotted knapweed in the diet of grazing sheep was lower in June and July than August. Percent spotted knapweed in the diet of grazing ewes averaged 5% in mid-July and 21% in mid-August. A subsample of five ewes was adequate for estimating percent spotted knapweed in the diets of ewes in the band. Our data suggest that sheep will consume spotted knapweed when herded to areas with high concentrations and that the best time to graze spotted knapweed–infested pastures would be in late July or early August, when spotted knapweed is flowering but before viable seed production.

Establishment potential is one of the primary components of invasive species risk assessment. Models that predict establishment of potentially invasive ornamental crops often ignore differences among cultivars and the variability in plant response to site-specific factors. The objective of this study was to determine the extent to which differences among cultivars and habitat characteristics affect establishment of 5 cultivars of ornamental cleome. Experiments were conducted to compare germination, survival, and growth of cultivars in cultivated (gardens) and noncultivated (roadsides and prairies) environments and, in prairies, the effects of competition (low, intermediate, and high). In the first experiment, germination, survival, and growth were recorded in gardens, prairies, and roadsides in four locations in Minnesota. In the second experiment, the effects of competition with resident species were studied in five seed lots from three cultivars in three prairie sites. Additionally, a quantitative description of germination and transplantable seedling quality, when grown under greenhouse production standards, was obtained and compared with results from the cultivated and noncultivated outdoor environments. Germination in greenhouse conditions was significantly greater (78%) than in garden, prairies, or roadsides (< 46%). Mortality was greater in noncultivated than in cultivated environments (3% wk⁻¹ and 1.4% wk⁻¹, respectively). Survival was affected by competition, which reduced population establishment. Cultivar differences were most pronounced at seedling emergence, whereas habitat characteristics were more influential at later stages of the life cycle. Germination and plant height were similar among noncultivated environments. Variability in seedling emergence, survival, and growth in response to cultivar, habitat, and competition are important determinants of establishment potential. Among the cultivars studied, the native cleome, roughseed clammyweed, has a greater establishment potential than the nonnative cleome, spiderflower.

Nomenclature: Roughseed clammyweed, Polanisia dodecandra (L.) DC. PONGR; spiderflower, Cleome hassleriana Chod. CLEHA

Interpretive Summary: Gardens can be sources of nonnative, invasive, ornamental crops. Cleomes are popular ornamental garden plants that have escaped cultivation and naturalized in several countries including the United States. Native and nonnative cleome cultivars were evaluated in this study. The objective of this study was to compare establishment potential in gardens, roadsides, and prairie environments; effects of competition in prairies were also assessed. Garden settings are most suitable for reseeding, although germination is also possible in roadsides and prairies. Mortality was greater in noncultivated than in cultivated environments (3% wk⁻¹ and 1.4% wk⁻¹, respectively). Survival was affected by competition, which reduced population establishment. Cultivar differences were most pronounced at seedling emergence, whereas habitat characteristics were more influential at later stages of the life cycle. Germination and plant height were similar among noncultivated environments. Variability in seedling emergence, survival, and growth in response to cultivar, habitat, and competition are important determinants of establishment potential. Thus, for flower-breeding programs to assess invasive potential, specific target environments—in which a potentially invasive species could invade—are more critical than testing in cultivated sites. Competition plays a significant role in decreasing population establishment at the seedling stage. Native North American species may be more likely to establish as weeds than nonnative species.

The interactions between native and exotic species occur on a continuum from facilitative to competitive. A growing thrust in invasive species science is differentiating where particular native species occur along this continuum, with practical implications for identifying species that might reduce the invasibility of ecosystems. We used a greenhouse experiment to develop a competitive hierarchy of 27 native species with red brome, an invasive annual grass in the arid lands of the southwestern United States, and a field study to assess in situ responses of brome to native perennial species in the Mojave Desert. Native species most competitive with brome in the competition experiment included the annuals Esteve's pincushion and western fiddleneck and the perennials eastern Mojave buckwheat, sweetbush, and brittlebush, which reduced brome biomass to 49 to 70% of its grown-alone amount. There was no clear difference in competitive abilities with brome between annual and perennial natives, and competiveness was not strongly correlated (r  =  0.15) with the biomass of the native species. In the field, sweetbush and brittlebush supported among the least cover of brome, suggesting congruence of the strong early competitive abilities of these species with in situ patterns of brome distribution. At the other extreme, brome attained its highest average cover (19%) below littleleaf ratany, significantly greater than all but 3 of the 16 species evaluated. Cover by brome was only weakly related (r  =  0.19) to the area of the perennial canopy, suggesting that factors other than the sizes of perennial plants were linked to differences in brome cover among species. Results suggest that (1) interactions with brome differ substantially among native species, (2) these interactions are not as closely linked to biomass production as in more temperate regions, and (3) there is potential for identifying native species that can reduce invasion of desert ecosystems.

Nomenclature: Red brome, Bromus rubens L.; brittlebush, Encelia farinosa A. Gray ex Torr.; eastern Mojave buckwheat, Eriogonum fasciculatum Benth.; Esteve's pincushion, Chaenactis stevioides Hook. & Arn.; littleleaf ratany, Krameria erecta Willd. ex Schult.; sweetbush, Bebbia juncea (Benth.) Greene; western fiddleneck, Amsinckia tessellata Gray

Interpretive Summary: Identifying native species, for use in revegetation and plant-community augmentation projects, capable of reducing the fitness of invasive species is a major thrust in invasive species science and management. We sought to identify native species competitive with Bromus rubens (hereafter Bromus), an exotic annual grass increasing fuel loads and facilitating fires devastating to resources in the arid lands of the southwestern United States. In a greenhouse experiment screening the competitive abilities of 27 native species, we found that the native annuals Chaenactis stevioides and Amsinckia tessellata and the perennials Eriogonum fasciculatum, Bebbia juncea, and Encelia farinosa mostly strongly competed with Bromus. In a companion field assessment of Bromus below different native perennial plants in the eastern Mojave Desert, Bromus cover varied more than ninefold among 16 species. Species such as Thamnosma montana, B. juncea, E. farinosa, and E. fasciculatum showed promise in their ability to support l

Broom snakeweed is a native weed widely distributed on rangelands of western North America. It often increases to near monocultures following disturbance from overgrazing, fire, or drought. This paper presents an up-to-date review of broom snakeweed toxicology, seed ecology, population cycles, succession, and management. The greatest ecological concern is that broom snakeweed displaces desirable forage for livestock or wildlife and greatly reduces biodiversity. It also is toxic and can cause abortions in all species of livestock. Propagation usually is pulse-driven in wet years, allowing large expanses of even-aged stands to establish and dominate plant communities. Snakeweed can be controlled by prescribed burning or spraying with herbicides. A weed-resistant plant community dominated by competitive grasses can prevent or minimize its reinvasion.

Nomenclature: Broom snakeweed, Gutierrezia sarothrae (Pursh) Britton & Rusby GUESA

Interpretive Summary: Broom snakeweed is an invasive native subshrub that is distributed widely across rangelands of western North America. In addition to its invasive nature, it contains toxins that can cause death and abortions in livestock. It establishes in years of above-average precipitation following disturbance by fire, drought, or overgrazing. This allows widespread, even-aged stands to develop that can dominate plant communities. Although its populations cycle with climatic patterns, it can be a major factor impeding succession of plant communities. Snakeweed can be controlled with prescribed burning and herbicides; however, a weed-resistant plant community should be established and/or maintained to prevent its reinvasion. Proper grazing management to maintain competitive grasses is essential for suppression of this invasive weed.

The introduced exotic vines pale and black swallowwort rapidly have become invasive throughout regions of the northeastern United States and adjoining areas of Canada. Preliminary studies have reported that the species are allelopathic, possibly contributing to their competitive ability and invasiveness. Results from our laboratory assays indicated that swallowwort root exudates caused significant root length reductions (e.g., 40% for butterfly milkweed and 20% for large crabgrass) and reduced germination (e.g., 25% for lettuce) of indicator species. Additional bioassays with dried swallowwort tissues demonstrated that tissue leachates caused varied responses in indicators, with both significant stimulatory and inhibitory effects. In particular, significant congeneric interactions were noted between the two swallowwort species. Evidence from this study of swallowwort tissue phytotoxicity has important implications for developing effective management and habitat restoration strategies for the two invasive species.

Nomenclature: Annual bluegrass, Poa annua L.; barnyardgrass, Echinochloa crus-gallis L. P. Beauv.; black swallowwort, Cynanchum nigrum (L.) Pers., non Cav.; butterfly milkweed, Asclepias tuberosa L.; common milkweed, Asclepias syriaca L.; large crabgrass, Digitaria sanguinalis (L.) Scop.; orchardgrass, Dactylis glomerata L.; pale swallowwort, Cynanchum rossicum (Kleopow) Borhidi; lettuce, Lactuca sativa L.; tomato, Solanum lycopersicum L

Interpretive Summary: Our laboratory studies provided evidence for the negative impact of pale (PSW) and black swallowwort (BSW) tissue leachates and root exudates upon the growth and germination of indicator species, in particular common and butterfly milkweed. Additionally, we found important congeneric interactions between the two species, with results suggesting a possible competitive advantage for BSW. Our study findings indicate that allelopathic interference might help to facilitate the dominance of swallowworts in cases of interspecific competition. However, field studies are needed to demonstrate whether this interference occurs under natural conditions, and whether it ultimately has an ecological significance in the invasiveness of the swallowworts in natural settings.

With regard to management, these results suggest that there could be unexpected ramifications to what is currently one of the recommended control strategies (in addition to chemical controls) for the two species—mowing. Mowing can be effective at preventing populations from spreading if timed to coincide with seed set and repeated throughout the growing season. Our research suggests that the abundant swallowwort leaf and stem material remaining on the soil surface after mowing potentially could release biologically active compounds into the soil rhizosphere as it decomposes. These leachates have the potential to inhibit the germination and seedling growth of sensitive plant species in the seedbank, or those used to restore treated sites. Consequently, if mowing is used to control invasive swallowwort populations, mowed biomass should be collected and removed from the treated site if possible.

Many perennial bioenergy grasses have the potential to escape cultivation and invade natural areas. We quantify dispersal, a key component in invasion, for two bioenergy candidates:Miscanthus sinensis and M. × giganteus. For each species, approximately 1 × 10⁶ caryopses dispersed anemochorously from a point source into traps placed in annuli near the source (0.5 to 5 m; 1.6 to 16.4 ft) and in arcs (10 to 400 m) in the prevailing wind direction. For both species, most caryopses (95% for M. sinensis and 77% for M. × giganteus) were captured within 50 m of the source, but a small percentage (0.2 to 3%) were captured at 300 m and 400 m. Using a maximum-likelihood approach, we evaluated the degree of support in our empirical dispersal data for competing functions to describe seed-dispersal kernels. Fat-tailed functions (lognormal, Weibull, and gamma (Γ)) fit dispersal patterns best for both species overall, but because M. sinensis dispersal distances were significantly affected by wind speed, curves were also fit separately for dispersal distances in low, moderate, and high wind events. Wind speeds shifted the M. sinensis dispersal curve from a thin-tailed exponential function at low speeds to fat-tailed lognormal functions at moderate and high wind speeds. M. sinensis caryopses traveled farther in higher wind speeds (low, 30 m; moderate, 150 m; high, 400 m). Our results demonstrate the ability of Miscanthus caryopses to travel long distances and raise important implications for potential escape and invasion of fertile Miscanthus varieties from bioenergy cultivation.

Nomenclature: Eulaliagrass, Miscanthus sinensis Anderss.; giant miscanthus, Miscanthus × giganteus Anderss

Interpretive Summary: Eulaliagrass (Miscanthus sinensis) has already escaped from ornamental plantings to form large naturalized populations hundreds of meters from original planting locations (Quinn et al. 2010). Our results suggest that these new populations could have established following long-distance seed dispersal. Because the potential for long-distance dispersal and subsequent establishment has been demonstrated, it is important to take preventive measures to avoid further propagule pressure and gene flow into naturalized populations. As has been pointed out, breeders of bioenergy and ornamental varieties can take steps to minimize the potential for escape by selecting for nonshattering seedheads, engineering glabrous seeds, and inducing sterility (Quinn et al. 2010). Our results indicate a strong effect of wind speed on dispersal distance in M. sinensis. Growers should be aware of the need to monitor for escaped plants, particularly if producing fertile varieties. Because we show that most giant miscanthus (Miscanthus × giganteus) and M. sinensis seeds were trapped near the source, exhaustive monitoring efforts should be conducted on a regular basis within 50 m of production fields. In addition, because we know that a small proportion of seeds can disperse several hundred meters from a source, growers should also coordinate efforts with local land managers to ensure early detection and control of escaped plants in nearby natural areas.

We investigated the single and combined effects of two biological control agents, the stem-mining weevil Hadroplontus litura and the pathogen Pseudomonas syringae pv. tagetis, with a herbicide (reduced or full application of glyphosate: 0.63 kg ae ha⁻¹, or 3.78 kg ae ha⁻¹, respectively) on the growth of Canada thistle, Cirsium arvense. We hypothesized that first, although each control method would have a negative effect on Canada thistle shoot biomass, root biomass, and shoot number, the integration of more than one control method would have greater impact than individual control methods. Second, we hypothesized that the order in which control methods are applied affects the outcome of the management program, with a pathogen application following weevil infestation being more effective than one prior to it. Although control methods impacted Canada thistle growth (P < 0.001, expect for a nonsignificant impact of glyphosate on shoot number), the combined effect of the three control methods behaved, generally, in an additive manner. A marginal interaction between the pathogen and the herbicide (P  =  0.052) indicated a slight antagonistic interaction between these control methods. An interaction between the two biological control agents tested (P < 0.001) indicated that application of a pathogen prior to the release of weevil larvae could be more deleterious to Canada thistle than a late application. The observed, mostly additive, relationship between biological control agents and herbicides implies that integrating control methods rather than using a single approach could lead to greater Canada thistle control.

Nomenclature: Glyphosate; Pseudomonas syringae pv. tagetis (Hellmers) Young et al.; stem-boring weevil, Hadroplontus litura (Fabricius); Canada thistle, Cirsium arvense (L.) Scop

Interpretative Summary: Canada thistle (Cirsium arvense) is one of the most problematic invasive plant species infesting crops, pastures, rangelands, roadsides, and noncrop areas in the United States, Canada, northern Europe, and New Zealand. Although many single methods have been developed to minimize Canada thistle spread and impact, this persistent invasive species continues causing problems in temperate regions of the world. Field data and mathematical models have shown that, if properly designed, integrated weed management can provide sustainable, economically viable, and successful weed control. In an integrated weed management program, control methods can interact synergistically, additively, or antagonistically, but few studies have specifically evaluated the existence and patterns of such relations. In repeated greenhouse experiments, we investigated the potential for suppressing Canada thistle using a combination of the stem-mining weevil Hadroplontus litura, the pathogen Pseudomonas syringae pv. tagetis, and reduced or full-rate applications of glyphosate (0.63 kg ae ha⁻¹, or 3.78 kg ae ha⁻¹, respectively). In general, individual control methods impacted Canada thistle growth and the combined effect of the three control methods behaved in an additive manner. A marginal interaction between the pathogen and the herbicide indicated an antagonistic interaction between these control methods. An interaction between the two biological control agents tested (P < 0.001) indicated that application of a pathogen prior the release of weevil larvae could be more deleterious to Canada thistle than a late application. The observed mostly additive relationship between biological control agents and herbicides implies t

Black henbane is a poisonous, invasive plant in the family Solanaceae, and is typically associated with highly disturbed environments, such as pipelines, roadsides, and mammalian burrows. Often, such disturbances require reseeding for successful restoration; thus, the potential exists for competition between henbane and perennial grasses commonly used in restoration projects. These competitive interactions have not, to our knowledge, been evaluated. We conducted a greenhouse study to compare the response of henbane when grown alone and in combination with three common, cool season, perennial, northern mixed prairie grass species. We examined both seedling and mature grass response to the presence or absence of henbane and the response of henbane to the grasses. Using the relative neighbor-effect index, black henbane was found to be a very poor competitor with mature grasses and two out of three seedling grasses tested. All measures of henbane growth were significantly lower among plants grown with a mature grass pot companion. Total biomass of henbane was up to 99% lower when grown with mature grasses. Mature grasses were not negatively affected when grown in combination with henbane. Western wheatgrass (Pascopyrum smithii) was the only seedling grass that was competitive with henbane but was also the only seedling grass negatively affected by henbane in both biomass and tiller production. These experiments suggest that henbane is not well suited for invasion of mature grass stands but may negatively influence some perennial grass seedlings in restoration situations.

Nomenclature: Black henbane, Hyoscyamus niger L.; western wheatgrass, Pascopyrum smithii (Rydb.) Á. Löve

Interpretive Summary: Black henbane is a poisonous, invasive annual or biennial in the family Solanaceae that appears to be an opportunistic invader, proliferating when competition is minimal. Black henbane can be successful in situations with limited competition and high nutrient and moisture availability. As such, henbane tends to be associated with disturbed conditions in which reseeding of native vegetation may be required. Revegetation with perennial grasses is an excellent method for long-term control of invasive plants but is often species and site specific. The results from these experiments indicate that black henbane is a poor competitor with Sandberg bluegrass and Idaho fescue seedlings, but it does negatively affect western wheatgrass seedlings. However, once established, all three grasses strongly suppressed henbane. For reclamation efforts along disturbance corridors, such as gas lines where black henbane often appears, multiple grass species may be effective in suppressing henbane.

Invasive plants are negatively affecting the ecological and economic production of rangelands by reducing resource productivity, decreasing biodiversity, displacing native vegetation, and altering ecosystem processes and functions. However, despite these well-known negative effects, once invasive plants are regionally established, limited effort is directed at preventing their continued spread across rangelands. Most efforts are directed at restoration at specific locations while additional rangelands are invaded. Restoring native plant communities invaded by exotic plants is frequently unsuccessful, especially in more arid environments, and is often too costly to apply at the scale required to make meaningful progress in reducing invasive plant populations relative to their expansion. Of the few prevention efforts being implemented, most are a second priority to control and restoration efforts. Integrating strategies to prevent new infestations and restrict the expansion of existing populations in invasive plant management programs is critical to limiting the negative effects of invasive plants in rangelands. However, we are “missing the boat” on this issue by not providing sufficiently developed and validated management actions. Limited information is available for developing management strategies to prevent the spread of invasive plants, although it has been suggested that land managers need to increase biotic resistance of desired plant communities, decrease invasive plant propagule pressure, and eradicate small incipient infestations to prevent the continued expansion of invasive plants. Thus, instead of scientifically validated methods developed to limit the spread of invasive plants, managers are often left with vague suggestions for preventing the continued spread of invasive plants. We suggest that if prevention is going to be successful, researchers are going to need to conduct more applied research to provide land managers with specific prevention strategies and quantify the benefits of various prevention strategies.