To validate carboxyterminal processing protease of D1 protein (CtpA) as a target for herbicide discovery, CtpA sense mRNAs were overexpressed to suppress the internal level of CtpA protein in mouseearcress plants. Using antibodies raised against recombinant CtpA protein, we demonstrated that we have generated transgenic mouseearcress plants with reduced levels of CtpA protein and plants with elevated levels of CtpA protein. Transgenic plants with severely reduced levels of CtpA protein exhibited a bleached and chlorotic phenotype and stunted growth. The mutant phenotypes were enhanced by bright illumination. However, plants with a slight reduction of CtpA protein did not exhibit the mutant phenotype and could not be distinguished from wild-type plants under normal growth conditions. Several CtpA enzyme inhibitors were shown to have herbicidal activity in planta. Interestingly, plants producing excessive amount of CtpA protein were shown to be resistant to these inhibitors. Our results suggest that CtpA is essential for plant growth and development, but a reduced amount of CtpA is sufficient to carry out its essential function. CtpA may be a good target for herbicide development, but very high levels of inhibition may be required to produce a herbicidal effect. In addition, overexpressing CtpA in target plants might provide a mechanism for producing plants resistant to the herbicide.

Nomenclature: CtpA, Carboxyterminal processing protease of D1 protein (also referred to as D1 protease); mouseearcress, Arabidopsis thaliana (L.) Heynh

Because of its ability to induce contact dermatitis, the establishment and spread of poison ivy is recognized as a significant public health concern. In the current study, we quantified potential changes in the biomass and urushiol content of poison ivy as a function of incremental changes in global atmospheric carbon dioxide concentration (CO₂). We also examined the rate of new leaf development following leaf removal to simulate responses to herbivory as functions of both CO₂ and plant size. The experimental CO₂ values (300, 400, 500. and 600 µmol mol⁻¹) corresponded approximately to the concentration that existed during the middle of the 20th century, the current concentration and near and long-term projections for this century (2050 and 2090), respectively. Over 250 d, increasing CO₂ resulted in significant increases in leaf area, leaf and stem weight, and rhizome length relative to the 300 µmol mol⁻¹ baseline with the greatest relative increase occurring from 300 to 400 µmol mol⁻¹. There was a nonsignificant (P  =  0.18) increase in urushiol concentration in response to CO₂; however, because of the stimulatory effect of CO₂ on leaf biomass, the amount of urushiol produced per plant increased significantly for all CO₂ above the 300 µmol mol⁻¹ baseline. Significant increases in the rate of leaf development following leaf removal were also observed with increasing CO₂. Overall, these data confirm earlier, field-based reports on the CO₂ sensitivity of poison ivy but emphasize its ability to respond to even small (∼ 100 µmol mol⁻¹) changes in CO₂ above the mid-20th century carbon dioxide baseline and suggest that its rate of spread, its ability to recover from herbivory, and its production of urushiol, may be enhanced in a future, higher CO₂ environment.

Nomenclature: Poison ivy, Toxicodendron radicans, (L.) Kuntze TOXRA

A hand-held hyperspectral radiometer was used to measure differences in reflectance characteristics of 24 Palmer amaranth and 15 pitted morningglory accessions collected from the central and southern United States. A hyperspectral reflectance reading was collected from two mature leaves at 24 and 27 d after emergence (DAE) for each accession. Two analysis techniques, linear discriminant analysis and best spectral-band combination (BSBC) analysis, were performed using various vegetation indices, spectral bands, and individual wavelengths. Differentiation of individual accessions was difficult. Palmer amaranth accession classification accuracies were < 50% using both analysis techniques, except one accession collected in South Carolina (63%), when pooled over data acquisition dates. Pitted morningglory accession classification accuracies were also generally < 50%. Classification accuracies were higher using BSBC analysis at 24 DAE; however, at 27 DAE only one accession resulted in classification accuracy > 30%. These results suggest there are only slight reflectance differences within Palmer amaranth and pitted morningglory accessions. These differences may not be predictable based upon accession origin because of the genetic diversity of Palmer amaranth and pitted morningglory. However, differentiation between Palmer amaranth and pitted morningglory was 100%. Thus, spectral sensors used to differentiate between Palmer amaranth and pitted morningglory do not need to be calibrated for a particular region of the United States, and differentiation between these two species can be made using reflectance characteristics.

Nomenclature: Palmer amaranth,Amaranthus palmeri S. Wats. AMAPA, pitted morningglory, Ipomoea lacunosa L. IPOLA

Old-growth forest in Indiana consists of a few remnant stands in a fragmented agricultural landscape. Old-growth stands can differ substantially from more recently disturbed stands in species composition and may be less susceptible to invasion by exotic plant species. Herbaceous species were sampled in quadrats placed in a repeating pattern along two perpendicular transects in six old-growth stands in Indiana in spring and summer of 2005. Transects extended from forest edges to the center of each stand. Woody exotic species were sampled in 10-m-wide belts centered on each transect during the summer sampling run. Herbaceous exotic species were a minor component of the understory community in all stands and accounted for less than 1% of total herbaceous cover in all six stands. Exotic woody species generally followed a similar pattern. Four sites contained no more than two woody exotic species, and these were encountered at relatively low frequencies and densities. However, European privet and multiflora rose were present in 27 and 48% of quadrats in one stand, respectively. Thus, although both herbaceous and woody exotic species were relatively rare, heavy infestations of woody exotic species are possible. This suggests that, for some old-growth sites, exotic species can overcome both local and landscape factors that limit their abundance.

Nomenclature: European privet, Ligustrum vulgare L, multiflora rose, Rosa multiflorum Thunb. ex Murr

The introduced species weeping alkaligrass, and the native species Nuttall's alkaligrass, two of the most salt-tolerant C₃ grasses found in arid and semiarid environments of western North America, occur within the Grande Ronde valley of eastern Oregon. Both species occur as weeds within Kentucky bluegrass seed fields and subsequently as grass seed contaminants. Two separate germination experiments were conducted to understand better the seed germination biology of these two species compared to Kentucky bluegrass under negative water potentials or high temperature conditions. Results of these studies indicate that although all three species benefited from an ionic enhancement associated with NaCl, weeping alkaligrass was the most drought and salt tolerant of the three species. Dry seeds of weeping alkaligrass were also particularly tolerant to high temperatures with no differences in germination at temperatures below 50 C, indicating that seed viability under nonirrigated field conditions should be unaffected by high soil temperatures. Under soil temperature conditions as high as 40 C, moist Kentucky bluegrass seeds had the greatest germination rates, indicating that this species should benefit from irrigation more than the other two species.

Nomenclature: Nuttall's alkaligrass, Puccinellia nuttalliana, weeping alkaligrass, Puccinellia distans, Kentucky bluegrass, Poa pratensis L

Species richness and diversity are important indicators of ecosystem function and may be related to plant community resistance to invasion by nonindigenous species. Our specific objective was to determine the influence of clopyralid plus 2,4-D, glyphosate, and fosamine, at different application rates and timings, on richness and diversity of total species, total native species, and total nonnative species within a Russian knapweed–infested plant community. Twenty-eight treatments (3 herbicides by 3 rates by 3 application timings, and an untreated control) were applied to two sites located along the Missouri River riparian corridor in Montana. Clopyralid plus 2,4-D, glyphosate, and fosamine were applied in June (spring rosette stage of Russian knapweed), July (bud to bloom stage of Russian knapweed), and August (flowering stage of Russian knapweed). Herbicide rates were clopyralid plus 2,4-D at 0.08 (clopyralid) 0.42 (2,4-D), 0.13 0.67, and 0.18 0.92 kg ai ha⁻¹; glyphosate at 0.6, 1.2, and 1.8 kg ai ha⁻¹; fosamine at 3.6, 7.2, and 10.8 kg ai ha⁻¹. Density of each species was recorded during June and August of 2001 and 2002. Species richness and Simpson's Reciprocal Index (1/D) were calculated. By August 2002, only the glyphosate treatment (4.6 species m⁻²) yielded greater total richness over that of the control (3.5 species m⁻²). At that time, diversity after applying clopyralid plus 2,4-D remained similar to that of the control (1.4), but glyphosate (2.3) and fosamine (2.0) increased total species diversity. Nonnative grasses and forbs accounted for the increases in richness and diversity. Glyphosate may be appropriate for enhancing ecosystem function and possibly niche occupation to preempt reinvasion by Russian knapweed, but restoring native species seems unlikely using any of these herbicides alone.

Nomenclature: Clopyralid, fosamine, glyphosate, 2,4-D, Russian knapweed, Acroptilon repens (L.) DC. CENRE

Giant reed is an extremely aggressive riparian invader in California. Little is known about its response to nitrogen, which is often elevated in watersheds downstream from agricultural fields and wastewater treatment facilities. Two pot-experiments were conducted to quantify physiological responses of giant reed, and a co-occurring riparian species, common threesquare, to added nitrogen and to investigate a possible enhancement effect of nitrogen on the ability of giant reed to spread laterally belowground into a competitive environment. The first experiment measured shoot height, tissue biomass, and leaf area of giant reed and common threesquare, both herbaceous perennials, grown in pots with and without added nitrogen. The second experiment measured lateral rhizome growth, tissue biomass, and tiller production of giant reed in planters subjected to four possible treatments: with or without competition and with or without added nitrogen. Competition planters had previously been colonized by common threesquare and no-competition planters were unoccupied. Nitrogen-treated plants from the first experiment had greater overall shoot height. With added nitrogen, giant reed produced more root and shoot biomass, whereas common threesquare produced more rhizome and shoot biomass. In the second experiment, added nitrogen resulted in significantly greater rhizome length and greater production of tillers by giant reed regardless of competition. In competition plantings without added nitrogen, giant reed tiller production was reduced, whereas the addition of nitrogen nearly restored tiller production to levels attained without competition. Neither nitrogen nor competition significantly affected giant reed biomass production. Results of these experiments indicate the positive response of giant reed and a native riparian species to nitrogen enrichment and suggest that nitrogen can compensate for the effects of competition on giant reed in some cases. As a result, this species might be able to penetrate some environments without negative impacts from competing vegetation.

Nomenclature: Common threesquare, Scirpus americanus Pers. SCPAM, giant reed, Arundo donax L. ABKDO

Anecdotal reports suggest waterhemp is becoming more difficult to control with glyphosate. Waterhemp accessions collected primarily in Illinois before 1996, in 1998 to 1999, and in 2003, were evaluated for differential responses to glyphosate treatment. The pre-1996 group consisted of 10 accessions, whereas the 1998 to 1999 and 2003 groups consisted of 45 accessions each. Plants 10 to 12 cm in height from each waterhemp accession were treated with 0 or 220 g ae ha⁻¹ glyphosate. Although shoot dry biomass 14 d after treatment, expressed as a percentage of the untreated, varied within and among accessions, the mean responses of the three groups were similar. However, when glyphosate responses were divided into 10 arbitrary classes (0 to 10, 11 to 20, …, and 81 to 90; > 91% of untreated), the number of plants within each class differed among groups. Most notably, the proportion of plants in the range of 0 to 30% of the untreated control decreased with time when progressing from the pre-1996 to the 2003 accessions. The relative frequency of waterhemp plants that were ≥ 100% of the untreated plants (i.e., uninjured by glyphosate) in the pre-1996, 1998 to 1999, and 2003 groups were 5, 6, and 5%, respectively. In a separate trial, waterhemp populations that exhibited the least sensitivity to glyphosate at 220 g ha⁻¹ were controlled by 870 g ha⁻¹ glyphosate. These results suggest that waterhemp, as a whole, within Illinois has not become less sensitive to glyphosate over the past few years. However, the proportion of plants that are most sensitive to glyphosate may be decreasing within populations.

Nomenclature: Glyphosate, common waterhemp, Amaranthus tuberculatus (Moq.) Sauer var. rudis (Sauer) Costea & Tardif AMATA, tall waterhemp, Amaranthus tuberculatus (Moq.) Sauer var. tuberculatus (Sauer) Costea & Tardif AMATU

Dandelion is a cosmopolitan weed of economic and environmental significance because of its negative effects on crop yield and the large amount of herbicides used for its control in agricultural, residential, and recreational areas in North America. Asexual dandelion plants, which are apomictic and produce genetically identical seeds, provide a great experimental model for biological studies, such as examination of its reproductive responses to global environmental changes. In a growth chamber experiment, we investigated how elevated CO₂ affected reproduction and seed dispersal properties in dandelion. Dandelion plants were grown at ambient (370 µmol mol⁻¹) or elevated (730 µmol mol⁻¹) CO₂ until reproductive maturity. Results showed that dandelion plants examined in our study produced 83% more inflorescences and 32% more achenes, i.e., single-seed fruits, per plant at elevated than at ambient CO₂. Seeds from elevated CO2-grown plants were significantly heavier and had a higher germination percentage, leading to larger seedlings and earlier establishment in the subsequent generation. Furthermore, achenes from plants grown at elevated CO₂ had characteristics, such as higher stalks at seed maturity, longer beaks, and larger pappi, which would increase the distance of seed dispersal by wind. In addition to these elevated CO₂-induced changes in reproductive properties, dandelion has a number of inherent characteristics, e.g., high competitiveness and adaptations to disturbance, that would increase its vegetative and reproductive success in a higher CO₂ environment. Consequently, dandelion can potentially become more widespread and noxious as atmospheric CO₂ continues to rise because of human activities.

Nomenclature: Dandelion, Taraxacum officinale G.H. Weber ex Wiggers TAROF

The objective of this research was to develop a model to predict common cocklebur seedling emergence in spring tillage and no-spring-tillage systems in the presence and absence of a soybean canopy. A Weibull function was used to accumulate heat units (i.e., growing degree days) at a 2.5 cm soil depth on days when mean soil temperature, soil water potential, and soil thermal fluctuation were above established thresholds. The base temperature, soil water potential, and soil thermal fluctuation thresholds used for model development were 17 C, −100 kPa, and 7.5 C, respectively. A single function adequately described common cocklebur seedling emergence in the presence and absence of drill-seeded soybean from data combined over an artificial (2004) and natural seedbank (2005) (R²  =  0.986). Model parameterization differed between the artificial and natural seedbank in the absence of spring tillage, but emergence was adequately described, regardless of soybean presence. Separate parameter estimates for the artificial and natural seedbanks were needed to adequately describe emergence in the system without spring tillage (R²  =  0.975 to 0.984). The ability of the model to account for reduced emergence when soil moisture is limited or when daily thermal fluctuation requirements are not met could assist practitioners with assessments associated with field scouting for weeds as well as other management decisions.

Nomenclature: Common cocklebur, Xanthium strumarium L. XANST, soybean, Glycine max (L.) Merr

Previous evidence indicates that changes in well-defined phases of dormancy in underground adventitious buds of leafy spurge in late summer and autumn are regulated by complex sensing and signaling pathways involving aboveground sugar signals. However, little information exists concerning seasonal photosynthesis and carbohydrate partitioning of leafy spurge, although such information would help to elucidate the involvement of sugar in controlling bud dormancy. An outdoor study was conducted over two growing seasons to determine and model seasonal patterns of photosynthesis and aboveground carbohydrate partitioning and their relationship to underground adventitious bud carbohydrate status. Photosynthesis and total nonstructural carbohydrate (TNC) content of aboveground tissues was greatest during vegetative growth. Photosynthesis gradually declined over the growing season, whereas TNC decreased sharply during flowering, followed by a gradual decline between midsummer and autumn. Leaf starch increased dramatically to midsummer before declining sharply throughout late summer and early autumn, whereas sucrose content responded inversely, indicating a mobilization of starch reserves and export of sugars to overwintering belowground sink tissues. Because newly formed underground adventitious buds showed a continuous increase in TNC from midsummer through autumn, export of sugars from aboveground tissues likely contributed to the increase in TNC. These results may facilitate new strategies for biological control of leafy spurge.

Nomenclature: Leafy spurge, Euphorbia esula L

Vineyard weed communities were examined under the influence of an organic weed control practice, soil cultivation with a Clemens cultivator, and applications of the herbicide glyphosate. Experimental treatments (winter–spring glyphosate, spring cultivation, fall–spring cultivation, fall cultivation–spring glyphosate) were carried out in a California wine grape vineyard for 3 yr. Cultivation alone was not as effective as glyphosate, based on lower weed biomass in the glyphosate-only treatment in 2 of 3 yr. However, given that two passes with the Clemens cultivator decreased weed biomass relative to one pass, it is possible that additional passes could bring about further reductions. Pairing fall cultivation with glyphosate was as effective at reducing weed biomass as two glyphosate applications in 2 of 3 years, suggesting that substituting a glyphosate application with cultivation may be an effective method of reducing herbicide use in vineyards. Canonical correspondence analysis revealed significant treatment effects on community structure. Weed composition in the spring cultivation treatment was significantly different from that of all other treatments. Based on our findings of high relative abundance of field bindweed and sowthistle species, which are problematic vineyard weeds that grow into the vine canopy and disrupt canopy management practices, it is possible that either the presence of soil disturbance or the absence of herbicides favored these species.

Nomenclature: Glyphosate, annual sowthistle, Sonchus oleraceus L. SONAL, field bindweed, Convolvulus arvensis L. CONAR, spiny sowthistle, Sonchus asper (L.) Hill SONAS, wine grape, Vitis vinifera L. ‘Merlot’

Bengal dayflower (also known as tropical spiderwort) is one of the most troublesome weeds in peanut in Georgia, United States. Field studies conducted in 2004 and 2005 evaluated the relationship between the duration of Bengal dayflower interference and peanut yield in an effort to optimize the timing of weed control. In 2004, the critical period of weed control (CPWC) necessary to avoid greater than 5% peanut yield loss was between 316 and 607 growing degree days (GDD), which corresponded to an interval between June 8 and July 2. In 2005, the CPWC ranged from 185 to 547 GDD, an interval between May 30 and July 3. Maximum yield loss in 2005 from season-long interference of Bengal dayflower was 51%. In 2004, production of peanut pods was eliminated by interference with Bengal dayflower for the initial 6 wk (495 GDD) of the growing season. Robust Bengal dayflower growth in 2004 shaded the peanut crop, likely intercepting fungicide applications and causing a reduction in peanut yield. Therefore, the competitive effects of Bengal dayflower are likely complicated with the activity of plant pathogens. In spite of higher Bengal dayflower population densities, greater Bengal dayflower growth, and greater peanut yield losses in 2004 than in 2005, the CPWC was a relatively similar 4-wk period that ended during the first week of July, for peanut that was planted in the first week of May.

Nomenclature: Bengal dayflower (tropical spiderwort), Commelina Bengalensis L. COMBE, peanut, Arachis hypogaea L

The objective of this study was to test the potential for using a single-entry procedure to revegetate Russian knapweed– and green rabbitbrush–infested rangeland. I hypothesized that simultaneously applying an herbicide and seeding in the fall would produce the highest establishment and growth of desired species. For Russian knapweed, treatments included three seeding rates (zero, low, and high) and two herbicides (with and without clopyralid plus 2,4-D) applied in either the spring (2003) or fall (2004). The seed mixture included pubescent wheatgrass, Siberian wheatgrass, and alfalfa. Grasses were seeded on October 21, 2003 (fall dormant), and April 5, 2004 (spring), using a no-till rangeland drill. The seeding rates consisted of 3.4, 6.8, and 9.3 (low) or 5.0, 9.0, and 12.3 (high) kg ha⁻¹ of alfalfa, Siberian wheatgrass, and pubescent wheatgrass, respectively. For green rabbitbrush, treatments included two seeding levels (13.5 kg ha⁻¹ of Siberian wheatgrass and not seeded) and three herbicides (clopyralid, dicamba, and 2,4-D) and a control applied in July. Clopyralid plus 2,4-D (0.21 plus 1.12 kg ha⁻¹) and dicamba at 2.1 kg ha⁻¹ gave 61 and 66% control of Russian knapweed and green rabbitbrush, respectively. Herbicides interacted with seeding to provide the highest density of seeded species on the Russian knapweed site. These data support the hypothesis that simultaneously applying an herbicide and seeding in the fall would produce the highest establishment and growth of desired species. Conversely, only seeding affected Siberian wheatgrass establishment on the rabbitbrush sites. It may be reasonable to seed Siberian wheatgrass without controlling rabbitbrush if forage production is the primary objective.

Nomenclature: Clopyralid, 2,4-D, green rabbitbrush, Ericameria teretifolia (Dur. & Hilg.) Jepson ERITE, Russian knapweed, Acroptilon repens (L.) DC. ACRRE, alfalfa, Medicago sativa L, pubescent wheatgrass, Agropyron intermedium var. trichophorum (Link) Halac, Siberian wheatgrass, Agropyron fragile (Roth) Candargy

A 3-yr field study was conducted to determine the effect of a biological control agent, Sclerotinia minor Jagger, and a common herbicide, Killex, on the population dynamics of dandelion and other broadleaf species and on the dandelion seed bank. Treatments were applied as one spring, one early autumn, or a spring plus an early autumn treatment per year. The response of the dandelion population to a spring herbicide treatment was similar to two applications per year (spring and early autumn). Significantly less dandelion control occurred after the first early autumn application of the herbicide. Two weeks after application, spring or early autumn treatments with S. minor were equally effective in suppressing dandelions. In the second year of the two applications per year of S. minor treatment, weed control was equivalent to the herbicide. By the third year of the one spring application of S. minor, weed control was equivalent to the herbicide. Generally over the study period, the early autumn application of S. minor was less effective than the spring or the spring and early autumn applications. The S. minor treatments significantly reduced the dandelion seed bank, and this effect was not significantly different from the Killex herbicide treatment. The rate, frequency, and seasonal timing of application had no effect on the dandelion seed-bank size, but terminating the application would gradually replenish the seed bank. Populations of white clover, broadleaf plantain, birdsfoot trefoil, and common ragweed were similarly suppressed by either the S. minor or the herbicide treatments. Yellow woodsorrel significantly increased after 1 yr of herbicide treatment compared with the S. minor and untreated control treatments, indicating a possible weed species shift. Turf quality was improved because of the herbicide and S. minor treatments, but grass injury and smooth crabgrass invasion were recorded in 17% of herbicide-treated plots.

Nomenclature: Killex (2,4-D, mecoprop, dicamba), birdsfoot trefoil, Lotus corniculatus L. LOTCO, broadleaf plantain, Plantago major L. PLAMA, common ragweed, Ambrosia artemisiifolia L. AMBEL, dandelion, Taraxacum officinale Weber ex Wiggers TAROF, smooth crabgrass, Digitaria ischaemum (Schreb.) Schreb. ex Muhl. DIGIS, white clover, Trifolium repens L. TRFRE, yellow woodsorrel, Oxalis stricta L. OXAST

Russian thistle is the most problematic broadleaf weed for spring-sown crops in the low-precipitation (< 340 mm yr⁻¹) region of the inland Pacific Northwest of the United States. A 6-yr field experiment was conducted at Lind, WA, to evaluate three postharvest control strategies for Russian thistle in continuous annual spring wheat. Postharvest treatments were (1) tillage with low-disturbance overlapping undercutter V-blade sweeps; (2) paraquat diuron at the labeled rate, which is widely used by farmers; and (3) an untreated check (letting Russian thistle grow unhindered). The undercutter V-sweep consistently killed all Russian thistle with essentially no residue burial, and no seed was produced. In contrast, the paraquat diuron treatment halted Russian thistle dry biomass production, but plants continued to extract soil water and produce an average of 310 seeds m⁻² on the lower branches. In the check, Russian thistle produced an average of 700 kg ha⁻¹ postharvest dry biomass and 5,670 seeds m⁻². The undercutter V-sweep treatment had significantly more water in the 180-cm soil profile at time of wheat harvest, after a killing frost in October, and in mid March as well as greater spring wheat grain yield compared with the herbicide and check treatments. Results show that postharvest tillage with an undercutter V-sweep consistently achieved 100% control, retained ample wheat residue on the surface to control erosion, and was by far the most effective treatment in this experiment.

Nomenclature: Diuron, paraquat, Salsola iberica Sennen & Pau SASKA, wheat, Triticum aestivum L

Because of the large number of potentially invasive species, and the time required to complete weed risk assessments (WRAs) with the use of the current, mandated system in the United States, species need to be prioritized for assessment and possible listing as Federal Noxious Weeds. Our objective was to rank the potential invasiveness of weedy or pest plant species not yet naturalized in the United States. We created a new model of invasiveness (hereafter the U.S. weed-ranking model) based on scoring factors within four elements: (1) invasiveness potential, or likelihood to exhibit invasive behavior; (2) geographic potential, or habitat suitability; (3) damage potential, or likely impact; and (4) entry potential, or likelihood to be introduced. The ranking score was the product of the four elements. We scored 250 species satisfactorily, from a list of 700 . We analyzed model sensitivity to scoring factors, and compared results to those from a WRA model for Hawaii. For species not in cultivation in the United States, the top 25 species included a mix of annuals, perennials, sedges, shrubs, and trees. Most had exhibited invasive behavior in at least several other countries. Because of greater entry potential scores, the highest-scoring species were weeds in cultivation. Twenty-nine such species, out of 44 total, had scores greater than the highest scoring species not in cultivation. In comparison to the Hawaii WRA model, correlation and regression analyses indicated that the U.S. weed-ranking model produced similar, but not exact, results. The ranking model differs from other WRAs in the inclusion of entry potential and the use of a multiplicative approach, which better suited our objectives and United States regulations. Two highly ranked species have recently been listed as Federal Noxious Weeds, and we expect most top-tier species to be similarly assessed.

Nomenclature: Invasive, exotic weeds, invasiveness model, weed risk assessment