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