The biology of Canadian weeds. 3. Lepidium draba L., L. chalepense L., L. appelianum Al-Shehbaz (updated)

Abstract

This paper updates an earlier paper by Mulligan and Findlay (1974) on the cruciferous weeds Lepidium draba, L. chalepense and L. appelianum (treated in the previous paper as Cardaria draba, C. chalepensis and C. pubescens, respectively). Known collectively as hoary cresses, these species reached North America in the 19th and early 20th centuries as contaminants in seed or in ballast. The three species spread extensively in the western parts of the continent by the mid 20th century, partly through seed dissemination, but also by establishment of clonal patches and rhizomatous spread. In the prairie provinces the three species are weeds in grainfields, hayfields and forage crops, and have been largely under control in those crops since the 1970s. By the 1970s all three species had spread into agricultural regions of south central British Columbia and remain a serious problem there. They have also spread into forest, range, and riparian areas of Alberta and British Columbia and are among weedy threats to native ecosystems. Populations in Saskatchewan and Manitoba appear to have stabilized, but remain on noxious weed lists. In the western United States, the species have also spread from cultivated areas into rangelands, and L. draba in particular competes successfully with native vegetation, resulting in reduced local biodiversity. Lepidium draba can also reduce forage production, quality and palatability. Biocontrol measures for L. draba are under investigation. Mechanical controls have proved to be generally ineffective in large populations. Herbicides are effective in the short term in some agricultural crops, but do not provide long-term control, and are of restricted use on rangelands and at riparian sites. Potential beneficial uses of Lepidium draba in phytoremediation and as a source of disease-fighting chemical derivatives are reported. Key words: Lepidium draba, Lepidium chalepense, Lepidium appelianum, Cardaria draba, Cardaria chalepensis, Cardaria pubescens, hoary cress, white-top