Currently, affordable, long-term methods of weed control are lacking for many of the most problematic weeds on western coal mines, including such infamous species as cheatgrass (Bromus tectorum), Japanese brome (Bromus japonicus), Canada thistle (Cirsium arvense), and whitetop (Cardaria draba). Herbicides and mechanical approaches provide short-term weed control, but can be difficult and expensive to apply over large spatial scales, require reapplication for long-term control, and often do not promote establishment of desired vegetation. Finding safe, affordable, and ecologically-sound methods to control invasive, exotic plants on western mined lands is imperative for achieving post-mining land-use objectives. Planting a cover crop to compete with invasive, exotic species has been explored as a potential approach for reducing exotic species abundance and increasing desired species abundance in agriculture and ecological restoration (Blackshaw et al., 2006; Landhausser et al., 1996; Ledgard & Davis, 2004; Morgan, 1997; Perry & Galatowitsch, 2003; Sheley et al., 2006; Shirley, 1994; Singh et al., 2003). Unfortunately, cover crops often fail to improve desired species success, mainly because they do not act selectively. Cover crops that are strong enough competitors to control invasive, exotic species also tend to suppress desired species, whereas cover crops that don’t suppress desired species also don’t tend to suppress invasive, exotic species (De Haan et al., 1994; Hoffman et al., 1993; Lanini et al., 1991; Ledgard & Davis, 2004; Perry & Galatowitsch, 2003).

Allelopathic cover crops, however, might be more likely to act selectively against exotic weeds while allowing native, desired species to establish. Allelopathic plants produce phytotoxins (i.e., allelochemicals) that reduce the growth or survival of neighboring plants. In natural, undisturbed plant communities, allelopathy may be relatively ineffective, since plant species that frequently interact with allelopathic plants would be expected to develop resistance to the allelochemicals over time (Fitter, 2003). In contrast, in novel interactions between native and exotic species, allelopathy may be more intense, since neither species has had time to develop resistance to the novel allelochemicals (Rabotnov, 1982). This logic forms the basis of the novel weapons hypothesis, which posits that some exotic species are so successful because they are allelopathic and produce phytotoxins that are novel to the native species in their invaded range (Callaway & Ridenour, 2004). By the same logic, however, native species that are allelopathic may be particularly effective against exotic species, which have not had time to develop resistance to native allelochemicals. Those same native allelochemicals would be expected to have relatively little effect on other native species, which have had the time to develop resistance.  Many North American rangeland species are thought to be allelopathic, including annual ragweed (Ambrosia artemisiifolia), common sunflower (Helianthus annuus), Canada goldenrod (Solidago canadensis), and littleleaf pussytoes (Antennaria microphylla).

We tested the hypotheses that native, allelopathic species would (1) reduce exotic, invasive species growth and (2) increase native, desired species growth by freeing desired species from competition with exotic species. Specifically, we examined the effectiveness of annual ragweed, common sunflower, Canada goldenrod, and littleleaf pussytoes as cover crops to control cheatgrass, Japanese brome, Canada thistle, and whitetop, and to favor desired, native species establishment in competition with those exotic species. Further, we used activated carbon, which adsorbs allelochemicals in soil, to examine whether allelochemicals were responsible for effects of the cover crops on the exotic and desired species.

Effects of the four potential cover crops on interactions between four invasive, exotic species and four native, desired reclamation species were examined in a greenhouse competition experiment.  Key findings include:

• Our hypotheses that native, putatively-allelopathic species would reduce exotic species growth and increase desired species growth in competition with exotic species were supported. In particular, ragweed and sunflower appeared to improve desired species growth by reducing competition from some of the exotic species.
• Goldenrod and pussytoes also reduced growth of some exotic species and improved growth of some desired species in competition with exotic species. The connection, however, between their negative effects on exotic species and positive effects on desired species was less clear.
• Our hypothesis that the cover crops would inhibit exotic species via allelopathy was not supported.  In a few instances, the activated carbon treatment suggested that organic compounds were responsible for negative effects of some cover crops on some exotic species.  In an equal number of instances, the activated carbon treatment suggested that organic compounds moderated the negative effects of some cover crops on exotic or desired species.  Cover crop effects on resource availability, rather than allelopathy, may explain the negative effects of ragweed and sunflower on exotic species and positive effects on desired species.

Management implications: Our results suggest that planting ragweed, sunflower, goldenrod, or pussytoes may reduce cheatgrass, Japanese brome, Canada thistle, and whitetop invasion and improve desired species success in competition with those exotic species in western grassland restorations. The mechanisms behind these effects are unclear, particularly for goldenrod and pussytoes, and may operate differently under field conditions. Further research is needed to explore the conditions necessary for these cover crops to be effective in the field.