Canada thistle (Cirsium arvense [L.] Scop.) is one of the most problematic weeds of temperate regions and is found throughout North America, Europe, Africa, and across central Asia. Canada thistle's ability to spread quickly and recover from many control methods makes managing Canada thistle a significant challenge for land managers. Herbicide application can be effective, but mixed results, toxicity concerns, and the need for re-application demand new, more efficient strategies that reduce herbicide use.

A greenhouse study tested effectiveness of clipping and grass seeding for Canada thistle control. Grasses used included two natives (western wheatgrass [Pascopyrum smithii {Rydb.} A. Löve], streambank wheatgrass [Elymus lanceolatus {Scribn. & J.G. Sm.} Gould ssp. lanceolatus]) and one sterile hybrid (common wheat [Triticum aestivum L.] x tall wheatgrass [Thinopyrum ponticum {Podp.} Z.W. Liu & R.C. Wang]) called Regreen™. Grasses were seeded alone or in combination (Regreen+western wheatgrass) in pots with Canada thistle.

Field Study I tested combinations of mowing, herbicide, and grass seeding across two habitats (wetland, upland) and three different local climatic regimes for control of Canada thistle. Grass treatments involved seeding western wheatgrass (upland sites) or prairie cordgrass (Spartina pectinata Bosc ex Link) (wetland sites) alone or in combination with Regreen (upland and wetland sites). Six sites (three wetland, three upland) were paired geographically across Colorado with each wetland site in close proximity to an upland site.

Field Study II tested combinations of mowing, herbicide, soil amendment addition (organic matter, manganese), and grass seeding (western wheatgrass, intermediate wheatgrass [Thinopyrum intermedium (Host) Barkworth & D.R. Dewey] on a Colorado mesic site.

In greenhouse trials, clipping inhibited Canada thistle growth, while grass seeding did not. In Field Study I, herbicide application produced effective control. In Field Study II, tilling enhanced herbicide effectiveness. Organic matter or manganese alone did not reduce Canada thistle growth. Manganese addition reduced herbicide effectiveness. In both field studies, neither mowing nor grass seeding enhanced herbicide effectiveness, and tilling did not increase Canada thistle biomass.

Future research should address restoration of infested wetland sites, the importance of irrigation during drought for restoration, and the mechanism through which manganese sulfate inhibits herbicide effectiveness.