Ecological Impacts
Eurasian marsh thistle's habitat requirements appear to be quite similar to those of the much more congenial native swamp thistle, and marsh thistle seems likely to compete directly with and even displace it. Its often dense colonies may displace other plant species as well. Its extreme spininess makes it unpalatable to deer and other wildlife. Large colonies are unpleasant to walk through and detract from enjoyment of the outdoors.
Ignored for years, marsh thistle is now considered a potentially serious pest in British Columbia (Martin 2001). It has also become locally abundant in the northeastern US (Voss 1996, see also USDA-NRCS 2002). In the upper Great Lakes region, marsh thistle is a relatively new invader, being first collected here in 1934 (Voss 1996). The fact that it has since spread rapidly and aggressively to a variety of disturbed and undisturbed, moist to wet habitats in Upper Michigan, Lower Michigan, and Wisconsin provides strong evidence that this species is on its way to becoming a major pest in the upper Great Lakes region.
Status in Area
Marsh thistle is well-established and becoming common in the UP and northeast Wisconsin (WIS 2002, Voss 1996). It is apparently still rare or nonexistent in Minnesota (MINN 2002). It is considered "moderately invasive" in Upper Michigan (MIPC 2002), but only "naturalized" in Wisconsin (WIS 2002). It has become locally abundant in the northeastern US (Voss 1996, see also USDA-NRCS 2002), and if left unchecked it will presumably continue to increase in range and abundance in the upper Great Lakes region.