Management and Control
Apparently little if any information exists on the control of Eurasian marsh thistle in North America. Agencies and individuals engaged in marsh thistle control are encouraged to share their experiences with GLIFWC's invasive plant staff.
Several native thistles occur in the upper Great Lakes region, and a few of these are listed as rare in Michigan, Wisconsin, and/or Minnesota. One of them, Pitcher's thistle [Cirsium pitcheri (Eaton) T. & G.] is globally rare and listed as federally "threatened". Additionally, the fairly common swamp thistle (C. muticum Michx., also native) occurs in similar habitats as marsh thistle, and appears superficially similar. Care should be taken to verify the identity of Eurasian marsh thistle before attempting control measures.
Manual Control
For small stands and isolated plants.
As a biennial or monocarpic perennial (flowering only one season, then dying), marsh thistle has a weakness - if populations can be prevented from producing seeds, they will rapidly diminish and eventually die out. Preventing a large colony from producing seed takes persistence, but has the potential to greatly reduce populations (and control effort) within just a few years.
Cutting or mowing: While cutting the stems off near the ground before flowers are produced presumably reduces seed production, it will not eliminate it entirely. As with the related musk and plumeless thistle (Carduus spp., see Hoffman and Kearns 1997), marsh thistle plants are able to send up new branches from buds in the axils of the rosette leaves (pers obs). Close cutting a second time usually prevents Carduus spp. from producing seeds, though, and should greatly reduce or eliminate marsh thistle seed production as well.
Yearly monitoring and treatment is probably necessary for several years or more, until the rosettes all attempt to flower and die, and the seed bank is completely exhausted.
Digging & Hand Pulling: Marsh thistle plants have relatively shallow root systems and can often be dug up with a shovel. For larger populations these methods can be quite labor intensive and somewhat destructive to neighboring species, but for small infestations they can be quite time- and cost-effective.
Chemical Control
For use on small to medium stands.
Herbicides: Glyphosate (available commercially as "Roundup" and "Rodeo") is a nonselective, systemic herbicide that is effective on nearly all green plants. Late spring may be the best time to spray, before rosettes attempt to flower. Plants that are in full bloom may produce viable seed even after spraying.
In July of 2001, this typist came upon a population at a remote site in western Upper Michigan that probably numbered several dozen flowering plants plus a substantial number of rosettes. In July 2002 and 2003 the flower stalks were cut near ground level with a knife, and a little glyphosate solution was sprayed into the remaining cut hollow stems with a spray bottle. The hope was that this would kill the basal part of the plants, making a second cut unnecessary. Unfortunately no follow-up observations were possible either summer. By the summer of 2003, though, and with relatively little effort, this population had already been reduced to 3-4 flowering plants and only scattered rosettes.
Whenever using herbicides to control invasive plants, care should be taken to avoid spraying nontarget vegetation and to minimize human exposure.
Cultural Control
Fire: Effectiveness of fire as a tool to control marsh thistle is unknown. The fact that marsh thistle often occurs in moist to wet habitats, where use of fire is probably not feasible or appropriate, probably precludes its use against this plant.
While disturbance is not required for marsh thistle's spread, it does appear to facilitate it. Minimizing or avoiding ditching, road-building, logging, and other earth-moving disturbance will slow its colonization of semi-natural and natural areas.
Biological Control
Biological control may be effective for smaller populations, and is probably the only effective long-term solution for controlling marsh thistle populations in eastern North America. While apparently no research has been done to find biocontrols for marsh thistle in North America, a number of insects have been introduced in an attempt to control other Eurasian thistles [other Cirsium spp., along with closely related Carduus and Silybum (milk thistle) spp.] (McClay et al. 2001).
A major problem in finding suitable biocontrols for Eurasian thistles in North America has been the lack of specificity of insects and other organisms associate with Cirsium spp. in Europe (McClay et al. 2001). Many of the insects tested so far attack not only Eurasian thistle species, but native thistles as well. Of the 92 thistle species native to North America, few if any are considered significant weeds, and several are rare and endangered.
The European seedhead fly Terellia ruficauda (Diptera: Tephritidae) was unintentionally introduced to North America. This seedhead fly attacks marsh thistle in Britain (Masters et al. 2001), but so far has had little effect on Canada and other Eurasian thistles in North America. The native painted lady butterfly (Vanessa cardui: Lepidoptera) is a generalist feeder that may cause significant defoliation of Canada thistle (Cirsium arvense (L.) Scop.) in some years, but has generally not caused significant decline in this species.
For more informationTNC Wildland Weed Management and Research Library
Wisconsin Manual of Control Recommendations for Ecologically Invasive Plants