Canada Thistle Cirsium Arvense (L.) Scop.______

Michigan Department of Natural Resources Michigan Natural Features Inventory, March 2018 Canada thistle Cirsium arvense (L.) Scop.____________________________________________________________________________ Canada thistle is native to southeastern Europe and the eastern Mediterranean region, and was introduced to North America before 1800. It is probably the most widespread of all thistle species and was likely introduced in ship ballast and as a seed contaminant. Canada thistle spreads rapidly; its extensive root system and clonal nature allow it to form monocultures, altering community structure, outcompeting native species, and reducing biodiversity. It also can threaten rare and endemic species. Healthy natural communities offer some resistance to invasion, but Canada thistle poses a particular threat to degraded sites and newly restored grasslands. Canada thistle also has a significant economic impact on crop and rangeland. Most livestock will not eat thistle and their selective grazing facilitates its dominance in pastures. It can even scratch grazing animals, resulting in small infections. It reduces crop yield through competition for resources including sunlight, nutrients and water. It is a host for a number of agricultural pests Eric Coombs, Oregon Department of Agriculture, Bugwood.org such as bean aphid, sod‐web worm, which attacks corn, and stalk insects that damage tomatoes. Flowers: Canada thistle’s lavender, Identification pink or white flowers are Habit: held in a structure called Canada thistle is a spiny herbaceous perennial that an involucre, composed of ranges from 0.3–1.5 m (1–5 ft) in height. It has an bracts called phyllaries. extensive network of creeping roots that reach deep Some phyllaries may have into the ground, and it forms extensive clones. tiny spines. Male and female flowers are held on Chris Evans, Illinois Wildlife Leaves: separate plants and clones Action Plan, Bugwood.org Canada thistle has lobed leaves with spiny teeth. Plants consist of a single sex. that grow from seed initially develop a basal rosette of leaves. Later in their first year, they send up a stem with Fruits/Seeds: alternate leaves. The leaves are highly variable. Canada thistle has small dry tan seeds called achenes. They are attached to a feathery bit of fluff which helps them disperse in the wind. Only female plants produce seed. Julia Scher, USDA APHIS PPQ, Bugwood.org Leslie J. Mehrhoff, University of Bruce Ackley, The Ohio State Connecticut, Bugwood.org University, Bugwood.org Habitat: Bark/Stems: Canada thistle thrives on sunny disturbed sites such as Canada thistle’s stems are smooth, with shallow roadsides, ditches, pastures and hayfields. It also lengthwise ridges. They branch near the top of the invades natural areas including prairies, barrens, plant. They do NOT have spines and are not winged. savannas, glades, sand dunes and meadows. Invasive Species–Best Control Practices–Canada Thistle Page 1 Similar Species The native swamp thistle (Cirsium muticum) is not Canada thistle’s perennial status, clonal growth and clonal and has larger, flower heads with cobwebby, small flower heads distinguish it from other thistles in somewhat sticky involucres. The flowers are a deeper Michigan. It is also shorter; typically knee to waist high, shade of purple. although it can reach six feet. It does not have winged or prickly stems. D. Kausen, controd‐ Liana May dev.newenglandwidl.or Eric Coombs, Oregon Department Leslie J. Mehrhoff, University of Swamp thistle (native) Swamp thistle (native) of Agriculture, Bugwood.org Connecticut, Bugwood.org Canada thistle Canada thistle Reproduction/Dispersal Canada thistle is anchored by a deep tap root up to 6 The invasive European marsh thistle (Cirsium palustre) feet long and reproduces through perennial, creeping has small clustered flowers, however it is a much taller, rhizomes and prolific seed production. The lateral roots scraggly plant and its stems are prickly. can grow as much as 12 feet in a year and new plants can also regenerate from root fragments. Most clones are either female or male, both of which flower from midsummer to early fall. Flowers are pollinated by a variety of insects, including honey bees and wasps. Female flowers produce small, light brown, tapered seeds in the fall that are dispersed by wind, animals, humans, and vehicles. Germination rates are very high and seeds have been shown to remain viable in the soil for twenty years. Best survey time Canada thistle is easiest to detect when in flower and P. Higman P. Higman European marsh thistle European marsh thistle fruit. However, its dense clonal clusters, non‐prickly stems and relatively short stature distinguish it from The invasive bull thistle (Cirsium vulgare) has much other thistles any time during the growing season. larger flowers and its stems are also prickly. Planning a control effort Resources for invasive species control invariably fall short of the actual need, so it is important to prioritize sites for treatment and plan carefully. Assessing the scope of the problem is a critical first step: Minimize intensive disturbance where Canada thistle is undesirable. Map known populations. Identify leading edges and outliers. Identify potential dispersal pathways and monitor Loke T. Koke, VA Polytechnic them; is the population along a pathway or stream? Inst & State Univ, Bugwood.org L. Wallis Bull thistle Bull thistle Invasive Species–Best Control Practices–Canada Thistle Page 2 Does it lie in the path of road‐mowing crews that treatment sites for multiple years, perhaps indefinitely, might spread it further? Are there construction sites to ensure depletion of the existing seed bank and in the area where it might be introduced in fill dirt? prevent replenishment of the seed bank from other How is the species behaving in your area? Is it sources. spreading rapidly? It is also important to assess whether there are enough Does it occur in high quality habitat or on important desirable plants in the seed bank to replace the thistle recreational, hunting or fishing lands? after treatment. Long term treatment efforts should be Given this information, develop a strategy for control: planned accordingly so that resources are not wasted 1. Prioritize high value sites where treatment success where successful control cannot be achieved. can be achieved. Hand pulling 2. Prevent further spread by monitoring the leading Hand pulling over repeated years can be effective for edges and outliers. controlling small populations, but requires diligent, 3. Choose appropriate control methods, given site repeated monitoring to ensure the seed bank is conditions and available resources. depleted. It is usually not practical in large, well 4. Determine whether any permits are required (e.g., established populations, particularly since seeds can herbicide application in wetlands, prescribed remain viable in the soil for twenty years. Hand‐pulling burning). can be used as a follow‐up to other control methods 5. Eradicate smaller satellite populations. such as mowing or herbicide application. 6. Treat larger core infestations. 7. Monitor to ensure desired results are being Hand‐pulling is only effective if the root is extracted, achieved; adapt management to improve success. otherwise plants can re‐sprout. It should be conducted prior to seed production or it will facilitate seed Documenting occurrences dispersal. Practitioners recommend pulling when it is in In order to track the spread of an invasive species on a full bud and repeatedly thereafter at 3‐4 week intervals, landscape scale, it is important to report populations until dormancy in the fall. It may only take several years where they occur. The Midwest Invasive Species to eradicate emerged plants; however, depletion of the information Network (MISIN) has an easy‐to‐use seed bank will take many more years. interactive online mapping system. MISIN I‐phone and Soil disturbance by hand‐pulling also stimulates seed android phone apps are also available. Links to the germination. Deliberate stimulation in this way may MISIN and its phone apps are provided in the “On‐line expedite depletion of the seed bank, thereby speeding resources” section. Herbaria also provide a valuable and up long‐term control. However, this will only be authoritative record of plant distribution. The University effective if follow‐up management of new sprouts is of Michigan Herbarium’s database can be searched undertaken before new seeds are produced. Follow‐up online for county records of occurrence, for example. treatment will be required for many years. When Canada thistle is first encountered in a county where it has not been known previously, specimens Pulled plants should be bagged and taken to a landfill or should be submitted to the Herbarium to document its dried and then burned or buried deep into the ground. presence. Check the “Online Resources” section for Protective clothing, including gloves, should be used to links to both of these resources. avoid serious skin scratching by the plant’s prickles. Control Mowing Canada thistle is notoriously difficult to control due to Mowing Canada thistle is similar to hand‐pulling, but its’ deep, perennial tap root, fast growing lateral roots where compatible with management goals, is usually and abundant, highly dispersible seeds. The primary easier and can be conducted over larger areas. It is not goals for control are to prevent seed production, an effective control method alone; however, it can deplete the seed bank, and prevent

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