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12 CANADA THISTLE (Cirsium arvense) perennial PLANT DESCRIPTION Canada thistle, or creeping thistle, is a broadleaf perennial weed that can be seen from spring to fall as single- stemmed, upright, 3–5 foot tall flowering plant with purple flowers and spine tipped leaves. It can also be seen as a seedling, or as a shoot from its spreading root system in spring or fall. Canada thistle is smaller than other thistles but forms dense colonies that are easily noticed when in flower or in seed. LEAF: Canada thistle leaves are dark green with a whitish midrib and are covered in fine hairs on the upper surface. The leaves are wavy and lobed with a prominent spine on each lobe. STEM: Stems are hairless with vertical ridges that develop branches as the weed prepares to flower. FLOWER: Purple, upright flowers with green oval bases bloom at the ends of branches in early summer. Flowers look like small shaving brushes or pom-poms. SEED: Cottony, white, and fluffy seeds develop by midsummer. REPRODUCTION: Seeds are primarily dispersed by wind, but this perennial weed also spreads by sending up vegetative root sprouts from a large, spreading root system. A mature Canada thistle plant is capable of producing thousands of seeds that remain viable in the soil for up to 20 years. WHY IT IS NOXIOUS: Canada thistle produces a prolific amount of long-term, viable windblown seeds. With a high tolerance to mowing and a wide range of habitat preference, Canada thistle presents a maintenance problem for roadside vegetation management as well as agricultural and neighboring lands. SEASONAL IDENTIFICATION WHEN TO FIND: Canada thistle can be found from spring until snowfall but is easiest to notice in flower from early to late summer. Dead plants with seeds stand into fall. SPRING IDENTIFICATION = EASY TO MODERATE Seedlings or root sprout leaves are dark green, slender, and spine-tipped. The upper surface of the young leaves are covered with fine white hairs. SUMMER IDENTIFICATION = EASY As single-stemmed plants grow tall, flower branches develop near the top of the plant. Purple flowers emerge at the ends of branches and resemble a shaving brush. Seeds are produced by midsummer to late summer. FALL IDENTIFICATION = EASY Canada thistle dies aboveground, turning brown and usually holding cottony seed heads until they detach and are taken by wind. Dense colonies are easy to spot. WINTER IDENTIFICATION = EASY TO MODERATE In unmowed areas, dead plant skeletons can be seen standing until snowfall. SIMILAR-LOOKING SPECIES BULL THISTLE (Cirsium vulgare) has spiny leaves like Canada thistle, but bull thistle is much larger at maturity. Covered in hairs on upper and lower surfaces, the leaves are much more pointed and deeply lobed and flowers are pink. MUSK THISTLE (Carduus nutans) also has spiny leaves but has much larger pink flowers compared to Canada thistle. Stems are covered in spine-tipped leaf wings. Leaf lobes may feature several wings and several spines per lobe. Flowers are globe shaped and often nod or bend under their own weight. LOW HIGH ohio distribution roadside distribution growth seeds ability to dominate difficulty of control WHERE TO FIND: Canada thistle is common and widespread in Ohio. It is found growing in moderately dry and dry soils with full sun. Look at the edge of the road under rails, in mowed areas, on infrequently mowed slopes, and adjacent properties. CONTROL METHODS: MECHANICAL: Mow to reduce seed production and to maintain visibility if needed; expect regrowth. CHEMICAL: Make directed or broadcast applications from spring to early summer with selective herbicide. Use residual selective herbicide with dense colonies or along guardrails. CULTURAL: Plant tall fescue in early fall, allow to grow tall in spring. RECOMMENDATION SUMMARY: Canada thistle is a difficult-to-control perennial weed that spreads by wind-blown seeds and an extensive, spreading root system. For best control results, make herbicide applications in late spring before flowers bloom. If ideal timing is missed, fall applications before hard frost will provide adequate control. Make directed applications to dense colonies or patches growing away from the road edge and broadcast applications to Canada thistle growing in turf or under guardrails and signs. Because Canada thistle is a perennial and can sprout from root fragments, hand pulling or mowing has little effect. Only mow for height reduction and seed reduction if necessary. Plant tall fescue in fall and allow to grow tall in spring. Failure to control Canada thistle can result in new infestations from wind-blown seed or formation of dense colonies by spreading root rhizomes. Dense colonies are difficult to control and seeds can remain viable in soil for up to 20 years. RIVM LEVEL OF CONCERN

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