INVASIVE PLANTS OF OHIO
Fact Sheet 14 - Factsheet in .pdf format
Canada thistle is a slender, herbaceous, non-native perennial plant reaching a height of 2-4 feet. The leaves are simple, alternate, irregularly lobed, and taper towards the tip. The underside of the leaf is normally smooth with the margin bearing many sharp spines.
Stems are grooved, hairy, and branched at the top. The root system is comprised of a deep taproot that may extend 6 feet down and an extensive creeping rhizome that other thistles in Ohio lack. Numerous fragrant, lavender-pink, one-inch flowers adorn the plant from June to September. A single plant may produce up to 5,300 seeds, each of which is attached to a hair-like tuft making them easily dispersed by the wind.
Canada thistle occurs in nearly every open habitat within its range and tolerates nearly any soil type that is not waterlogged. In natural areas, it is a particular problem in old fields, prairies, savannas, and early successional forests. It can also be a problem in wet sedge meadows where it invades areas above the waterline.
Despite its name, Canada thistle is not native to Canada or even to North America. It is native to eastern and northern Europe and western Asia, and was introduced to North America in the 1600s. It has spread throughout all of the United States except the southeast. It is found throughout Ohio.
The extensive root system of Canada thistle allows it to out-compete and displace many native species, especially in degraded prairies where native species are not well established. Spreading both by seed and rhizome, Canada thistle can create monocultures covering large areas. The wind-dispersed seeds may remain viable for 20 years or more, allowing it to spread quickly and making it difficult to eradicate.
Prescribed burning, especially in the spring, can be effective by reducing thistle density and allowing native species to compete for resources. Mowing will temporarily reduce the amount of Canada thistle, but will not kill it unless mowing is repeated often for many years - which can also harm native plants as well. Hand pulling is usually ineffective since small portions of broken taproot can easily regenerate.
Foliar spraying of a systemic herbicide such as RoundupÂ®, GlyproÂ®, or TranslineÂ® is an effective control method. Fall and spring are normally the best times to treat Canada thistle to maximize the herbicide absorption into the deep taproot. Several applications will usually be needed.
There are currently no effective biological controls for Canada thistle.
ADDITIONAL INFORMATION SOURCES:
Doll, J.D. 1997. Controlling Canada Thistle. North Central Regional Extension. Publication No. 218.
Evans, J. E. 1984. Canada Thistle (Cirsium arvense): a literature review of management practices. Natural Areas Journal 4(2): 11-21.
Nuzzo, V. 1987. Element Stewardship Abstract for Canada Thistle. The Nature Conservancy.