Invasive Species of Concern in Maryland

ANNAPOLIS, MD (April 12, 2005) Canada Thistle

photo: Peggy Greb, USDA ARS, www.invasive.org

With spring here, flowers will soon be out, but weeds too will be upon us. One of the worst is Canada thistle (Cirsium arvense), a misnomer, because it is actually native to Europe and Asia. It is one of the most difficult to control weeds we have and many other (less printable) names come to mind when trying to manage this plant!

Now well established in northern United States, it is thought to have been introduced here sometime in the 1600s, perhaps in contaminated grain seed, has been a serious problem in Maryland for over 100 years. A Garrett County law was passed in 1882 stating that no landowner, tenant or roads supervisor shall allow the growth of Canada thistle. They were serious, with a fine of $10.00 a day if not controlled. That was a lot of money for the time!

Many invasive weed species are a problem of natural areas but Canada thistle is a serious problem not only in natural areas but also in agriculture, landscapes, lawns and right of ways. Canada thistle is especially a problem in the establishment of conservation sites and natural areas. Given a free rein, Canada thistle can out-compete many desirable plants or crops by shading with its dense growth and by tapping deep into the soil to strip water and nutrients, preventing other plants from growing. In addition, Canada thistle may also have an allelopathic effect on the surrounding plants, giving it yet another advantage. All these factors add up to a very aggressive, difficult to control weed, and has earned Canada thistle the designation of Invader of the Month for April by the Maryland Invasive Species Council.

Canada thistle has dark green leaves which are oblong, variably lobed with spiny edges and are attached to the stem alternately. Canada thistle has grooved slender stems which are green or black. It grows to a height of 3-5 feet tall, branching at the top with numerous rose-purple flowers clustered on the top of them.

Unlike the other thistles which start from seed each year, Canada thistle can spread not only from seed but also through an extensive deep horizontal perennial root system which has vegetative buds that producing new plants as it creeps through the soil. These horizontal and vertical root systems can reach depths of 6-15 feet. Due to this extensive root system, Canada thistle generally grows in distinct patches. Introduction of this weed to new areas can be from the windborne seed, spreading root systems, movement of contaminated hay or soil, or equipment.

In Maryland, emergence generally occurs in early to mid April starting first as a rosette then with rapid growth culminating with flowering and seed production in mid May through June. After seed production, Canada thistle will continue to send nutrients to the roots to build its reserves and in the heat of summer will dry down some. As the fall weather turns cooler and moisture is available, a second flush of Canada thistle will occur, but with limited flowering.

The numerous seeds, generally 1,000-1,500 per plant, can have a high germination rate. They can remain viable in the soil for up to 20 years, but most germinate within one year. The seeds are attached to a cottony pappus commonly known as thistle down which allows it to blow with the wind. These blowing seeds sometimes have the appearance of snow in the summer. Canada thistle is diecious, meaning male and female flowers occur on separate plants, with required cross pollination being done primarily by honey bees.

To clarify another misnomer, the thistle seed sold in stores as bird seed is not thistle seed, but Sterilized Niger Seed, and does not pose a problem for the garden.

Canada thistle and three other thistle species, Musk, Plumeless and Bull thistles, are listed as prohibited noxious weeds in the State of Maryland and must, by law, be controlled. Twenty of Maryland’s counties have noxious weed control programs to help residents control Canada thistle and the other Maryland noxious weeds. In addition, Canada thistle is considered a noxious weed throughout the U.S, with millions of dollars spent annually on its control. Left uncontrolled, it can reduce yields of row crops by as much as 90 percent.

Management and control of Canada thistle is difficult at best so it is essential to develop a long term plan of control. Preventing an infestation is a good place to start but if you are trying to control a existing infestation an integrated program is recommended. An integrated program might include cultural (establish a cover), mechanical (mowing or tillage), chemical (labeled herbicides) and biological (natural enemies) control methods. Good control of this plant will allow more desirable plants to flourish.

In Maryland, advisory assistance with how to control this noxious weed is available through your local county noxious weed control program or through the Maryland Department of Agriculture at (410) 841-5871.

For more information about other Invasive Species of Concern, visit www.mdinvasivesp.org or call MDA at 410.841.5920.

photos available electronically on request.