ANNAPOLIS, MD (April 12, 2005)
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
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
Marylandís 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
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.