Temperate regions of Eurasia
is an herbaceous perennial with erect stems 1½-4 feet tall, prickly
leaves and an extensive creeping rootstock. Stems are branched, often slightly
hairy, and ridged. Leaves are lance-shaped, irregularly lobed with spiny,
toothed margins and are borne singly and alternately along the stem. Rose-purple,
lavender, or sometimes white flower heads appear from June through October,
generally, and occur in rounded, umbrella-shaped clusters.
The small, dry, single-seeded
fruits of Canada thistle, called achenes, are 1-1½ inches long and
have a feathery structure attached to the seed base. Many native species
of thistle occur in the U.S., some of which are rare. Because of the possibility
of confusion with native species, Canada thistle should be accurately identified
before any control is attempted.
communities that are threatened by Canada thistle include non-forested plant
communities such as prairies, barrens, savannas, glades, sand dunes, fields
and meadows that have been impacted by disturbance. As it establishes itself
in an area, Canada thistle crowds out and replaces native plants, changes
the structure and species composition of natural plant communities and reduces
plant and animal diversity. This highly invasive thistle prevents the coexistence
of other plant species through shading, competition for soil resources and
possibly through the release of chemical toxins poisonous to other plants.
Canada thistle is declared a "noxious
weed" throughout the U.S. and has long been recognized as a major agricultural
pest, costing tens of millions of dollars in direct crop losses annually
and additional millions costs for control. Only
recently have the harmful impacts of Canada thistle to native species and
natural ecosystems received notable attention.
IN THE UNITED STATES
Canada thistle is distributed throughout the
northern U.S., from northern California to Maine and southward to Virginia.
It is also found in Canada, for which it was named. Canada thistle has
been identified as a management problem on many national parks and on
preserves of The Nature Conservancy in the upper Midwest, Plains states,
and the Pacific northwest.
IN THE UNITED STATES
Canada thistle grows in barrens, glades, meadows,
prairies, fields, pastures, and waste places. It does best in disturbed
upland areas but also invades wet areas with fluctuating water levels such
as streambank sedge meadows and wet prairies.
was introduced to the United States, probably by accident, in the early 1600s
and, by 1954, had been declared a noxious weed in forty three states. In
Canada and the U.S., it is considered one of the most tenacious and economically
important agricultural weeds, but only in recent years has it been recognized
as a problem in natural areas.
BIOLOGY & SPREAD
thistle produces an abundance of bristly-plumed seeds which are easily dispersed
by the wind. Most of the seeds germinate within a year, but some may remain
viable in the soil for up to twenty years or more. Vegetative reproduction
in Canada thistle is aided by a fibrous taproot capable of sending out lateral
roots as deep as 3 feet below ground, and from which shoots sprout up at
frequent intervals. It also readily regenerates from root fragments less
than an inch in length.
of Canada thistle can be achieved through hand-cutting, mowing, controlled
burning, and chemical means, depending on the level of infestation and the
type of area being managed. Due to its perennial nature, entire plants must
be killed in order to prevent regrowth from rootstock. Hand-cutting of individual
plants or mowing of larger infestations should be conducted prior to seed
set and must be repeated until the starch reserves in the roots are exhausted.
Because early season burning of Canada thistle can stimulate its growth and
flowering, controlled burns should be carried out late in the growing season
for best effect.
In natural areas where Canada
thistle is interspersed with desirable native plants, targeted application
of a systemic herbicide such as glyphosate (e.g., Roundup® or Rodeo®), which
carries plant toxins to the roots, may be effective. For extensive infestations
in disturbed areas with little desirable vegetation, broad application of
this type herbicide may be the most effective method. Repeated applications
are usually necessary due to the long life of seeds stored in the soil.
USE PESTICIDES WISELY: ALWAYS READ THE ENTIRE PESTICIDE LABEL CAREFULLY, FOLLOW ALL MIXING AND APPLICATION INSTRUCTIONS AND WEAR ALL RECOMMENDED PERSONAL PROTECTIVE GEAR AND CLOTHING. CONTACT YOUR STATE DEPARTMENT OF AGRICULTURE FOR ANY ADDITIONAL PESTICIDE USE REQUIREMENTS, RESTRICTIONS OR RECOMMENDATIONS.
NOTICE: MENTION OF PESTICIDE PRODUCTS ON THIS WEB SITE DOES NOT CONSTITUTE ENDORSEMENT OF ANY MATERIAL.
For more information on
the management of Canada thistle, please contact:
- Daniel Roddy, U.S. National
Park Service, Wind Cave National Park, SD, dan_roddy at nps.gov
- U.S. Geological Survey,
Biological Resources Division, Flagstaff, AZ http://www.nbs.nau.edu/
Gwendolyn Thunhorst, The Nature
Conservancy, Arlington, VA
Jil M. Swearingen,
National Park Service, Washington, DC
John M. Randall, The Nature
Conservancy, Davis, CA
Evans, J.E. 1984. Canada
thistle (Cirsium arvense): a literature review of management practices.
Natural Areas Journal 4(2):11-21.
Hutchison, M. 1992. Vegetation
management guideline: Canada thistle (Cirsium arvense (L.) Scop.).
Natural Areas Journal 12(3):160-161.
Swearingen, J. 2009. WeedUS Database of Plants Invading Natural Areas in the United States: Canada Thistle (Cirsium arvense). http://www.invasive.org/weedus/subject.html?sub=2792.
The Nature Conservancy. Canada
Thistle: Element Stewardship Abstract. In: Wildland Weeds Management & Research
Program, Weeds on the Web.
USDA, NRCS. 2009. The PLANTS Database (http://plants.usda.gov). National Plant Data Center, Baton Rouge, LA 70874-4490 USA.
Plant Conservation Alliance, Alien Plant Working Group.