Common name:

Canada thistle, Canadian thistle, corn thistle, creeping thistle

Scientific Name:

Cirsium arvense (syns. Breea arvensis, Carduus arvensis, Cirsium incanum, Cirsium orchrolepideum, Cirsium setosum, Cnicus arvensis)

Noxious Weed Listing:

• WeedWise: Maintenance
• State of Oregon: Class B
• State of Washington: Class C
• 4-County CWMA: Class C
• Columbia Gorge CWMA: Class C

Description:

General:

Canada thistle is a tall, prickly, creeping rhizomatous perennial with multiple purple flowers. The erect plant stands around 2 to 4 feet tall on rigid, hairy, branched stems that usually die back during winter. This aggressive noxious plant forms dense unisex patches on disturbed land, and produces allelopathic chemical that inhibit growth of other plants.  Canada thistle is native to Europe, not Canada, and can be found throughout the United States.

Leaves:

Canada thistle leaves are oblong to lanceolate or lance-shaped, 2 to 7 inches long with deep and irregular lobes. Leaves are toothed with prickles that are 3 to 6 mm long. The green leaves are slightly hairy on their upper surface. Canada thistle leaves are alternate and are attached directly to the stem; do not have a leaf stem. Young leaves also alternate but are oval-shaped with no point, teethed with a softened prickle.

Flowers

The purple disk flowers of the Canada thistle plant are surrounded by rows of soft spine-tipped phyllaries. Flowers appear from June to October with 1 to 5 heads per branch. Petals are often purple but can be pink or even white. Both sexes have umbrella-shaped flowers, with female flowers being slightly bigger with longer pappus (male 10-14 mm, female 14-20 mm).

Fruits

Canada thistle has dry, small, single seed fruit called achenes. Oval, slightly curved, hairy and shiny; the achenes have feathery pappus attached at the base. Both the achenes (2-4 mm long) and the pappus bristles (12-20mm) are tan.

Roots

Canada thistle has a deep and extensive root system consisting of vertical and several horizontal roots extending as far as 15 feet. Roots are rhizomatous, with new stems sprouting in spring.  Roots are stiff and fragile but long-lived. In a years time, one plant’s root system can take over an area up to 25 ft². Seedlings start with deep taproot with creeping rhizomes developing between 2 and 4 months.

Reproduction:

Canada thistle reproduce both by seed and vegetatively through creeping roots. March through April, Canada thistle seeds germinate and grow to form rosettes. New plants can develop from underground shoots and roots longer than 1 cm. In May, new shoots bolt. June through August the plant flowers with seed production starting in August and continuing throughout September. Female flowers need to be pollinated to produce viable seeds which take 8-10 days to develop.  Plants produce 1,000 seeds on average and may produce up to 5,300 seeds. Seeds are dispersed by the wind.

Seeds are transported mostly by wind. The Canada thistle plant returns to the rosette stage near October and November.  Flower stems usually stay up for some time after seed reproduction. New root and shoot buds develop over winter.

New plants developed from vegetative reproduction are hardy, not sensitive to competition. New plants developed from seeds are slow-growing and sensitive to both competition and shading.

The seeds may remain viable in the soil for over 20 years.

Habitat:

Canada thistle can be found in disturbed areas with abundant sun and moist but not wet soils. Crop fields, forest openings, gardens, hillsides, logged areas, pastures, range land, roadsides, stream banks, vacant lots and waste places  are the usual infestation locations for Canada thistle. It is found throughout most of the continental United States.

Impacts:

• Canada thistle spreads quickly and forms dense infestations
• Reduces crop yields and pasture productivity
• Competes with and replaces native vegetation
• Reduces animal diversity
• Host plant for several crop-damaging insects

Introduction:

Canada thistle was introduced to the Willamette Valley between 1875 and 1899. Although it was reported to be not common when first collected, it is now very common throughout the area.

Distribution:

Clackamas County:

Canada thistle can be found throughout Clackamas County.  It is widespread and directly impacts properties throughout the county.  As an ubiquitous weed, this is not a species that is actively surveyed and the mapped distributions do not represent the full extent of the Canada thistle population in Clackamas County.

State of Oregon:

• Oregon iMapInvasives
• Oregon Weedmapper
• Oregon Flora Plant Atlas

United States:

• EDDMapS
• USDA Plants Database

Management:

Strategy:

The management of invasive weeds is best served through a process know as Integrated Pest Management (IPM).  IPM is a weed management methodology that utilizes:

• Management thresholds to determine when and if to initiate control,
• The ecology and life history characteristics of the targeted invasive weed,
• Site-specific conditions and land use considerations to inform management practices,
• The effectiveness and efficiency of various control methods.

An IPM based strategy ensures the maximum effectiveness of treatment measures.  IPM strategies typically use more than one management method to target one or more susceptible life stages.  It is adaptive to site conditions in the field and to the response of a plant to management.  The utilization of multiple management tools also inherently reduces the use of herbicides in a management plan.   The IPM process ultimately provides a framework for the establishment of Best Management Practices (BMP) which outlines the best approach for controlling a weed particular infestation.

Manual:

Only young plants can be removed by digging up the roots. More developed infestations have deep rhizomatous roots that greatly complicate removal. Any remaining root fragments can produce new plants. Do not compost flower heads, seeds or rhizomes as most home compost piles do not reach a temperature sufficient to kill all seed.

Mechanical:

Mowing can help te reduce the nutrients stored in the plants, but to be effective mowing must be repeated every few weeks for several growing seasons. Mowing can be an important tool, especially when combined with other management practice such as herbicide application.

Tillage increases Canada thistle infestations.  Rhizomes are cut and spread, allowing new plants to grow. Tillage can be effective in combination with chemical treatments.

Cultural:

Burning and grazing are not effective controls for Canada thistle management. Burning may reduce seed production.

Solarization of patches using plastic sheeting can be successful in controlling seedlings and small plants.

Chemical:

Herbicide application is an effective means to control Canada thistle infestations. Canada thistle is susceptible to several systemic herbicides. Be cautious when using herbicides on pasture land with grazing animals.

Before you Start:

• Before purchasing any herbicide product it is important to read the label.  The label is the Law.  Carefully review all parts of the label even if you have used the product before.  Select a product that is most appropriate for your site.  If you have questions, ask your vendor before purchasing a product.
• When selecting herbicides always use a product appropriately labeled for your site. Follow label recommendations and restrictions at all times.  If any information provided here contradicts the label, the label takes precedence.  Always follow the label!
• Protect yourself.  Always wear the recommended protective clothing identified on your label.
• When applying herbicides use spot spray techniques whenever possible to avoid harming non-target plants.
• Do not apply during windy or breezy conditions that may result in drift to non-target plants
• Avoid spraying near water.  Hand-pull in these areas, to protect aquatic and riparian plants and wildlife.
• Avoid exposure to pets, pollinators, and wildlife.  Remove animals from treatment areas to avoid exposure to herbicides. Follow the reentry instructions on your herbicide label and keep pets out of the area until the herbicides have dried.
• Avoid spraying blooming plants to minimize any effects to bees and pollinators, and makes applications at times of the day when insects and animals are not active.
• Be sure to store any chemicals, out of the reach of children and pets to keep your family safe.
• Product labels and formulations change regularly.  Check the Pacific Northwest Weed Management Handbook and the product label for current control recommendations.

Herbicides:

The mention of any brand name product is not, and should not be construed as an endorsement for that product.  They are included here only for educational purposes.  Suggested rates are generalized by active ingredient.  Specific rates will vary between products.  Be sure to review the label before application and use the recommended label rate at all times.

Biological:

Several different insects are have been used to target Canada thistle populations in Oregon. While effective in reducing infestations, biological control methods will never result in the complete eradication of a weed. Populations of all biological controls will vary from year to year.

• Canada thistle stem weevil larvae (Hadroplontus litura) mine the pith in stems of bolting plants. The adults feed on leaves causing minor damage.  This biological control was first released in 1981 and is now established in most eastern Oregon counties. The weevil is becoming more widespread but is still difficult to collect. The weevil does not seem to thrive in western Oregon.
• Thistle stem gall fly larvae (Urophora cardui) were specifically released in the United States to control Canada thistle. Larvae gall stems, which act as nutrient sinks, occasionally reducing seed production or plant height.
• The Canada thistle rust fungus (Puccinia punctiformis) was introduced into Oregon in 2018 to control Canada thistle. Spores germinate on leaves, travel down stems, and systemically infect the root system as a cryptic root parasitic fungus. The rust fungus often limits an infestations ability to increase and, in some cases, causes infestations based on one or two mother plants to experience colony collapse

Disposal:

Canada thistle flowers and seeds should be collected and put in trash or yard waste. Do not compost flowers and seeds. Non-flowering plants can be chopped up and composted or put in trash or yard waste.

Follow-Up:

Follow-up treatments are often necessary to control large infestations.

Best Management Practices:

Small Infestations:

• Consider the land use practices on site.  Identify, and site-specific considerations that should be taken into account before initiating control.
• Be sure you can properly identify Canada thistle.  There are several native thistle look-a-likes.  If you are unsure about your weed bring a sample to the Conservation District, and we can help to identify your particular weed.
• Identify any native or desirable plants nearby, and take precautions to minimize and negative impact on them.
• Dig individual plants or very young infestations.  Carefully remove any rhizomatous roots to prevent resprouting.
• Lay sheet plastic over infestations to solarize small and young plants.
• Spot spray Canada thistle plants using one of the recommended products labeled for use in your area.  Applying a directed spray of glyphosate or clopyralid at the flower bud stage before flowering, or in the fall before the first frost will control Canada thistle. Generally, fall treatments are more effective than spring treatments.  Clopyralid provides the best and most consistent control. Metsulfuron, aminopyralid or triclopyr products are also effective. You may need to repeat herbicide applications for several years.

Large Infestations:

• Consider the land use practices on site.  Identify, and site-specific considerations that should be taken into account before initiating control.
• Be sure you can properly identify Canada thistle.  There are several native thistle look-a-likes. If you are unsure about your weed bring a sample to the Conservation District, and we can help to identify your particular weed.
• Identify any native or desirable plants nearby, and take precautions to minimize and negative impact on them.
• Spot or broadcast spray Canada thistle plants using one of the recommended products labeled for use in your area.  Apply a directed spray of glyphosate or clopyralid at the flower bud stage before flowering, or in the fall before the first frost will control Canada thistle. Generally, fall treatments are more effective than spring treatments.  Clopyralid provides the best and most consistent control. Metsulfuron, aminopyralid or triclopyr products are also effective. You may need to repeat herbicide applications for several years.
• Mow or tilling populations can stimulate regrowth which may accelerate follow-up chemical control treatments.
• Revegetate sites by seeding competitive cover and prevent re-establishment.  Grasses can be an effective competitor and allow for the use of selective herbicides.

Fun Facts:

• Canada thistle is an excellent pollinator plant supplying nectar to bees for honey.
• Canada thistle has male and female flowers
• Leaves can be chewed to relieve a tooth ache.
• Roots are edible but often cause flatulence.
• Plants host agricultural pests such as sod-web worm, bean aphid, stalk borer, and cucumber mosaic virus.

Gallery

Canada thistle seedling

Canada thistle rosette

Canada thistle stems

Canadian thistle leaf

Canada thistle flowering plants

Large infestation of Canada thistle

Canada thistle flowers

Canada thistle inflorescence

Canada thistle flowering plants

Large infestation of Canada thistle

Canada thistle plant seeding

Canadian thistle pappus

Canadian thistle flower heads

Canadian thistle flowers

Canada thistle seeds

Additional Information:

• ODA Noxious Weed Profile
• Washington State Noxious Weed Control Board
• Oregon State University Extension: Canada Thistle 
• Pacific Northwest Weed Management Handbook: Canada Thistle Control
• Weed Research & Information Center: Canada Thistle Weed Report
• Columbia Gorge CWMA: Canada thistle BMP
• King County Washington: Canada Thistle Fact Sheet
• Center for Invasive Species and Ecosystem Health: Canada Thistle
• Invasive Species Compendium: Canada thistle Datasheet

References:

1. DiTomaso, J.M., G.B. Kyser, S.R. Oneto, R. G. Wilson, S. B. Orloff, L. W. Anderson, S. D. Wright, J.A. Roncoroni, T.L. Miller, T.S. Prather.  2013.  Weed Control in Natural Areas in the Western United States.  Davis, CA: UC Weed Research and Information Center.  pp 119-121.
2. Bossard, C. C., J. M. Randall, & M.C. Hoshovsky.  2000.  Invasive Plants of California’s Wildlands.  Berkeley, CA: University of California Press.  pp 106-111.
3. Kaufman, S. R. & W. Kaufman.  2007.  Invasive Plants: Guide to identification and the Impacts and Control of Common North American Species.  Mechanicsburg, PA: Stackpole Books. pp 301-303.
4. King County Noxious Weed Control Program.  2011.  King County: Canada Thistle Fact sheet.  https://your.kingcounty.gov/dnrp/library/water-and-land/weeds/Brochures/CanadaThistle_factsheet.pdf  (Retrieved Jan. 7, 2015).
5. Oregon Flora Project. 2013.  Oregon Plant Atlas: Cirsium arvense.  http://www.oregonflora.org/atlas.php .  (Retrieved May 22, 2013)
6. Peachy, E., D. Ball, A. Hulting, T. Miller, D. Morishita, P. Hutchinson. eds.  2019.  Pacific Northwest Weed Management Handbook: https://pnwhandbooks.org/weed/other-items/control-problem-weeds/thistle-canada-cirsium-arvense-selective-control-crops (Retrieved Aug 30, 2019).