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Canada Thistle

Cirsium arvense
Updated Sep 11, 2023
 
1

Make a Positive Identification

  • Canada thistle grows stems of 3–5 feet with pink thistle flowers.
  • Plants live many years (perennial). Stems die back in the winter. It regrows from robust root systems underground.
  • It has spiny stems and its leaves limit human activity where it grows.
Species: Canada Thistle
Canada thistle stems with leaves and flowers

Weston Miller, Oregon State University

Canada thistle grows slender stems 3–5 feet tall. Many stems grow together to form dense patches.

Species: Canada Thistle
Canada thistle leaves

Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

Mature leaves are 2–8 inches long. Leaves are alternately arranged on the stem. They are lobed with stiff spines.

Species: Canada Thistle
Canada thistle seeds

Theodore Webster, USDA Agricultural Research Service, Bugwood.org

Each seed has a white structure (pappus) that aids wind dispersal. Seeds sprout on or just below the soil surface. Seeds remain viable in the soil for many years.

Species: Canada Thistle
Canada thistle seeds on newspaper

"Cirsium vulgare" by the weed one is licensed under CC BY-NC-SA 2.0 (cropped).

Each Canada thistle flower produces hundreds of seeds as shown in the photo.

Species: Canada Thistle
Canada thistle growing in a box above ground in order to show its robust root system

Purdue Department of Botany and Plant Pathology (Merrill Ross, retired)

Canada thistle grows a large root system. The photo shows a box about 4 feet above the ground surface. A side of the box has been removed to show the roots. Roots grow laterally for dozens of feet and about 10 feet deep. The root system stores energy and supports vigorous shoot growth.

Species: Canada thistle
Canada thistle shoots emerging from mature roots in spring

Howard F. Schwartz, Colorado State University, Bugwood.org

New shoots emerge from the root system during warm periods.

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2

Canada Thistle Benefits

  • Canada thistle flowers provide food resources for pollinators such as honey bees.
 

Canada Thistle Risks

  • Canada thistle spreads quickly. It is difficult to control.
  • Canada thistle displaces desired vegetation. The plant degrades wildlife habitat and livestock forage.
  • Canada thistle will dominate an area if not controlled.
  • Its prickly leaves and stems limit human activity where it grows.
Risk Card
Does it cause harm?
Adults & Children
Some
Property
High
Pets
None
Annoyance
High
Environment
Some
Action Recommended
 
3

TAKE ACTION

If you have Canada thistle on your property, take action to control it.

Do I need to take action?
Yes. Remove individual plants and small patches of Canada thistle. Both small and large patches require several years of attention to control.

Does Canada thistle have any benefits?
Flowers attract pollinators, especially bees.

NEED HELP?

Consider a licensed pest control company. Learn How to Hire a Pest Control Company.
Your local Extension Specialist in Oregon  and other states  can suggest other methods.

 
4
Solution for Canada Thistle

Early Detection & Rapid Response

Watch for Canada thistle on property you manage. Remove it before it becomes a bigger problem.

Physical Removal of Plants & Non-Chemical Options

  • Hand tools are effective on individual plants and small patches. Dig out plants and remove as much of the root system as possible.
  • Use a string trimmer or mower to cut Canada thistle stems repeatedly during the growing season. Prevent it from flowering.
  • Grazing and biocontrol are other control options.

Herbicides (Weed Killers)

Herbicides effectively kill Canada thistle when used according to label directions.

Monitoring & Follow-Up

  • Return to the Canada thistle stand. Look for regrowth. Control it each year.
  • Look for Canada thistle shoots and seedlings in the spring.

Jump To

Method Does it work? Is it safe? Recommendation
A
Physically Remove Stems & Roots
Effective
Low risk
Recommended
B
Cut Stems Repeatedly
Somewhat effective
Low risk
Recommended
C
Targeted Grazing
Effective
Low risk
Recommended
D
Biological Control
Somewhat effective
Low risk
Recommended
E
Herbicides Triclopyr & Glyphosate
Effective
Moderate risk
Use if Necessary
F
If Using Herbicides, Protect Yourself & Minimize Risks
G
PREVENT CANADA THISTLE
 
A

Physically Remove Stems & Roots

Non-Chemical Method

Find permission

Physically Remove Stems & Roots

Recommended

Dig out individual plants and small patches. Remove the roots. Canada thistle will regrow if all the roots aren’t removed.

Does it work?
Effective
  • Several years of monitoring and effort are required to get rid of Canada thistle.
  • Use preventive measures for best results.
How much effort?
High effort
  • Cut and remove stems to access roots. Dig the roots out with tools.
  • Return to the area each year and take action as needed.
What's the risk?
Low risk
Possible risk of exposure or harm from chemicals
NONE

Soil Disturbance & Erosion

  • Minimize soil disturbance as much as possible when removing Canada thistle.
  • Regrade the soil after digging Canada thistle roots. Apply mulch (when appropriate).
  • Take steps to prevent erosion as needed.
  • Replant the area to shade Canada thistle seedlings.
 
B

Cut Stems Repeatedly

Non-Chemical Method

Weston Miller, Oregon State University

Cut Stems Repeatedly

Recommended
  • Cut stems in small patches. Kill seedlings. Remove flowers before seeds form.
  • Use a string trimmer or tractor-mounted mower or disk to cut down Canada thistle plants.
Does it work?
Somewhat effective
  • This method keeps Canada thistle seeds from forming. Plants will still come back from its vast root system.
  • Use preventive measures for best results.
How much effort?
High effort

Cut stems repeatedly during the growing season.
What's the risk?
Low risk
Possible risk of exposure or harm from chemicals
NONE
Worker using string trimmer

kn1, iStock

  • Use a string trimmer or hoe to cut stems or seedlings. Leave the cut stems on the ground.
  • Mowing suppresses stem growth and depletes root energy reserves.
Tractor with disk attachment cultivating field

freeteo, iStock

  • Tillage suppresses stem growth and kills seedlings. Till before the plants flower to keep them from setting seed.
  • Root fragments as small as an inch can create new Canada thistle plants.
  • When tillage equipment drags root fragments, it can increase the size of the patch and spread it to new areas.
  • After working in an area with Canada thistle, clean your equipment before moving to new areas.
 
C

Targeted Grazing

Non-Chemical Method

Scott Bauer, USDA Agricultural Research Service, Bugwood.org

Targeted Grazing

Recommended
  • Grazing animals are used by ranchers and land managers to reduce Canada thistle growth.
  • Goats, sheep, and cattle all eat Canada thistle to some degree.
Does it work?
Effective
  • Canada thistle plants will sprout new stems following grazing.
  • Use preventive measures for best results.
How much effort?
Moderate effort
  • Establish temporary fences to contain the livestock in an area with Canada thistle.
  • Move the livestock before they damage desired plants.
What's the risk?
Low risk
Possible risk of exposure or harm from chemicals
NONE

Grazing is an effective way to reduce Canada thistle canopy and keep it from flowering. For best results, follow up with other control activities.

Grazing Tips

  • Grazing animals eat Canada thistle rosettes and succulent stems. Goats may eat older stems. Repeated grazing throughout a season suppresses Canada thistle growth.
  • Grazing animals will eat desirable vegetation as well as the targeted weed.
  • If you do not have access to grazing animals, contact professional pest control companies with grazing animals for weed control. Or ask your local Soil and Water Conservation District for referrals.
  • Grazing and herbicide treatment work well together to get rid of Canada thistle.
 
D

Biological Control

Non-Chemical Method

L.L. Berry, Bugwood.org

Biological Control

Recommended
  • Several native and introduced insects feed on Canada thistle. They play a role in large-scale, long-term control strategies. Several plant pathogens also injure Canada thistle.
  • A crown-stem weevil and a gall fly (shown in the photo) are present in the Western United States. These insects feed on Canada thistle and suppress its growth.
Does it work?
Somewhat effective
  • Biological control on its own won’t get rid of Canada thistle. Combine this approach with other control methods.
  • Use preventive measures for best results.
How much effort?
Moderate effort
  • Collect insects from an established stand of Canada thistle.
  • Distribute the insects on Canada thistle plants at your site.
  • Monitor your site for biocontrol insect activity.
What's the risk?
Low risk
Possible risk of exposure or harm from chemicals
NONE

Biological Control Reference

For more information about biocontrol for Canada thistle, see Weed Biological Control  (Oregon Department of Agriculture).

 
E

Herbicides Triclopyr & Glyphosate

Chemical Method: Use with caution

iStock

Herbicides Triclopyr & Glyphosate

Use if Necessary

Herbicides that contain the active ingredients triclopyr and glyphosate effectively control Canada thistle when used according to label directions.

Does it work?
Effective
  • Several years of monitoring and effort are required to get rid of Canada thistle.
  • Use preventive measures for best results.
How much effort?
Moderate effort
  • Treat individual Canada thistle plants and patches.
  • Return to the area each year and take action as needed.
What's the risk?
Moderate risk
  • Herbicides come with real risks. ALWAYS read the entire label front to back. Review instructions even for brands you know.
  • Herbicides can run off your site into waterways and may harm wildlife. See How to Keep Pesticides Out of Waterways.
Possible risk of exposure or harm from chemicals
Using herbicides includes some amount of risk. The lowest risk comes with using alternative methods.

You may be exposed to an herbicide if you:

  • Get it on your skin
  • Breathe it in
  • Eat or smoke afterward without washing hands
  • Touch or eat plants that are wet with spray (you, pets, or children)
  • Bring it inside on your shoes or clothes

Follow directions closely to reduce risk.

Herbicides with active ingredients triclopyr and/or glyphosate, used individually or in a mixture, are effective chemical treatments for Canada thistle. Look for these chemical names in the “Active Ingredients” section of product labels.

Photo of herbicide label highlighting active ingredient triclopyr

Weston Miller, Oregon State University

  • The white box on this example product label highlights active ingredient triclopyr. The text on the label states “Kills completely.”
  • Triclopyr doesn’t injure most grasses. It is a good choice for treating Canada thistle growing next to desired grasses in lawn, pasture, and meadow areas.
  • Some products with triclopyr (ester formulations) become a vapor when applied on hot days. The vapor can damage nearby plants. Check the label for temperature limits.
Photo of herbicide label highlighting active ingredient glyphosate

Weston Miller, Oregon State University

  • The white box on the example label highlights active ingredient glyphosate. The text on the label states “Kills grass and weeds around flower beds, trees, shrubs....”
  • Glyphosate will damage most plants and grasses. Don’t let the spray contact plants you want to keep.

Herbicide Application Tips

  • Seedlings are killed with a single herbicide application.
  • Repeated applications over 2–3 years will eliminate established Canada thistle stands.
  • Herbicide applications are most effective in spring to early summer and mid to late fall.
  • Return to the treated area and look for regrowth and re-treat as needed.
  • Herbicide products that target Canada thistle more narrowly than glyphosate and triclopyr are available.
Minimize the potential impact of herbicides to bees and other pollinators. Treat Canada thistle plants before they flower. Or apply herbicides in the morning or evening when bees are less active. Avoid spraying pollinators directly. For more information, see OSU’s How to Reduce Bee Poisoning from Pesticides  
 

If Using Herbicides, Protect Yourself & Minimize Risks

Chemical Method: Use with Caution
Great blue heron in marsh

BrianLasenby, iStock

Why is it important to read herbicide labels?

  • They have detailed information on how to use the product correctly and legally.
  • They contain information on potential hazards of the product.
  • They provide instructions you should follow for poisonings and spills.
  • Following label instructions helps you to minimize the risks and maximize the benefits.

Key Herbicide Safety Tips

  • Read the entire label front to back.
  • Follow the instructions.
  • Review the instructions even for brands you know.
  • Only apply the product where the label says it may be applied.
  • Be precise in your application. More is not better.

The Label is the Law

ALWAYS read the label before using herbicide products. The label is a legal document that provides information on how to safely use the herbicide. This helps avoid harm to human health and the environment. Using an herbicide in off-label ways is illegal. It can result in legal enforcement actions.

READ THE LABEL & Follow Instructions
It has instructions to protect you and the environment.

  • Labels are different for every product and they often change over time.
  • Use a magnifying glass if needed.
  • Pay attention to CAUTION, WARNING, and DANGER statements.
  • Pay attention to the PRECAUTIONARY STATEMENTS.
  • The law states you must read and follow herbicide instructions.

Protect Yourself
Eye, skin & lung irritants

  • Wear the right protective gear. This often includes chemical-resistant gloves, safety glasses, a long-sleeve shirt, pants, socks, and shoes.
  • Mix outdoors or in a well-ventilated area.
  • Wash hands after mixing or applying, and before eating or smoking.
  • Take a shower immediately after handling herbicides.
  • Wash clothes worn while mixing or applying separately from other laundry.

Protect Children & Pets
Children and pets are at risk if they eat or touch the plants before it dries.

  • Keep them away during and after applying herbicides (read label for how long).
  • Remove toys and pet dishes from yard before applying.
  • Don’t track herbicide products into your home on shoes or clothes.

Protect Plants You Want to Keep

  • Glyphosate and similar herbicide ingredients damage both grass and broadleaf plants.
  • Minimize spraying of foliage, stems, exposed roots, or the trunks of desirable shrubs or trees to avoid harm.
  • Follow the label to avoid damaging the roots of trees and shrubs.

Avoid Wet, Windy, or Hot Weather
Use during favorable weather for best results.

  • Don’t spray when it’s raining or when rain is expected in the next 24 hours.
  • Wind causes spray to drift that can get on you and desired plants.
  • Herbicides may be less effective in hot weather if the target plants are moisture-stressed.
  • Some herbicides can turn into a vapor in hot weather and damage nearby plants.

Protect Pollinators

  • Apply in the early morning or evening when bees are less active.
  • Kill weeds before they flower. Avoid spraying flowering plants.
  • Do not spray on bees or insects.

Storage & Disposal

  • Store in a secure area away from children.
  • Don’t put unused herbicide products in the trash.
  • Never pour down any drain or waterway.
  • Take unused herbicides to a hazardous waste facility.

Call  1-800-CLEANUP (1-800-253-2687) to find out where to dispose of herbicides.

For the Portland metro region in Oregon, contact Metro’s Recycling Information. Call  503-234-3000, email   or visit Metro’s website  

More about:

About Using Pesticides on School Grounds in Oregon

If using pesticides on school grounds, there are special rules in Oregon. See School Integrated Pest Management  (Oregon Department of Agriculture).

NEED HELP?

The National Pesticide Information Center (NPIC)  can answer questions about pest control chemicals.
 1-800-858-7378 or npic@ace.orst.edu  

Consider using a licensed pest or weed control company. Learn How to Hire a Pest Control Company.

Your local Extension Specialist in Oregon  and other states  can suggest other methods.

 

PREVENT CANADA THISTLE

Established Canada thistle shoots emerging

L.L. Berry, Bugwood.org

Look for New Stem Growth Following Control Activity
  • Look for and kill seedlings.
  • Single Canada thistle plants form large patches from their strong root system. What appears as a seedling may be a shoot growing from roots.
  • Control it in early spring and fall. Herbicide treatments are most effective at these times.
  • Focus on stem reduction and flower removal in summer.
  • Continue to monitor the area each year. Control it as needed.
Gloved hand using metal brush to clean shovel

Weston Miller, Oregon State University

Remove Dirt from Shoes and Equipment
  • After working or traveling in an area that has Canada thistle seeds, clean your boots and tools. Use a wire brush to remove all soil that may contain seeds or root fragments.
  • If you drive into a Canada thistle stand, clean your vehicle before traveling into new areas. Seeds are easily transported to new locations on vehicles and equipment.
Landscape area with native plants growing densely together

Weston Miller, Oregon State University

Replant the Affected Area
  • After Canada thistle is removed, the desirable plants that were buried underneath Canada thistle stems often regrow.
  • Replanting is necessary when Canada thistle removal significantly damages a site and few or no desirable plants remain. Replanting stabilizes the soil surface and shades Canada thistle seedlings.
  • Plant with a variety of native species.
  • Check plantings yearly for new Canada thistle plants.
Area damaged by invasive plants growth and removal replanted with native plants

Carmen Hauser, iStock

Replant Larger Areas with Technical Support
  • Canada thistle is difficult to eliminate from an infested area. Replanting a previously infested area requires planning and effort.
  • Create a multi-year revegetation plan. Plans include site preparation and planting details, plant care, and follow-up control for Canada thistle and other weeds.
  • Plan for at least 2-3 years of monitoring and maintenance.
  • Your local Extension specialist, soil and water conservation district, or a professional revegetation specialist can suggest strategies for your area.

Invasive Species Alert

  • Invasives are non-native species that spread aggressively and alter the environment.
  • Controlling unwanted Canada thistle is costly.
  • Please do your part to control Canada thistle on property you manage. It can spread beyond your property and have an adverse impact on your neighbors.
static invasive map
Invasive species data @ 2022, iMapInvasives (NatureServe)

Canada thistle is already widespread in Oregon. There’s no need to report infestations of it.

SEE OTHER COMMON PACIFIC NORTHWEST WEEDS

Content provided by editor Weston Miller and writers Jessica Green and J. Jeremiah Mann. Pesticide safety information edited by Kaci Buhl.

 Peer reviewed by OSU Department of Horticulture.

Photo of Weston Miller

Weston Miller

Project Founder and Content Writer

Weston Miller served as Community and Urban Horticulture faculty for Oregon State University Extension Service for Clackamas, Multnomah, and Washington Counties. Weston is an author for content for this website. He developed funding partnerships with Portland area agencies to initiate and build out the Solve Pest Problems website focused on this goals:

Photo of Jessica Green

Jessica Green

Jessica Green has held various positions at Oregon State University for over 15 years. She was one of the original content contributors for Solve Pest Problems and now assists with maintaining the resource for the Oregon IPM Center. Jessica is a contributing author/editor for the PNW Weed and Insect Management handbooks, has designed and conducted research trials, and now serves as an educator for OSU's Pesticide Safety Education Program (PSEP).

J. Jeremiah Mann

J. Jeremiah Mann

J. Jeremiah Mann completed a Physical Science undergraduate degree at Humboldt State University, and M.S, Ph.D focusing on plant science topics at UC Davis. He went on to work for the Natural Resources Conservation Service and in a leadership position serving a private agricultural technology company. He currently lives in Sacramento California where he consults on pest and property management topics.

Photo of Kaci Buhl

Kaci Buhl

At the state level, I lead the Pesticide Safety Education Program (PSEP). The program hosts live recertification events around the state, serving over 1,000 licensed pesticide applicators each year. We also produce web-based training modules and license-preparation study manuals. Special training for unlicensed pesticide applicators is also available through a grant from the USDA Specialty Crop Block Grant Program. The PSEP at OSU works closely with the Oregon Department of Agriculture's Pesticides Division.

Canada Thistle References

Canada Thistle  
University of California

Control Practices for Canada Thistle  
Purdue University

Target Grazing Canada Thistle  
University of Idaho