-
Pest SolutionsPESTS + WEEDSUSING PESTICIDES OR HERBICIDES?
I should be a large image.
Canada Thistle
Cirsium arvense
Updated Oct 27, 2024
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
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
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
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
"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
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
Howard F. Schwartz, Colorado State University, Bugwood.org
New shoots emerge from the root system during warm periods.
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
Prickly spines hurt and limit access to areas
Property
High
Dominates an area when left unmanaged
Pets
None
Prickly spines also impact pets
Annoyance
High
Difficult to control
Environment
Some
Rapidly spreading to new areas & food source for pollinators
Action Recommended
Take action to control this invasive weed where acces is needed.
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.
PREVENT CANADA THISTLE
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.
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.
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.
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.
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.
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.
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 Control with Herbicides |
Effective |
Moderate risk |
Use if Necessary |
|
F If Using Herbicides, Protect Yourself & Minimize Risks
|
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
|
| How much effort? | High effort
|
| 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
|
| 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 |
kn1, iStock
freeteo, iStock
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
|
| How much effort? | Moderate effort
|
| 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
|
| How much effort? | Moderate effort
|
| 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
Control with Herbicides
Chemical Method: Use with caution
Chameleonseye, iStock
Control with Herbicides
Use if Necessary
| Does it work? | Effective
|
| How much effort? | Moderate effort
|
| What's the risk? | Moderate risk
|
| Possible risk of exposure or harm from chemicals | El uso de herbicidas conlleva cierto grado de riesgo. El riesgo más bajo se presenta con el uso de métodos alternativos. You may be exposed to an herbicide if you:
Follow directions closely to reduce risk. |
Some herbicides are available for home use. A few are listed below. These active ingredients, used in a mixture or individually, are suggested chemical treatments to control Canada thistle. Look for these chemical names in the “Active Ingredients” section of product labels.
- 2,4-D + Dicamba
- Diquat dibromide
- Glyphosate
- MCPP-P
- Quinclorac
- Sulfentrazone
Suggested active ingredients are from the Pesticide Information Center Online (PICOL) Database. They are permitted for HOME use in Oregon. Read the label for application directions and effectiveness information.
ONLY professional pesticide applicators can use Restricted-Use Pesticides (RUP) that may be more effective. For larger stands of Canada thistle contact a licensed pesticide applicator with experience controlling Canada thistle.
Herbicide Application Tips
- Most of these ingredients will damage most plants and grasses. Don’t let the spray contact plants you want to keep.
- 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. Re-treat as needed.
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
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 for small print.
- 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.
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.
Invasive species data @ 2022, iMapInvasives (NatureServe)
Canada thistle is already widespread in Oregon. There’s no need to report infestations of it.
Content provided by editor Weston Miller and writers Jessica Green and J. Jeremiah Mann. Pesticide safety information edited by Kaci Buhl.
Peer review facilitated by OSU Department of Horticulture.
Canada Thistle References
Canada Thistle
University of California
Control Practices for Canada Thistle
Purdue University
Target Grazing Canada Thistle
University of Idaho