Canadian thistle is an invasive weed that can be very difficult to control due to its extensive root system and prolific seed production. However, with persistence and using multiple control methods together, it is possible to get rid of Canadian thistle in your yard.
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What is Canadian Thistle?
Canadian thistle (Cirsium arvense) is an aggressive, colony-forming perennial weed that grows in a variety of conditions. It is native to southeastern Eurasia and was likely introduced to North America in the 1600s. It is now widespread across most of Canada and the northern United States.
Canadian thistle has an extensive underground root system that spreads horizontally. Its stems are slender, green and grooved, growing 1 to 4 feet tall. The leaves are oblong, irregularly lobed, with spiny margins. Flower heads are small, urn-shaped, and pink-purple in color. A single plant can produce thousands of seeds that allow it to spread rapidly.
This invasive weed infests many different habitats, including cropped fields, pastures, roadsides, ditch banks, gardens, and lawns. It crowds out desirable forage plants and reduces crop yields. Its sharp spines can cause injury to grazing animals. Canadian thistle is difficult to control due to its root system that can extend 15 feet deep and spread laterally through the soil.
Cultural Control of Canadian Thistle
Cultural practices alone will not eliminate Canadian thistle but can help reduce infestations when integrated with other control methods.
Promote Vigorous Grass Growth
A dense, healthy lawn is less susceptible to invasion by Canadian thistle. Follow proper mowing, fertilization, watering and cultivation practices to encourage grass to grow thick and tall, leaving little room for weeds. Mow the lawn regularly at a height of 3-4 inches. Apply a nitrogen fertilizer in early fall to boost grass growth. Water deeply and infrequently to promote deep root growth. Dethatch and aerate turf periodically to alleviate soil compaction.
Mulch Garden Beds
Spread a 2-4 inch layer of mulch such as wood chips, leaves or straw around flower beds and gardens. This blocks light that weed seeds need to germinate. Replenish the mulch as it decomposes. Take care not to pile mulch against the stems of plants.
Plant Competitive Ground Covers
Establish competitive, spreading ground covers to prevent Canadian thistle from invading open areas of soil. Good options include native violets, wild strawberry, creeping thyme, ajuga, and wintercreeper. Plant ground covers densely to quickly form a thick cover that suppresses weeds.
Clean Equipment
Canadian thistle seeds can be transported on contaminated equipment and in soil. Clean lawn mowers, tillers and other tools after use to help control the spread. Wash vehicles that drive through infested areas to remove seeds stuck in tire treads.
Mechanical Control of Canadian Thistle
Manual and mechanical techniques can effectively remove established patches of Canadian thistle but generally provide only temporary control. The extensive root system allows it to regrow after removal of aboveground growth. Persistence and repeat applications are required.
Hand Pulling
Hand pulling and digging can eliminate small, isolated patches of Canadian thistle if done repeatedly throughout the growing season. Grasp stems low and pull slowly to loosen the root system. Remove as much of the network of roots as possible. Sever any remaining roots 2-3 inches below ground with a soil knife. Alternately, vigorously hoe plants every 2-3 weeks to sever roots near soil surface.
Mowing
Frequent mowing to a height of 2-3 inches through the growing season can suppress Canadian thistle by removing photosynthesizing foliage and forcing the plant to expend resources regrowing leaves. It may take 1-2 seasons of diligent mowing to exhaust root nutrients. Time mowing to prevent seed production.
Cutting
Repeatedly cutting back stems and foliage to ground level with a brush cutter, mower or hoe can gradually starve the roots. Cutting is most effective when done every 10-21 days in spring, or whenever new growth appears. Make sure to bag and remove cut material to prevent re-rooting.
Tilling
Shallow tilling repeatedly during the growing season can help control Canadian thistle by cutting up its root system. Till to a depth of 3-4 inches and repeat every 3 weeks. This works best in loose, cultivated soil. Avoid spreading pieces of root to uninfested areas.
Chemical Control of Canadian Thistle
Herbicides provide the quickest means to get rid of heavy Canadian thistle infestations. Systemic herbicide translocated to the roots offers more complete control than only spraying foliage. Herbicides are most effective when the plants are actively growing and not under drought stress.
Glyphosate
Glyphosate products like Roundup can provide good control of Canadian thistle. Apply as a foliar spray when plants are in the bud to bloom stage. Use a 2% solution for best results. Glyphosate is a non-selective herbicide that kills any vegetation it contacts, so take care to avoid spraying desirable plants. Products containing glyphosate include:
- Roundup
- Kleenup Grass & Weed Killer
- Compare-N-Save Concentrate Grass & Weed Killer
Dicamba
Dicamba is a systemic herbicide selective to broadleaf weeds like Canadian thistle. Spray on actively growing foliage for uptake by the plant. Being a selective herbicide, it will not harm lawn grasses. However, it can damage many garden plants and trees. Dicamba products include:
- Banvel
- Clarity
- Sterling Blue
2,4-D
The systemic herbicide 2,4-D provides good postemergent control of Canadian thistle. As a selective herbicide, it does not affect grasses, but may injure many broadleaf plants. Apply as a foliar spray when thistle is actively growing. 2,4-D products include:
- Weedar 64
- Ortho Weed B Gon
- Spectracide Weed Stop
Triclopyr
Triclopyr is effective on Canadian thistle while doing less damage to certain broadleaf plants compared to 2,4-D. It is sold under brand names like:
- Turflon Ester
- Ortho Max Poison Ivy Killer
- Bonide Poison Ivy Killer
Aminopyralid
Aminopyralid provides excellent postemergent control of thistles when applied as a foliar spray. It is considered one of the best herbicides for Canada thistle control, particularly in pastures and grass hay fields where it will not harm grasses. Look for products such as Milestone.
Clopyralid
Clopyralid is a pyridine herbicide often combined with 2,4-D that offers very effective control of Canadian thistle. It translocates well to the roots. Products include Curtail and Stinger.
Biological Control of Canadian Thistle
Biological weed control uses living organisms such as insects, diseases or grazing animals to suppress weed growth. Biological control options for Canadian thistle include:
Insects
Certain insects can provide limited control of Canadian thistle by damaging foliage and flowers. Potential biocontrol agents include the thistle-head weevil (Rhinocyllus conicus) and thistle-stem weevil (Ceutorhynchus litura). These species are naturalized in North America but typically do not provide complete control.
Fungal Diseases
Fungal pathogens such as Puccinia punctiformis, Sclerotinia sclerotiorum and Pseudocercosporella herpotrichoides can infect and damage Canadian thistle but do not kill the plant. Their effects are usually minor and sporadic.
Grazing Animals
Goats and sheep will readily graze on Canadian thistle if given access, allowing them to suppress growth. The foliage contains spines, so animals must be trained or get used to grazing thistle. Do not rely on grazing alone for control.
Vinegar
Regular applications of horticultural vinegar may suppress Canadian thistle growth somewhat by drying and burning foliage. Use a product containing at least 20% acetic acid. Results are variable but vinegar is inexpensive and safe for gardens.
Prevention of Canadian Thistle
Preventing introduction and spread is crucial to avoid Canadian thistle becoming established. Follow these tips to keep it out of your yard:
- Inspect new plant purchases, soil amendments and seed mixes closely for signs of Canadian thistle.
- Remove plants quickly from gardens and landscaped areas at first sighting.
- Clean mowing and garden equipment after use on infested areas.
- Do not move soil, gravel or compost from a site with Canadian thistle to an uninfested area.
- Promptly re-seed bare patches in lawns and pastures to prevent infestation.
- Scout regularly for new plants along fence lines, ditches and edges of fields.
Conclusion
Controlling Canadian thistle takes persistence and using multiple control methods together. Focus first on preventing introduction and spread into new areas. For existing patches, combine cultural practices like mowing with herbicide applications for best results. Hand pull isolated plants before they can spread. Follow-up monitoring and maintenance will be required as plants try to re-establish from roots and seeds. But with diligence, you can get rid of Canada thistle and reclaim your yard.