Canada thistle (Cirsium arvense) is a difficult-to-control weed in lawns and garden beds. This plant spreads aggressively by seeds and underground runners. Despite its common name, Canada thistle is native to Europe and northern Africa. It is considered a noxious weed in Iowa and should be controlled as soon as it is noticed.
A cool-season perennial, this weed grows primarily in spring and fall. It begins growth in spring and will set flowers in early to mid-June in Iowa, with seeds fully developed by July. Growth slows during the summer months and plants resume growth in September, where much of the growth goes to the underground rhizomes that help it spread. Growth will die back over winter, but new leaves and stems emerge in spring from the underground stems.
Control of Canada Thistle in Lawns
Non-Chemical Control Options
Canada thistle is difficult to control by digging and pulling as the entire plant, including the extensive rhizome system, must be removed to get complete eradication. Any small pieces left behind will continue to grow and spread, often leading to even more sprouts.
The most effective means of control is to start early, before plants establish an extensive network of underground stems and before they flower and set seed. Frequent mowing can prevent bloom and seed set. At a minimum, these weeds should never be allowed to set seed, which can exacerbate the issue. Mowing frequently can exhaust young plants, causing them to die. Established stands frequently are unphased by mowing as new growth can return from the rhizomes.
Maintain Healthy Turf
Promoting a thick and healthy lawn helps prevent new Canada thistle populations from establishing. Follow sound cultural practices to promote a healthy lawn. This includes a good nitrogen fertility program, appropriate watering, and proper mowing. Learn more in these articles: Lawn Fertilization (PDF), Watering Home Lawns, and Mowing Your Lawn (PDF).
Herbicides
In most situations, herbicides are the only practical method of weed control. Effective broadleaf herbicides include the active ingredient(s) 2,4-D, MCPP, MCPA, dicamba, triclopyr, carfentrazone, and/or quinclorac. The timing of the application for best control is critical. Canada thistle is most susceptible to herbicides when applied in the fall when plants are actively growing. A fall application can help reduce the growth of the underground roots, making the weed less likely to reemerge later. Applications can be made late into the season, even after the first frost, as long as leaves are still green. Early summer, when flower buds are starting to form, is another effective time to apply herbicides. Applications made at this time can help prevent bloom and seed set. Often, applications will need to be made at both times over several growing seasons to get this weed under control. Persistence is key.
Once Canada thistle populations are under control, diligent scouting and early removal will be necessary. Plants establish readily by seed and even with the best of weeding efforts, new plants will appear in subsequent years. Regular and persistent removal of Canada thistle each time they are noticed will be needed every growing season to keep these weeds under control - remember, they are much easier to eradicate when the plants are young.
Control of Canada Thistle in Garden Beds
Control of Canada thistle in garden beds presents some unique challenges. Plants are not easily dug as new sprouts emerge readily from any piece of root left behind. The underground network of roots for this perennial weed is extensive with roots extending as much as 20 feet out and 6 feet down. The new growth often comes back even stronger, making the problem even worse.
Mowing or cutting is beneficial to prevent flowering and seed set. At a minimum, plants should never be allowed to flower and set seeds. Frequent and persistent cutting can exhaust young plants, causing them to die. Established stands frequently are not controlled by cutting, and new growth can return from the rhizomes (even stronger than before!).
Herbicides are typically the only effective solution. However, nearly all herbicides used to control Canada thistle are not labeled for use in garden beds, as they will also negatively affect the desirable plants in the area. Broadleaf herbicides with active ingredients like glyphosate, 2,4-D, MCPP, MCPA, dicamba, triclopyr, carfentrazone, and/or quinclorac can be spot-sprayed, applying the herbicide only to the leaves of the weed. Always apply herbicides when winds are calm and temperatures are cool to prevent drift and damage to desirable plants. Protect garden plants with barriers like buckets, boxes, or plastic sheets to further reduce problems with drift. Herbicides can also be applied with a sponge and wiped onto the leaves of the weed to prevent collateral damage to nearby plants.
The timing of the application is important. Control is best done in the fall and in the early summer, just as plants are setting flower buds. Often, applications will need to be made at both times over several growing seasons to get this weed under control.
More Information
- A brief history of Canada thistle in Iowa
- Types of Weeds in Landscapes, Gardens, and Lawns
- Weed Control in Home Lawns
- Managing Weeds in Landscapes, Gardens, and Lawns
- How to Control Crabgrass
- Control of Ground Ivy (Creeping Charlie)
- Control of Dandelion
Photo credits: 1: Bediha Duru Öksüz/AdobeStock; 2: CreativeSuburb/AdobeStock; 3: Ilona Lablaika/AdobeStock