How to Control Canada Thistle

Canada thistle is an invasive plant not native to the United States. The plant was accidentally introduced to the country nearly 400 years ago and today it is a noxious weed in 43 states. Canada thistle is a perennial weed growing from 1 to 4 feet tall. The purple-pink flowered prickly-leaf plant spreads by wind-blown seeds and spreading rootstock. Canada thistle thrives in terrain from prairies to wetlands and the noxious plant crowds out native species and reduces forage for wildlife and livestock. As the plant spreads, it invades crop lands and costs millions of dollars in crop losses. It threatens ecosystems and parklands by destroying native plant habitat and damaging animal diversity. It also invades lawns and gardens. Canada thistle is controlled in several ways.

Garden sprayer

Broadleaf weed killer

Pruner

Shovel

Chemical. Canada thistle can be controlled with systemic herbicides such as those containing glyphosate or clopyralid. Check the label of weed killers at garden centers and hardware stores. Those that kill dandelions will often control Canada thistle.

Application. Systemic herbicides kill the plant by carrying toxins to the roots. This is critical as the perennial thistle can regrow from rootstock. An herbicide or broadleaf weed killer with 4-D is effective if used while the plant is young and has its broad leaves. It can be easily applied with a hand or tank sprayer.

Biological. Some insects attack Canada thistle. They do not eliminate the weeds but can help control the plants by destroying leaves and flower buds. It may take 5 to 10 years to develop a dense insect population that has a measurable effect on large areas.

Hand-cutting. Hand weeding may work on individual plants. Preferably cut back the plant before it flowers and seeds. Otherwise cut off the seed head then cut the plant as low as possible. If digging, go deep enough to cut out the crown.

Mowing. Mow the plants in spring or late summer when they are heading to flower. Mow as close to the ground as possible. If the plants are mowed too early, they will continue growing and develop flowers. If mowed too late, the seeds may continue developing in the cut-off seedhead. Repeated mowing may be needed for several years to control these perennials.

Burning. Controlled burning is best done late in the growing season. Early in the spring, it can stimulate thistle growth.

Caution. Canada thistle is difficult to control. Because the seeds may lie dormant for up to 20 years, repeated treatments may be needed.

If you spot Canada thistle in your yard, the earlier you apply an herbicide to the growing weed, the more effective it will be.

Always wear gloves and eye protection when working around Canada thistle.