Expert Advice

Canadian Thistle


Lobed leaves that are prickly but have a smooth upper surface with woolly undersides. Leaves can be between 4 and 8 inches long with prickly thin stems that can grow from 1 to 4 feet tall. Small purple flowers bloom from June to October. The weed germinates through seeds they produce attached to a tuft of hairs which spread easily on windy days. Canadian thistle flourishes in moist areas, with rich, heavy soil.

Life Cycle

Canadian thistle is a perennial weed that grows in cool climates. Seeds germinate throughout the season usually within 2 weeks from the time the seed matures and falls to the ground. The plant above ground gets killed by the first frost, but the root stocks that grow horizontally through the soil can be up to 15 feet from the plant and survive harsh weather conditions, waiting to bloom again in the spring. New plants can sprout from the root stocks and grow to bloom within 8 weeks.


The best defence against Canadian thistle is a healthy lawn. If your lawn is fertilized regularly, mowed accurately and irrigated properly it will be able to withstand and choke out a weed invasion. First and foremost, watering deep but less frequent will allow the roots of your grass to deepen and in turn prevent the growth of all kinds of weeds. If we think about how seeds germinate like a newly planted lawn, the more often we water the more likely new grass seeds would germinate. So, anything we can do to avoid providing constant moisture to a weed seed bed by watering deeply enough that the existing plants have water in their root zone and even if a new weed germinated, it wouldn’t be able to source enough water long enough to survive.

Always keep your lawn around 3” long, this will give your soil some shade and will prevent any weeds from germinating, especially along the edges. All seeds regardless of type need sunlight at the soil where the seed is present to germinate, so remembering that like mulch in your gardens, if we can prevent the sunlight from reaching the seed when the conditions are favorable for germination, the seed won’t germinate.

An annual aeration is also recommended to loosen compacted soil, this gives your lawn a great advantage by opening the root system, allowing for air, nutrients and moisture to flow optimally, minimizing Canadian thistle germination.

If you do get the occasional weed on your lawn, the best solution would be pulling out the weed with its roots or by applying a selective weed control product that is available at your local retail outlet. If you feel like you’ve tried everything and need professional help, we offer a 6 visit program with weed control to help your lawn withstand weed invasion. If you have any questions about weeds on your lawn, don’t hesitate to give us a call and a lawn care specialist will be happy help.