Dissecting Canada thistle

By Shauna Dobbie

I bet you wish you could actually dissect all the Canada thistles out there and trash them. This plant is not native to Canada, despite the name; it is from Europe and Northern Asia but has become naturalized throughout temperate regions of the world. It is considered invasive, even in some countries where it is native.

It is not unattractive. I remember my grandfather out mowing the lawn, then telling my grandmother he had very carefully avoided her pretty purple flower. Guess what it was. Thistles do have a certain charm, with the tuft of purple above a head of perfectly spaced, diamond-shaped bracts. Then, when they’re pollinated, turning into dense clusters of silky puffs, the seeds blown away, one by one, on the wind. Sounds lovely, doesn’t it?

The problem is that this plant is too successful. It spreads seeds far and wide by feathery bristles on the pappi, but that is not the main way it spreads. It spreads by adventitious roots, underground, like turf grass. 

Interestingly, the plant is dioecious: some plants are male and some are female. New plants that grow from adventitious roots are clones of the parent, so they are the same sex as the parent. When you come across a patch of Canada thistles, they will all be the same sex. Its other weaknesses are an inability to tolerate shade or wet roots. 

It grows best in disturbed areas and over-grazed meadows. When the seeds find bare soil, they usually germinate rapidly; if they don’t, they can live up to 20 years in a dormant state. If you try to dig this plant up, heaven help you: the roots are very good at becoming a new plant from just a tiny piece. 

To get rid of Canada thistle, recommendations are to mow it frequently. Cutting it above ground will weaken the plant eventually and will prevent it from developing seeds in the meantime. If you opt to use some kind of chemical, use it in the fall, when the plant is moving nutrients from above ground to the roots.

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