Family Asteraceae

Canada Thistle isn’t actually from Canada – it’s native to Europe, western Asia and northern Africa.  Here in the U.S. it’s a very aggressive, invasive weed.  It’s a perennial that forms dense clonal colonies.  Its flowers are very attractive to insects, including bees and butterflies.

7/14/2008  Canada Thistle flowers with a Banded Hairstreak

 

It prefers disturbed soil, so when we plant a new prairie area, we see lots of Canada Thistle plants for the first few years.

For years we tried to figure out strategies to deal with it.  We would:

•  Spot spray with Milestone herbicide – preferably when the plants were still small.
•  Mow dense colonies just before it went to seed.

Both strategies helped some – there were fewer plants the following year.

This is a large patch of blooming Canada Thistle.

The mowing was partly successful – we mowed it for several years in a row, just as it was coming into full bloom.  There are still thistle plants growing there, but fewer than before.

The Milestone spraying is more of a problem.  Because it also kills some natives and is persistent in the soil, spraying also reduced the numbers of natives.

Then I listened to a presentation about a different strategy – to just leave the thistle alone and rely on competition with natives to reduce the number of thistle plants.  I’ve been trying this since I learned about it, in the summer of 2025.  I sometimes cut off and remove the tops of large plants, but mostly I try to ignore them, and to be sure there are many native plants growing nearby to provide competition.

In the meantime, I enjoy the many native butterflies and bees the flowers attract.

7/30/2017  Painted Lady butterflies lay their eggs on thistle, including Canada Thistle.

 

8/5/2017    And their caterpillars eat the leaves.