Invasive and Misnamed

6 June 2010

The annual onslaught has begun.  Canada Thistle is blooming in southwestern Pennsylvania.

Though we call it Canada Thistle, Cirsium arvense is badly misnamed.  Native to Europe and northern Asia, it’s now found as far away as Australia and New Zealand, and it’s never welcome.  It’s invasive nearly everywhere it grows.

Some of Canada Thistle’s other names are more descriptive:  Hard Thistle, Creeping Thistle, Cursed Thistle.  It spreads by seeds and through its “creeping” root system that extends horizontally for 15 feet or more.  Each plant produces only male or female flowers so a clump can be isolated and not be pollinated.  No matter.  It’s perennial, its roots spread, and it chokes out less aggressive plants.

Watch for Canada Thistle’s violet flowers by roadsides, in fallow fields and in disturbed sunny patches.  When you find it, there’s one bit of good news to keep in mind.  Canada Thistle provides food for Painted Ladies and American goldfinches.

Goldfinches nest when the thistle blooms.

(photo from Wikipedia. Click on the image to see the original.)

10 thoughts on “Invasive and Misnamed

  1. We were weeding our garden and I think this is what we’ve been pulling out for the past two days! If it has a creeping root system, how do I completely eradicate it from my garden? I hate the spikes.

  2. According to Urban ECO Stewards you should try to remove as much of the rootstock as possible, preferably when the soil is moist. Then watch for resprouting next year! Also remove before they bloom as their seeds can remain viable for over 20 years.

  3. Thanks for the advice everyone! We left the garden alone for a day and already the weeds are coming back up.

  4. yes we also have been cursed w this weed..i like goldfinches tho so if its a food source i forgive the weed long as its out of my yard..

  5. Dan and I keep pulling this out, along with the mustard garlic….keep up with it and get the roots by digging deep, but do not chop or rototiller since you just make MORE plants. Pull it BEFORE it goes to flower with thick gloves…

    If you want thistles for the goldfinches and painted ladies, I keep a FEW of the purple thistles(pasture?)…very jaggy, but the butterflies love the flower, and then the goldfinches the fuzzy part. They are HUGE this year along with the common mulleins.

  6. Does Round-up or some such localized pesticide work? Even though I wear garden gloves, the “jaggers” get through them. Our invaders are in a difficult place to access, between a very thick yew and a wrought-iron fence, so we’ve been reduced to cutting them off from between the railings as they grow. Clearly, we are not addressing the root system, which Round-up claims to kill.


  7. Anne,

    I would definitely try the Round-up in the place where the thistle can’t be pulled up. I’d be desperate to get rid of it, because I wonder if constantly cutting the stuff makes the roots even stronger. I hardly ever use Round-up or any other chemicals in my garden, since I prefer to pull weeds by hand whenever I can, but I think that in some extreme cases, it’s OK to use it in moderation. Whenever poison ivy starts growing in my yard, I use Round-up because I don’t even want the oil from that plant on my garden gloves.

    I can’t stand Canada Thistle! I didn’t know what it was called (thanks for the info, Kate), but I’ve been hating the stuff for years. I didn’t know it’s a perennial, and also that it isn’t even a native plant! I agree with Anne, the thorns on this weed seem to get through even my thickest garden gloves. Except for the thorns, at least it’s relatively easy to pull, especially when the ground is wet. That’s about the only nice thing I can say about it. If the goldfinches want to find a meal in my yard, they’ll have to use the bird feeder.

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