Canada thistle.

Well, we’re digging ourselves out of a “nearly spring” storm here in Calgary – we received approximately 20 cm (8″) of snow yesterday and flakes are still falling as I write this. Just two days earlier, we were basking under +12°C (53.6°F) sunshine, which is pretty typical of the way the weather goes around here at this time of year. I was delighted to get out on Friday morning and take a walk up on Nose Hill, where I ended up sharing the sunrise with five deer and a grumpy porcupine (you can see one of my photos of him here).

While I was up on the hill, I noticed that the City’s war on Canada thistle (Cirsium arvense) in the park is ongoing.  I don’t like the fact that herbicides are sprayed annually to control this nasty invasive, but if something wasn’t done about them, the whole park would be covered in thistles. Pulling them simply isn’t a good solution – and it’s not just because they are thorny!


Canada thistle (winter)


  • Canada thistle is not a native of Canada. It actually has its origins in Mediterranean Europe.
  • Another common name for Canada thistle is creeping thistle…as in, it “creeps you out” with its insane root system. 😉
  • Each plant can produce up to 5,000 seeds.
  • Each seed can remain viable in the soil for up to 20 years.
  • Just 8 to 10 days after flowers emerge, plants can already produce viable seed.
  • Canada thistle reproduces by seed AND by vegetative cloning – a double whammy.
  • New plants can form from the tiniest of root segments – just 3-6 mm (1/8 – 1/4″) thick and 8 mm (3/8″) long. This is why pulling and digging don’t do diddly.
  • The tap root of each plant can reach 6 m (20 feet) underground in a single growing season.  Isn’t that incredible?¹

I am sooooo glad I don’t have to battle Canada thistle in my flowerbeds! I have a severe problem with quackgrass (aka couchgrass, Elymus repens), however, which I have struggled with for over a decade. While I have made significant inroads, I cannot ever let my guard down….

What are your worst plant enemies?



  1. We used to have a plot at a community garden, or allotment as they say in the UK. The worst sin you could commit was to allow canada thistle to get a foothold in your plot. If this happened, the plot was taken out of use for a couple of years and covered with thick black plastic to solarize the soil. Good to know that Canada thistle is not actually a North American native.

    • I wonder what the group leaders of the community garden I belong to would do if they found Canada thistle growing in one of the plots – it hasn’t happened so far, fortunately. We do have a “weed patrol” and they check the plots regularly for nasties… that’s comforting!

  2. Worst plant enemies….dandelions and anything that blows in from the neighbor next door…..I’ve had to go to raised beds that are 3ft high to control their lack of yard maintenance. I am hoping a big fence in the near future may help.

    • Oh, isn’t that so frustrating? It’s difficult enough to keep the garden weed-free, but when your neighbours “contribute” as well, that’s just really annoying. Hopefully the fence does the trick!

      I agree, dandelions are really troublesome – while they’re not quite as prevalent as the quackgrass in my garden, it seems I’m still always digging them out. And I can never seem to get the entire root…. 🙂

  3. Interesting post. I tried to get rid of lily of the valley in a very dry corner where nothing else would grow. I all but sifted the soil to get all the roots out. About 5 years later they started coming back. In those 5 years, not even weeds grew in this area. 🙂 It wasn’t a double whammy because they didn’t have the seeds – but I understand the root problem.

    • Lily-of-the-valley can be such a nuisance (even though it is a really pretty plant!). That is really interesting about the weeds not growing in the same site for five years – I know lily-of-the-valley is poisonous to humans and animals, but apparently it’s allelopathic, too. I’m sorry to hear that it came back.

    • Hi Pat
      In Australia, some gardeners would die for a patch of Lily of the Valley. I didn’t know it had weed potential as it is really hard and an expensive plant to get going here. I always thought it needed a damp spot.

  4. 20 ft deep root! And to think I always thought Bermuda grass with its ability to come up even under 6 ft of dirt was the worst! Makes me (almost) happy to have to battle Bermuda grass instead of this thistle!

    • Yes, it has quite the history, doesn’t it? 🙂 I’ve never seen it trying to invade gardens here, but I’ve seen a lot of it while I’ve been out hiking in the mountains west of the city.

  5. I had a neighbour who let his grown as tall as me. I’d go over and tear them out and burn them in our fire pit. Very invasive. It was almost a full time job. Sorry you got all that snow, saw it on the news and thought of you.

    • Eesh, who needs enemies when you have neighbours that let their thistles grow unchecked?

      Yes, that storm was a doozy, we’re still digging out. But the sun was out today and it was actually pretty nice. The surprising thing was how much snow fell in such a short amount of time. At least it happened on a Sunday and not a weekday morning – a few less traffic accidents that way. I’m glad you were lucky to escape it!

      • Somehow we also missed a Calgary Snow Storm last night….I’m probably jinxing it, but we’ve been very lucky. You’ve got such a sunny outlook, I love that. Yes Monday morning snowstorm would’ve been so much worse.

  6. 6 metres is pretty scary! I have the occasional thistle in my garden and they are very tough, but my worst weed is ground elder (Aegopodium podagraria). Like thistles, it has roots that spread along and down up to 30 feet! Digging and chopping roots only stimulates each tiny piece of remaining root to produce a shoot, so it gets worse, not better! 😉

    • Oh yes, that stuff is pure evil! I actually think the variegated types are rather pretty, but once it gets going, it’s nearly impossible to stop. People here are always planting it underneath trees and then wondering why it gets into the lawn and eventually starts eating the house…. 😉

  7. I battled the thistles for 2.5 years at my last house. We didn’t use herbicides, I just went out and pulled them by hand. If you go out after it rains when the ground is still wet, you can get them out root & all. It’s a ton of work but if you keep on it, it can be done. I think the key is to catch them when they are still small and haven’t grown large taproots.

  8. I’ve a couple of garden enemies – Dandelion, Lily of the Valley and Kerria japonica – both Lily of the Valley and Kerria come in from next door – under fences it’s an ongoing battle.
    i dread when she shows me her latest purchases – there are a few she purchased last year which I know will become a problem for me sooner or later!

  9. Garden…lawn enemy – Creeping Charlie…in the lawn, in the flower beds and it smells bad as you pull it. I think it came in with purchased perennials. And we do fight Canadian thistles in our pastures, pull, cut and burn – don’t like herbicides.

    • Going the no-chemical route to fight thistles is definitely the best option, if you can! 🙂 And I’m so glad I’ve never had a problem with creeping Charlie (yet) – it’s absolutely horrible. I feel for you!

  10. In Australia, Victoria, a plant called Patterson Curse is a huge problem in Victoria but in South Australia, is a savoir for the bee industry. I have even seen it in the Melbourne in the last ten years. Usually is a country paddock problem.

  11. We have some Canada thistle here in Virginia. but it’s not a problem, at least on our farm, but we have plenty of our very own Virginia Thistle that comes up everywhere in poor soil. I enjoyed your blog and am now a follower!

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