Canada thistle, the plague of my peonies

June 23rd, 2007 by Kathy Purdy · 11 Comments

Canada thistle weaves throughout the peony bed

Observant readers may have noticed the prickly-leaved weed sidling up to ‘Rozella’ in my last post. That dastardly villain is Canada thistle, aka Cirsium arvense, and it is one nasty customer. According to the University of California Cooperative Extension,

Once established, Canada thistle spreads rapidly by horizontal roots, up to several meters per year. The extensive horizontal root system assures long-term persistence and spread by vegetative means. A segment of root as small as 1/8 to 3/8 inch (3-6 mm) in length and 1/16 inch (1 mm) in diameter is able to propagate a new plant. . . . Once established, Canada thistle is a fierce competitor for nutrients and water needed by crops or native plants. It produces allelopathic chemicals that assist in displacing competing plant species

Okay. It’s obvious this mess didn’t happen overnight. This peony bed was created in 2002, and my weeding practices have been, at best, inconsistent over the years. I’m really not sure what year the Canada thistle showed up, but since I wasn’t aware of how extensive its roots could get, I didn’t give this weed the immediate attention it apparently so richly deserved. I’m sure many times I looked at those spines and thought to myself, I’ll pull that the next time I have gloves handy. And even if I had gone to get gloves, the odds that I would have been distracted from my task by some other domestic duty or child-induced problem would have been pretty high. I’m not saying I never pulled the Canada thistles, I’m just saying I never pulled them frequently enough.

This year, I am trying to reform. I am making weeding my top priority in the garden. The peony bed was the very first thing I weeded in early spring, and I removed every single thistle plant as far down into the soil as I could manage. That might have been a mistake. According to the Colorado State Cooperative Extension,

Canada thistle allocates most of its reproductive energy into vegetative propagation. New shoots and roots can form almost anywhere along the root system of established plants. Tillage segments roots and stimulates new plants to develop. Shoots emerge from root and shoot pieces about 15 days after disturbance by tillage. Small root pieces, 0.25 inch long by 0.125 inch in diameter, have enough stored energy to develop new plants. Also, these small roots can survive at least 100 days without nutrient replenishment from photosynthesis.

As Purdue University explains further,

Removal of shoots and severe damage to established plants stimulate new growth from underground buds. It is the buds on the creeping roots of established Canada thistle plants which largely account for re-establishment after attempts at control. Buds on creeping roots can generate new shoots a year or more after top-growth has been destroyed.

So I think by pulling and digging, I actually encouraged the thistles to make more shoots and more roots. It’s one devil of a weed, I’m telling you. Those roots have to be deprived of nutrition over and over again to eradicate it.

In established plants of Canada thistle, carbohydrates move from the root system up to the newly forming shoots as growth starts in spring. As leaves on the shoots develop, photosynthates start moving to newly developing roots and flowerheads. The developing flowerheads take more and more of the energy (photosynthates) produced by the leafy stems and stored in the roots. Carbohydrates in the root system are at their lowest when the plant begins flowering.

Ideally, you’d remove each stem when it was in flower, so that the roots had the least amount of reserves. The largest shoots in this infestation were budding up, and so I decided to remove all the shoots today. I wore leather gloves, but I didn’t pull the shoots out. I cut them off at soil level, and threw them straight into the wheelbarrow:

It took me an hour or two to snip all the thistles in the peony bed

After the research I’ve recently done on Canada thistle, I know this is just the first round in a long fight. The peony bed looks a lot better at the moment, but I have no illusions about the future.

It’s prickly.

Stumble it!

Categories: Pests, Plagues, and Varmints

Keywords: peonies · peony · thistles · weeding · weeds

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11 responses so far ↓

1 Carol // Jun 23, 2007 at 10:58 pm

I have some Canada thistle “colonies” in a few places, as does my neighbor. I hate the stuff. I guess after reading this, it sounds like a bit of procrastination on pulling until they are just ready to bloom is one way to slow them down. Thanks for all the research!

I’ve also used a product called Perfectly Natural Weedkiller, which was primarily concentrated vinegar, to kill off the tops. However, I can’t find this anywhere this year, and haven’t had a chance to do an Interent search to find out who is selling it (I bought it at Walmart last year.)
2 Verdant Heart // Jun 24, 2007 at 1:00 am

Welcome to my world. Over the past decade my CRP land has been infested slowly, at first, with canada thistle. After a lot, I mean one hell of a lot of struggle, I settled on the herbicide ‘Stinger’ . I really hate to use the chemical method but I consulted with an expert (university expert) on what to do. This herbicide really works on Cirsium arvense. After using it on a large field of thistle I now keep a small bottle (16 oz) of the chemical at the ready. Whenever I find a Cirsium arvense I make sure it isn’t too near other plants (I force it flat on the ground) and then spray it. I leave a marker (a stake or flag) so I can go back to catch any new sprouts. Any attempt to physically remove plants larger than 4 or 5 inches will result in more plants from the root pieces. The plant is insidious with it’s underground roots but also with the airborne seed spread.

Stinger is not inexpensive but it is the only thing that I have found that works except clearing the area and using glyphosate, Round Up, for a year or more. You can only find it at a ag chemical distributor lie a local farmer’s co-op.

Good Luck really & Cheers… Verdant Heart
3 Robin (Bumblebee) // Jun 24, 2007 at 5:44 am

I feel your weeding pain! We have similar persistent weeds with underground roots here in Maryland. If they get into a place where I won’t harm the other plants with it, I pour on boiling water. It kills everything. And no chemicals.
4 Kelly // Jun 24, 2007 at 7:58 am

I made the mistake of pulling for a few seasons at the house we’re still trying to sell. Too late, I did some reading on the subject and now fully understand the grave mistake I made. That mistake was highlighted when I went out last weekend to deal with the lawn and saw that what once were beds with bulbs, climbing rose, moon flower and spiderwort, are now beds of 5-foot tall Canada Thistle. I cried.
5 Kathy Purdy // Jun 24, 2007 at 10:13 am

Carol,Verdant Heart, and Robin–the trouble with herbicides (or boiling water) is that they are not selective. They kill everything they touch. Before I spray Canada thistle with anything, I want to be positive the peonies aren’t going to be harmed. That will take some thought. I also have self-sowing poppies and larkspur in that bed, and I would probably lose some of them. Most of the sites I looked at recommended a multi-pronged approach that involved both spraying and cutting down. I didn’t want to use images without permission, but if you went to some of those web sites you probably saw pictures of how extensive the root system is.

Kelly, if you’ve truly lost your other plants, you might want to spray with one of the herbicides mentioned above, just to keep your house looking presentable. It’s hard to see something you’ve put so much work into ruined like that, but you need to view your house objectively, like a prospective buyer, and most of them are more concerned about the landscape looking neat, not pretty–as I’m sure you already know.
6 LostRoses // Jun 24, 2007 at 10:12 pm

That’s a bunch of nasty-looking customers you’ve got in that wheelbarrow! I get a few of them here and there and even let some flower (for the goldfinches) but for some reason they’ve never been prevalent in my garden. A couple of years of pulling them out by the roots does seem to discourage them, but that’s not very practical advice when you have lots of them to deal with. And prickly? Ouch!
7 Gardenista // Jun 24, 2007 at 10:51 pm

I’m sorry to see that dreadful problem you have there. I’m glad to have never known the Canada thistle on my property. I have read in blogs however, about otherwise organic-type gardeners resorting to glyphosate (Roundup) for this particular weed. Of course, you have to be careful to paint or sponge it on, avoiding other plants. I have heard that glyphosate breaks down quickly, so is not a harm to ground water, etc. Thus, I don’t feel entirely terrible using it if I need to. That horrible thistle seems to warrant some herbicidal help.
8 Jane // Jun 25, 2007 at 10:14 am

My thistles were primarily located between a chainlink fence and garden shed - a 6 or 8 inch space that it was impossible to get into and weed by hand so I gave up and started using roundup on them. I have a spray bottle where the spray can be directed fairly well (not much overspray) and every time I see a thistle pop up I isolate it as best I can from surrounding plants and spray it.

Going to roundup was a huge measure of my frustration - I spend 15 minutes every night digging dandelions out of my lawn rather than use herbicides on them, and even the ex-neighbour’s goutweed (the current occupants of that house and I are of one mind in the elimination of that scourge) and the creeping bellflower haven’t pushed me that far yet (although I’m getting close…).
9 Tracy // Jun 25, 2007 at 11:56 am

Kathy: I know exactly where you’re coming from. Just this weekend I pulled out some big thistles that I had sprayed with Roundup about 2 weeks ago, and I was planning to blog about my conflicted feelings about it this week. In the 8 years I’ve lived in this house, the only things I’ve used Roundup on are canada thistle, creeping charlie in my gravel paths, and little tree seedlings growing out of my rock walls. Like Jane, I’ll spend hours digging out dandelions by the root (or ignoring them), but there are a few weeds - or places where they grow - when that just won’t work.
10 Ki // Jun 26, 2007 at 7:57 am

We have been pulling up thistles in the same spot for over 5 years. They never get to the flowering stage so I didn’t know how we kept getting more every year until I read your post. All those years I thought the seeds from a plant remained dormant and would periodically sprout! I guess we have a battle on our hands. Thanks for the informative post.
11 Garden Bloggers’ Bloom Day: July | Cold Climate Gardening // Jul 15, 2007 at 1:13 am

[…] Ah, yes, Garden Bloggers Bloom Day has rolled around again. I always seem to have trouble finishing these bloom day posts on time. Even though I take photos for days ahead, the actual middle day of the month always seems to be a busy time. I should probably make it short and sweet, but making anything with words short seems to be difficult for me. I wanted to start off with larkspur, just to astonish all those Austin bloggers who seem to think it is a spring blooming plant. Are you kidding? Mine hadn’t even germinated for the April bloom report. Johnny’s is one of the few seed merchants that sells larkspur by color, and I always get this dark blue-purple to sow in the peony bed along with peony-flowered poppies, so there will be some color here after the peonies are done blooming. That’s the plan, anyway. Germination of both poppy and larkspur is always sporadic, and the poppies often look spindly. Maybe the peonies are just too much competition. Or those dratted Canada thistles. […]