I’ve become an unwilling expert on the weed known as thistle. Living where I do in Pennsylvania, we’re besieged by the plant in spring, and when not controlled, thistle causes problems for years. Fortunately for you, I have a lot of experience successfully ridding thistle from my own and other gardens, and I am happy to report that it’s not an impossible task if you’re willing to do a little work when it first appears. Okay, I just lied right there – sometimes it requires a LOT of work. But no matter how good you are at exterminating this devil of a plant, you’ll never be completely free of thistle, so get used to it and learn its habits.
Thistle appears nearly everywhere in North America
Thistle – specifically Canada Thistle – appears over a wide area of the U.S. and, obviously, Canada (Bull Thistle and Creeping Thistle are closely related). This cool season perennial (Zones 2-8), known for its difficulty to kill and its desire to take over your garden completely and totally, is a scourge for gardeners. But once we get over our thistle anxiety and learn how to keep it in check, it becomes a routine weeding task.
Thistle is a “pioneer” plant
Thistle is what’s called a “pioneer” plant, like dandelion: Give them bare earth, and they will fill it in promptly. This “pioneering” provides a great service in the wild by protecting the soil from erosion until larger plants can take over and by feeding pollinators like bees and butterflies where little else may exist. By virtue of thistle’s deep tap roots and extensive root systems, many air pockets and water pathways are created in the hard soil, encouraging the soil ecosystem along. So don’t hate on thistle too much – it’s only doing its job.
How does thistle spread?
Canada Thistle spreads by seeds and creeping roots called rhizomes, which makes it especially invasive when left unchecked. Give it an inch or two of bare ground near your Joe Pye Weed, and it will fill it in eagerly, and also kill your Joe Pye Weed in the process.
When does thistle appear?
Thistle weeds appear in spring, show their flower buds in late May or early June, and by the end of June, they’re in full flower. Shortly after this, and definitely by mid-July, thistle flowers have produced seeds, which are dispersed by wind, wildlife, and ignorant landscapers (don’t get me started…). In September, after the dog days of summer have passed, new shoots emerge from the roots, which survive until cold weather sets in, usually in November. This new growth replenishes underground store reserves to give the plant lots of power to overtake more of your garden the following spring.
Why weed killers don’t always kill thistle
Many gardeners, frustrated at the seemingly perpetual battle with this garden tyrant, reach for chemical herbicides out of sheer desperation. Unfortunately, chemicals only kill the leaves and flowers. But the uninitiated gardener is at first satisfied since the scourge of her garden appears to be deceased.
What the gardener doesn’t realize is that the herbicide has done nothing to the thistle weed’s massive and deep root system, which will produce new shoots and foliage shortly after, confounding the chemical-wielding gardener in all manner. Of course, one could perpetually spray, but they would also wipe out all of the surrounding plants. Herbicides kill indiscriminately, not to mention their effect on wildlife, soil, and your health. Organic control of thistle is more effective than chemical control, although it requires some diligence and considerable elbow grease.
Best hand tool for weeding thistle: CobraHead Original Weeder & Cultivator Garden Hand Tool
How to control thistle weed organically (without herbicides)
Keep the flower heads of thistle, and hence the seeds, in check.
When flowers appear, yank them off immediately and toss them in the compost bin (unless they’re mature – a few weeks old and well-developed – then bag them and toss them in a trash bin). By keeping the plant from flowering and seeding, you’ll save yourself a lot of work next year. Much to every gardener’s chagrin, thistle seeds have been known to remain viable in the soil for up to twenty years, so get those flowers when they first appear, if not the whole plant.
Keep thistle in check and exhaust its energy by cutting off the foliage whenever it appears.
Foliage collects energy via photosynthesis from the sun. The energy is used to produce more foliage, roots, flowers, and shoots. Interrupt this flow and the plant will eventually give up. By eventually, I mean the end of that year’s season, so keep cutting the foliage whenever you see it (you’re in this for the long haul, right?). The roots will continue to send up new shoots each time you hack off its foliage, so keep checking the surrounding area – thistle is a persistent plant, to be sure. Also, attempt to dig the plant out wherever you can – use hand tools that extend below the surface and have a serrated blade to cut through the roots. If possible, remove the tap root intact.
Shade is by far the best way to control thistle.
When thistle pops up in your shade garden, it has little energy to pop up again if you nip it in the bud. When it appears early in the season near your tomatoes, don’t fret too much. By mid-season, the tomatoes will shade it out, and the thistle will give up, assuming you’ve been knocking it back while young.
If you give thistle a wide-open, sunny expense of clay soil, all hell breaks loose – it will fill up that space almost overnight. So fill in every possible space in your garden with plants you love to shade out and out-compete the thistle seed and young shoots. If you have any open space, pop an annual or a ground cover in there, or use a nice vegetative mulch to fill it in and smother the thistle. If you don’t allow thistle the opportunity to start, you’ll rarely see it.
How to control a massive thistle weed problem, like if you’ve neglected an open space.
The best way to control thistle, in this case, is to mow it down very early in the season and then smother it. Use old blankets, carpeting, sheet metal, untreated lumber, etc., and then cover that material with 3 inches of mulch. Make sure you extend the material about a foot in each direction beyond where you see the thistle, as the plant will attempt to send new shoots up from the fringes of its root system. A heavy mulch alone may work, but you’ll probably need 6 inches or more to keep the thistle from finding its way out.
If you’re especially concerned with its return, after mowing, soak the thistle and the soil with a white vinegar solution before covering it up. The vinegar should knock out the remaining foliage, and once deprived of light by the material on top, it will expire. But only soak the area with vinegar when dry weather is forecast for at least 3 days. The vinegar solution takes some time to fully work, and wet weather will dilute the vinegar, even when covered. Leave the area covered for an entire year before planting anything in that space. I do not recommend that you use a vinegar solution to control thistle in an actively growing garden, as it will make the soil surrounding the thistle very acidic, which may kill delicate plants and even shrubs.
Read more on controlling thistle from Cornell University.
I have a massive thistle patch and mowing it over is difficult because of rocks. I started hand pulling it but learned that can make it worse. I’m against any type of poisonous herbicides. I’m overwhelmed. I did find they are covered in thistle gall fly larvae but I think they’re not going to kill the thistles. Between the thistles, Japanese stilt grass and barberry I’m at my wits end.
Cynthia: Yes, getting rid of thistle can be very frustrating. But if you’re diligent about it, you will succeed. But do not mow it down, as it will return in droves – hand pulling is far and away the best solution.
I struggled with it for years in Pennsylvania and I’m proud to say that for the most part it’s no longer a problem. The gall flies may help, but the best thing to do is to buy a tool like a cobra heed weeder (available on amazon) or a Weed Digger Fork so that you can get the thistle out by its root as much as possible – that’s the key. I’ve used both of these tools over the years and they do a great job getting deep into the soil to extract the root. Which one you use will depend on the type of terrain you’re working in – in narrow areas, like rocky areas or near other plants, you’ll want the digging fork. In more open areas you can use the cobra. I’ve learned to keep both handy as sometimes, you need both. As you’ve learned, just cutting the top of the plant off doesn’t work, as the remaining root simply produces another stem.
Keep in mind why the plant is there – thistle is what we call a “pioneer” plant – it grows where other plants can’t or won’t and is an indicator of poor soil, rocky soil, dense clay, heavy foot traffic, etc. When you weed out the thistle, also improve the soil with compost and eventually you’ll have plants you want and not plants you have to weed.
As far as the barberry and stilt grass, do the same – the struggle is worth it.
Holy cow! I’m blown away by everything I learned from this post. Thank you for taking the time to share.
Weed control has definitely been the most challenging part of gardening for me….