I’ve become an unwilling expert on thistle. Living where I do in Pennsylvania, we’re besieged by the plant in spring, and when not controlled, causes problems for years. Fortunately for you, I have a lot of experience successfully ridding thistle from my own and other gardens, and 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 – specifically Canadian 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.
Canadian Thistle spreads by seeds and by creeping roots, 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.
Thistle appears in spring, shows its flower buds in late May or early June, and by the end of June is in full flower. Shortly after this, and definitely by mid-July, thistle flowers have produced seeds, easily and widely dispersed by wind, ignorant landscapers (don’t get me started…), and wildlife. 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.
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’s roots, 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 affect 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.
How to control thistle organically
You must 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 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 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 from the sun, which is then transported to the roots to produce more foliage and new 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 – it’s 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.
Shade is by far the best way to control thistle. When it 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. But give thistle a wide-open, sunny expense of clay soil, and all hell breaks loose, as it will fill up that space almost overnight. So the idea is to 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 it the opportunity to start, you’ll rarely see it.
But let’s say you have a massive thistle problem, like one may experience when moving into a new property, or if one has 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 paranoid of 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.
Thistle is what’s called a “pioneer” plant, like dandelion. Give them bare earth, and they will fill it in promptly. This “pioneering” actually provides a great service in the wild, by protecting the soil from erosion until larger plants can take over, and by feeding pollinators like butterflies where little else may exist. And by virtue of their 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.