How To Kill Thistle Weeds

Never judge a book by its cover – or should I say a plant by its blooms. Thistles, like so many of their weedy little peers, have the power to overtake your lawn if left unchecked. It doesn’t take long for them to get a stranglehold on your garden, either. They may look pretty with their spiky purple blooms, but these suckers infiltrate your garden and have no intention of releasing their hold. Learn how to kill thistle weeds with our in-depth guide.

What Are Thistle Weeds?

Thistle is a fast-spreading, difficult-to-control perennial plant with prickly leaves, considered an unwanted weed by most gardeners. They have a lot of underground roots and are difficult to kill either naturally or with the use of chemicals. 

The Canada thistle is usually the most noxious. It has small flowers and long triangular-shaped leaves with pointed tips. It appears over a wide area of the U.S. and is closely related to Bull Thistle. Known for being difficult to kill, this thistle has only one aim – to totally take over your garden. 

How Does Thistle Spread?

Thistles are difficult to get rid of because they have a very invasive root system. They appear in spring, bud in late May or early June, and are in full flower by the end of June. By mid-July, these blooms have produced seeds, which are easily and widely dispersed by wind, wildlife, etc. New shoots emerge from the roots in September. When the cold weather sets in around November, this new growth replenishes stored reserves beneath the ground to empower the plant to take over even more of your garden next spring.

How Do You Kill Thistle Weeds Naturally?

Out of sheer frustration, most gardeners use chemical herbicides in the hope that if they can just whack this garden tyrant they can take back control of their garden. They don’t realize is that the use of these chemicals does not guarantee penetration of the very powerful root system. It often just kills the leaves and flowers above the surface. The other problem with chemicals, of course, is that they can damage other organisms, including surrounding plants, wildlife, soil, and your health. 

Organic control of thistle is more effective than chemical control, although it requires some diligence and considerable elbow grease. Thistles can be controlled with homemade herbicides, uprooting, and prevention. 

Homemade Solutions

A homemade herbicide consisting of vinegar and salt may be effective at killing unwanted thistle plants. The vinegar needs to contain at least 20 percent of acetic acid to be effective at killing the weeds. Filling a spray bottle with a mixture of this type of vinegar and three tablespoons of table salt makes an effective homemade thistle herbicide. Saturating the unwanted plants once a week with this homemade mixture helps control the problem. Be cautious about spraying other plants with this mixture, because it will kill them as well. 

How Do You Keep Thistles From Coming Back?

Be diligent. Pluck weeds out as you see them. Using a spot chemical weed control spray in the Fall or Spring will help prevent the thistles from re-emerging.

To fully rid your garden of Canada thistle, uprooting the plants may prove to be the best option. Because thistle has sharp prickles, make sure you’ve covered any exposed skin to prevent injury. It is also best to wear gloves when removing thistles. Their rhizomes are strong and resistant, so you should first loosen the root system by digging around the plant. Make sure every part of the root is removed fully, or a new thistle plant will develop.

Once thistle has been killed or removed from your garden, use a few homemade prevention methods to keep it from coming back. Plants won’t grow if they don’t have light, so block the problem areas with some homemade mulch. Mulch is a material used to cover soil. Many household items, including newspaper and cardboard, work well as mulch Garden waste such as pine needles and wood chips, are also good.

Keep Thistles In Check

Tap out their energy by cutting the foliage back whenever it appears. The shade is by far the best way to kill thistle. It’s not an impossible task if you’re willing to put in a little effort when thistles first appear You could be the best weed exterminator on the planet, and still never be completely free of the thistle weed. 

Save yourself a lot of hard work next year and get rid of thistle flowers as soon as they appear. Lop them off and throw them in the compost bin. If they are mature, a few weeks old and well-developed, then toss them in the trash can. Keeping plants from flowering and seeding will save you plenty of heartaches. Thistles have been known to remain viable in the soil for up to 20 years, so you must rid yourself of those flowers when they first appear, if not the whole thistle plant. 

Photosynthesis is a process where plants turn energy from the sun into food. Plants use this food to produce more leaves, flowers, and roots. If you can interrupt this flow, the plant will eventually die. It won’t happen overnight, though, so keep cutting the foliage back whenever you see it. Think short-term intensive labor for long-term gains. The roots will continue to send up new shoots every time you lop off the foliage, so keep checking the surrounding area. These are persistent plants, so you also need to persist. Try to dig the thistles out wherever you can. Use hand tools that extend below the surface and have a serrated blade to cut through the roots.

If you nip it in the bud when the Canada thistle rears its head in your shade garden, it will have little energy to pop up again. 

Know Your Enemy

If you want to turn what can be a back-breaking struggle into just another routine weeding task, take a little time to learn about this ‘pioneer’ plant. Just like the dandelion, give the thistle some bare earth, and they won’t waste time filling it. In the wild, it actually provides a great service by protecting the soil from erosion until larger plants can take over. It also feeds pollinators like butterflies in areas where little else may exist. Its deep taproots and extensive root systems, form air pockets, and water pathways are created in the hard soil, encouraging the soil ecosystem.

So don’t hate the thistle too much. It’s only doing its job. Instead, familiarise yourself with its habits. Thistles are one of the most stubborn weeds, so they can be tricky to kill. It will likely take you two or three growing seasons to completely get rid of them.