How to Get Rid of Thistle Before It Takes Over Your Garden

Some plants are just bad news. Thistle is one of them, with many invasive varieties and a nasty tendency to quickly take over your garden. Worse, this weed's prickly leaves are a nuisance for humans and pets, too.

So how can you keep this pesky plant out of your yard? Fortunately, there are a few tried-and-true methods that experts swear by—and that will keep your garden thistle-free.

How to Identify Thistle

You may feel thistle's prickly leaves before you see it. However, there are a few telltale ways to identify these plants, beginning from their early growth stages. "They start out as a rosette—a rounded clump of leaves low to the ground—in the first year," says Damon Abdi, assistant professor of landscape horticulture at Louisiana State University. "The following year, a flower stalk bolts from the rosette and eventually sets seed."

Thistle has a few defining traits other than its lobed, pointy foliage. Its flowers, for example, are round and fluffy and bloom in shades of purple, pink, or white. In particular, there are three different invasive species of thistle to keep an eye out for, all with different defining features.

• Bull Thistle: Look for a large taproot that extends deep into the soil. Bull thistle will also have more prominently lobed leaves and purple flowers.
• Musk Thistle: Musk thistle closely resembles bull thistle and also has a prominent taproot. Its leaves, however, will have a silver or white margin.
• Canada Thistle: Unlike other varieties, Canada thistle spreads via horizontal roots. It's generally not as tall as other thistle and has small, pink or purple flowers.
  

Negative Effects on Your Garden

This weed can spread very aggressively, so it's important to always keep it under control. "They're strong competitors for available water, nutrients, and sunlight," says Aaron Steil, horticulture specialist at the Iowa State University Extension. "Reduced air circulation created by tall weeds encourages the development and spread of foliage diseases."

It can be a difficult plant to remove once it starts to spread its seeds as well. "Thistle seeds are lightweight and carried by the wind, allowing just one plant to spread across a wide area," says Alexander Betz, landscape designer and founder of Plant by Number.

Steil also notes that a garden filled with weeds can have more insect problems, as thistle is an attractive alternative host for pests.

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How to Get Rid of Thistle

If thistle has started to take root in your garden, don't panic—there are a few ways to get rid of it. Here are just a few that gardening experts swear by.

Dig Up the Roots

One of the best ways to remove a majority of thistle varieties is by hand-pulling them. "Early is better ... try to remove it when it's still in rosette form," says Abdi.

Before you start, make sure to wear thick gloves, as handling thistle's prickly leaves can be unpleasant. Both Abdi and Betz recommend weeding after the soil is wet. If it hasn't rained in a while, you can water it yourself; this will make removing the roots slightly easier.

"Use a long weeding knife, fork, or spade to dig several inches deep inside the root," says Betz. "Loosen the soil around the plant and extract as much of the root as possible in one piece."

After, inspect the hole you've made, and remove any root fragments. Keep an eye on it in the following days to ensure there's no regrowth.

The only downside? This process can be labor-intensive and tedious. Working while the soil is wet will help make it slightly less taxing, though.

Mow It Down

Another option is to remove or cut the flower stalks as they emerge. Repeated mowing can exhaust the plant—especially while it's still young—causing it to die. Unfortunately, this doesn't always work, so mowing often isn't a permanent solution—though it is less exhausting than hand pulling.

Canada thistle, in particular, can be especially persistent, notes Steil. Consistent mowing can work in some cases, but you may have to use herbicides if this weed doesn't go away.

Smother It

Betz recommends this method for garden beds or borders.

"Cut the thistle to ground level, and cover the area with thick, overlapping cardboard or newspaper," he says. "Weigh it down with 4 to 5 inches of mulch and leave it in place for an entire growing season."

How to Keep It From Coming Back

Mulching is a great way to prevent thistle from taking root. Doing so will help keep the plant's seeds from germinating. "After the removal process, cover the bare soil with 3 to 4 inches of organic mulch," says Betz. "Replenish as it decomposes, and pull up any seedings [you see] promptly."

However, most thistle prevention comes down to simply having a healthy lawn and garden. "Maintaining desirable vegetation that's healthy and vigorous is one of the best ways to prevent thistles from getting established," says Steil. "Dense vegetation makes establishment of these weeds less likely."