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Jessica Walliser

3 steps to a thistle-free garden

Jessica Walliser
| Thursday, May 24, 2018, 8:55 p.m.
Canada thistles (Cirsium arvense) are pernicious perennial weeds. They spread via an extensive network of underground roots as well as from seed.
Jessica Walliser
Canada thistles (Cirsium arvense) are pernicious perennial weeds. They spread via an extensive network of underground roots as well as from seed.

Question: I have a question about thistle. We moved to a new place last August, and the previous owners had a huge garden patch that had corn. I didn't disturb that area until this January when I went out and tried to weed as much as I could. There is an area that is about 50-by-100-foot that is full of thistle. Oiy and double oiy! There is no way I can go through that section and hand weed and dig all those thistles out. What is a girl to do? I am an organic gardener so I don't want any harmful chemicals, especially since that area might have veggies some day. I can't seem to find any information on the internet that would apply to a large area. I'm talking 1000s of thistles. I'd torch it if I could!

Answer: Canada thistles (Cirsium arvense) are pernicious perennial weeds. They spread via an extensive network of underground roots as well as from seed. In spring, the prickly young leaves poke out of the soil and look relatively harmless, but the plants can grow quite tall and spread over an entire garden in short order.

One thing you should never do when dealing with Canada thistles, or any other weed that spreads by underground roots (such as bindweed, quack grass or knotweed, for example), is till the area. Tilling breaks the roots into smaller pieces, each of which will generate a new plant, spreading the weed and creating even more trouble.

I recommend a three-step approach to organic Canada thistle control in a large area like yours.

Step 1: Go into the area soon after the start of the growing season and weed whack or mow the existing thistles all the way down to the ground. Every time the plants start to grow back, mow or weed whack them again, making sure to sever the plant from its root completely. Continual top-kill like this is one effective way to get rid of thistles, but it takes several years. So, I'd partner this initial early season top-kill with steps 2 and 3.

Step 2: Immediately after mowing, cover the area with sheets of cardboard to smother the thistle. If you don't have enough cardboard available to cover the entire bed, call a few local flower shops and ask them to save you some of the long boxes cut flowers are shipped in. You can also often get boxes from the liquor or grocery store, though they're far smaller than the flower boxes, so you'll need more of them. As long as the boxes aren't covered in wax, they're fine to use for this purpose. Once the cardboard is in place, cover it with an organic mulch. I suggest using straw, leaves (shredded or whole), or untreated grass clippings. Three to five inches of mulch on top of the cardboard is ideal.

Step 3: Grow pumpkins in the area. Canada thistles are susceptible to shading, which means they don't do well with vegetation shading them, especially when they're just emerging from the ground. Push aside the mulch in small circular areas, spaced every 5 to 8 feet apart, and cut a very small hole through the cardboard. Plant two pumpkin seeds in each hole and water them in well. Choose a big, full-leaved pumpkin variety, like “Howden,” “Atlantic Giant,” “Champion” or “Early Giant.” The cardboard and organic mulch will help keep the soil moist so you won't have to water much once the pumpkins are established, but when you do water, apply the water directly through the hole you cut in the cardboard when planting the seeds.

The pumpkin vines will spread all over the bed, shading out any thistles that happen to poke through. By the start of next growing season the cardboard will have been broken down by soil microbes. Don't disturb the bed by tilling or turning, but do follow the same three step protocol for a second season. If you'd like, the second year you can substitute the cardboard with a layer of newspaper, 10 sheets thick, topped with straw or untreated grass clippings. And, you can do winter squash, cucumbers, gourds or other vining plants as a substitute for the pumpkins during the second season.

After the second year of this process, the Canada thistle should be fully smothered and out-competed, but watch for new plants creeping in from the outer edges of the bed.

This process will allow you to still grow something productive in the garden while getting rid of the thistle at the same time. Good luck!

Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio with Doug Oster. Send your gardening or landscaping questions to or The Good Earth, 622 Cabin Hill Drive, Greensburg, PA 15601.

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