Invasive Canada thistle can overtake garden

Q. I have these terrible weeds with “pinchers” on them in my flower beds. How do I get rid of them without killing the plants I want?

Marguerite from Youngstown

A. Ah – Canada thistle! It’s one of the worst weeds for gardeners and farmers alike.

Besides the “short bristly hairs” that go through your gloves like a thorn, the plant has many features that allow it to outcompete the plants you want to grow. When you pull one plant, another plant seems to start the next day just a few inches away from the old one.

The roots of Canada thistle can extend down 3-6 feet into the ground. Each plant can produce nearly 20 feet of rhizomes per year. These rhizomes are the underground stems that allow the plant to spread across a landscape.

When the plant blooms, each plant can produce up to 5,000 seeds that last up to 20 years in the soil. The fluffy seeds are spread quickly by wind and other means to every place we don’t want it. It is an invasive plant, but the law prohibiting its cultivation and requiring control is not well enforced, leading to outbreaks along road banks and other areas that allow seed to be produced, which blows into your lawn and landscape.

So what’s a gardener to do? Simple! Deplete the roots and rhizomes of energy!

But it’s never simple with this weed. Constant cutting and pulling of the plants every 7-10 days will starve the plant and cause death. To send up a new shoot, the plant must use carbohydrates and nutrients from the roots and rhizomes. As these plant reserves continue to be used up through eliminating above ground portions of the plant, you eventually win. But it can take 4-6 months to kill the plant.

Smothering it with a plastic ground cover can work. You would need to remove all desired plants (without getting any roots/rhizomes of the thistle) and cover the area in plastic. The key is eliminating shoots at the outer edge of the plastic.

The only alternative to this dire task is a chemical application. Pre-emergent applications are not effective. A systemic herbicide is best, as it will get to and kill the roots and rhizomes. Be sure to read and follow all label directions on any herbicide – traditional or organic.

For pictures and extensive details on controlling this invasive weed visit http://go.osu.edu/Canadathistle. For details on organic options for controlling weeds visit http://go.osu.edu/organicweedtoolbox.

Eric Barrett is OSU Extension educator for agriculture and natural resources in Mahoning County. Call the office hot line at 330-533-5538 to submit your questions. Regular clinic hours are Mondays and Wednesdays from 9 a.m.-noon.