Eliminating Canada thistle

Q. What is this plant?

Bob from Lowellville

A. Bob sent a great photo of the most dreaded weed in the yard and garden. Of course, that is Canada thistle.

Every gardener has seen this weed at some point in time. You can pull it time and time again, only to see the new stems popping up in the general area that give rise to new plants. This is because of the extensive (up to 3 feet below the ground) root system. Each time you pull the weed, you create the opportunity for the underground stems to form new above ground shoots. With that, each shoot that comes out of the ground can produce up to 40 flower heads and 1,500 seeds in just one growing season.

The key to eliminating this weed from the garden is persistence. You’ll need to outlast the root system in order to be successful. There are systemic herbicides which will get to the root and eliminate this plant, but there will be some root pieces that survive and a second application will be necessary. If you use a herbicide, be sure to read the label and follow the directions. There are some organic herbicides that will work, but they burn the plant above ground and do not affect the root system. This is effective, but it will take many, many sprays before you deplete the plant’s reserves in the root system.

Cultivation can be troublesome, as you create more root pieces that can emerge as new shoots. Pulling can work. Just keep doing it until you deplete the root reserves. Some have been successful with covering the entire area with a tarp. This eliminates light for plant roots to make chlorophyll and thus energy to survive. If this is tried, all of the area must be covered. Many times, this is not an option.

You can read more about the plant and options at go.osu.edu/canadathistle.

Another common weed this time of year is the bull thistle. While not easily confused with Canada thistle, this biennial, invasive weed has a very large stem and brutal spines on the tips of its leaves. It stays low to the ground in the first year, then grows up to 3 feet in the second year to have a beautiful, purple bloom. Bull thistle has a tap root and is easily dug up as a method of removal – just watch out for those spines! Photos and details of this weed are at go.osu.edu/bullthistle.

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