Organic apple growers Chris and Juli McGuire were concerned about proliferation of Canada thistle in their Lafayette County, Wisconsin apple orchard. After years of struggling with the weed they received a U.S. Department of Agriculture-Sustainable Agriculture Research and Education grant to test organic methods for controlling it. And they were able to reduce Canada thistle populations during the 2019 and 2020 growing seasons.
The McGuires raise two acres of apples as well as other fruits. They market them through a community-supported agriculture program and to local grocery stores. They raise dwarf apple trees because they’re relatively easy to handle, they said. The trees also bear fruit quickly, providing a rapid return on investment.
But the dwarf trees also have shallow roots and can’t withstand weed competition. The McGuires spread a thick layer of bark mulch under the trees. That keeps many weeds at bay, but the orchardists struggled to control Canada thistle.
“Canada thistle is a perennial weed that spreads from deep roots,” said Chris McGuire. “Once a patch becomes established in mulch, it proliferates. It’s impossible to dig out without uprooting nearby fruit trees. And as certified-organic growers, we don’t spray systemic herbicides to kill it.
“The key is to repeatedly kill thistle shoots every three weeks. They grow back and then you kill them again three weeks later. We found that any method of killing the shoots is effective.”
McGuire said that in the past, they would chop thistle patches every five weeks to eight weeks. But they found that’s not often enough. The three-week interval exhausts the plants and depletes their strength, gradually killing them, he said. By the end of a single growing season, just a few stragglers remained in the plots. By the end of the second year they were eradicated, he said.
Killing thistles repeatedly can be time-consuming and expensive. The McGuires tracked their time using different techniques. They found that cutting thistles close to the ground with a gas-powered string trimmer and slicing them with a diamond hoe were the most cost-effective techniques.
Hand-pulling the shoots was more time-consuming. Spraying an organically-approved herbicide was quick, but more expensive.
The McGuires have shared their results with other farmers through the internet and conference presentations. They’ve also shared their findings. Visit twoonionfarm.com/research for more information.