Five thistles have prickly presences on Wisconsin farms.
If producers aren’t sure which is which, the first step is to determine if the thistle is a perennial or biennial, says Mark Renz, University of Wisconsin-Madison weed specialist.
Biennial thistles – those with two-year life cycles – tend to appear as individual plants. When dug, there’s only a taproot. Wisconsin’s biennial thistles are plumeless, bull and musk.
Northern Wisconsin also has the biennial European marsh thistle that’s spreading south. According to the Wisconsin Department of Natural Resources, marsh thistle prefers moist acidic soils and is somewhat shade-tolerant. It’s found along roadsides and forest edges, as well as in old fields and wetlands. Spontaneous hybrids between European marsh thistle and Canada thistle have been reported in Europe.
Biennial thistles only grow vegetatively the first year. In late May the second year, musk and plumeless thistle send up flower stalks. Bull thistle does the same a couple of weeks later. Because biennial thistles reproduce only by seed, control must be focused on preventing seed production.
“Mowing when plants just begin to flower prevents seed production,” Renz said.
Herbicide such as aminopyralid or Milestone is best applied to rosettes fall or spring.
Canada thistle is a competitive creeping perennial with an extensive horizontal and vertical root system. A single plant’s roots can spread 15 feet. Control is more challenging and timing is different, Renz said. Canada thistle should be mowed when plants have produced flower buds. Farmers may need to mow Canada thistle three to five times each year and do so for many years to eliminate populations.
Other techniques should be integrated with mowing. Grazing can be timed to suppress Canada thistles; graze when there are flower buds. Because livestock may avoid Canada thistle at that stage due to spiny leaves, rotational grazing with higher stocking densities can overcome the avoidance issue. If livestock eat most of the Canada-thistle shoots for two to three years, populations can be eliminated.
In wet years, a bacteria common on the landscape called Pseudomonas syringae tagetis turns Canada thistle tips white. But while it might suppress Canada thistle in wet years, rarely does the bacteria kill it. If farmers see symptoms they can spread the bacteria by mowing plants when foliage is wet. But Renz said additional management will be required to eradicate populations.
Herbicides can suppress Canada thistle in pastures. The key is applying at the correct stage of thistle development – when flower buds are present, but with few fully developed flowers. Applications before or after that growth stage are less effective.
Applications can also be made to Canada thistle rosettes in the fall. Plants should have first been aggressively mowed or grazed. Fall applications through October can provide equivalent control to applications at flower-bud stage. Aminopyralid or Milestone, or clopyralid or Stinger, appear effective. Spot-treating Canada thistle in pastures will enable more pasture legumes to persist.
Another option to avoid legume injury is weed-wiping glyphosate onto Canada thistles growing above the forage. Thistles should be at least a foot taller than desirable forage.
Gared Shaffer, South Dakota State University weeds field specialist, also does battle with thistles.
“In addition to knowing how each specie grows, (farmers) need to properly identify the problem thistle to know the best way to manage it,” he said.
He too suggests an integrated approach – chemical, mowing and grazing – to control thistles.
“Mob-grazing livestock ... has been shown to decrease some thistle species,” he said.
There’s interest in training livestock to eat undesirable plants like thistles. Calves, kids or lambs at their mothers’ sides can learn to eat thistles if their mothers eat them. They will remember later in life. Thus it’s best to train young females intended as replacements.
According to the Wisconsin Department of Natural Resources, Canada thistle is a regulated invasive specie with a “restricted” classification. European marsh thistle is also an invasive specie. It’s classified as “restricted” in Ashland, Bayfield, Chippewa, Clark, Door, Florence, Forest, Iron, Langlade, Lincoln, Marathon, Marinette, Menominee, Oconto, Oneida, Price, Rusk, Sawyer, Shawano, Taylor and Vilas counties in Wisconsin. It is “prohibited” elsewhere.