NRCS: Thistles a concern

All spring I imagined what beautiful flowers would soar up and dazzle my eyes. I pensively waited as large upright stalks pushed up from withered refuse. In June, I realized that I had a wild bed of thorny roses and spiny thistles. In my head I’d imagined something much better. So I bought a new pair of leather gloves, shed a tear, and donated a little blood to the thistle patch.

There are many different types of thistles. Four exotic species are currently listed on the Minnesota primary noxious weed list: bull, Canada, musk, and plumeless. They all look fairly similar, but since they’re all invasive pests, distinguishing between them won’t be a primary concern of this article. Thistles are extremely spiny on their leaf margins and stems. The leaves from a basal rosette and are deeply serrated. Thistles grow upright, from 1 feet to 5 feet, and are crowned with pretty pinkish-purple flowers. Thistles bloom from June to September. Thistle seeds are attached to a plume of white hair making them air-born and easily dispersed. Canada thistles are perennial. Bull, plumeless, and musk thistles are biennial.

Thistles invade pastures, cropland, waste areas, roadsides, lawns, and gardens. They prefer full sun but will still colonize under tree canopy. Their spiny leaves protect them from grazing animals, so once they get in a pasture, they are prone to take over. Rotational grazing can prevent thistle establishment because overgrazing weakens desirable species, making the pasture more susceptible to thistle invasion.

Commonly mechanical and chemical weed control should be used together to eradicate thistles. Thistles can be controlled with broadleaf herbicides; however, the thistles won’t show any effects from the herbicide for two weeks. In that time, their flowers could go to seed and disperse. If thistles are close to flowering, trim off their flower heads and spray the living leaves. Mowing can be used to keep thistles from flowering, but I’ve also seen Canada thistles learn to bloom below the mower blade height.

Thistles can re-grow and spread from root fragments. They have deep taproots that break easily, so it is very difficult to pull or dig them all the way out. At first I thought I’d go after my own patch of thistles with a shovel. I dug them out of the ground and was happy with the results for about a month. Then they started to grow back. The patch was smaller, but persistence is crucial when digging thistles.

After digging/pulling thistles, burn them or toss them in the trash. Avoid putting thistle roots and flowers in the compost pile; otherwise, you might grow a thriving pile of living thistles.

Before spraying thistles, be sure to ask the primary gardener in the house. Spray drifts onto desired plants is a major concern. On top of that, some plants may look like thistles such as the globe thistle or the artichoke. The globe thistle is an ornamental thistle that might have been planted on purpose. Its leaves look like any other thistle. Artichokes would be planted in a straight, evenly-spaced line in the garden. And before sneaking into your neighbor’s lawn in the middle of the night to spray, ask their permission. Asking first, promotes friendlier neighbor relationships.

Financial assistance for thistle control is available to agricultural producers on non cropland acres through the Environmental Quality Incentives Program. Assistance to other landowners may be available through the Lake of the Woods Soil and Water Conservation District. For more information, contact the SWCD/NRCS office at 218-634-2757 ext. 3.

For more information on thistles visit: http://www.ag.ndsu.edu/pubs/plantsci/weeds/w799w.htm

Knick is a soil conservation technician with the Natural Resources Conservation Service, Baudette.