Gaining victory over Canada Thistles

There are many varieties of thistle found in Ohio. Many are native and are beneficial to wildlife. But some thistles are non-native and are a very invasive plant! Native varieties Field Thistle (Cirsium discolor) which is the most common native thistle in the state. Tall Thistle (Cirsium altissimum) which grows up to 10 feet along woodland edges and in open fields. Swamp Thistle (Cirsium muticum) which is usually found in wet, boggy, or fen habitats. Carolina Thistle (Cirsium carolinianum) which is a smaller plant found primarily in southern Ohio that blooms earlier in the spring. Non-Native varieties Canada Thistle (Cirsium arvense), which is the most common invasive thistle in Ohio. It spreads rapidly via aggressive underground rhizomes, forming large colonies. It is listed as a noxious weed by the Ohio Department of Agriculture. Bull Thistle (Cirsium vulgare) this large, very prickly biennial thistle features large, vase-shaped purple flowers. Musk Thistle (Carduus nutans), known for its large, heavy, bright purple flower heads that often droop. Plumeless Thistle (Carduus acanthoides), a short, highly branched thistle with small purple or red flowers. Blue globe thistle (Echinops bannaticus 'Blue Globe') is a non-native perennial thistle that is clump forming and can be a beautiful addition to the garden. This is a tap rooted plant that is difficult to divide or transplant. Blue globe thistle is not invasive. Thistle plants are part of the largest plant family, Asteraceae. Plants in the Asteraceae family include daisies, sunflowers, coneflowers, dandelions and many more. Thistle plants have prickly spines on the leaves, with some varieties having spines on their stems. The spines help protect the plant from being eaten by grazing animals and other herbivores. Thistle plants are beneficial for bees, butterflies and other insects that need nectar and pollen from the large purple flowers. Birds, like the American Goldfinch, benefit from the nutritious seeds in the fall. Hummingbirds enjoy the sweet nectar and will often seek out the soft, white, fuzz from dried thistle seed heads to line and insulate their tiny nests. Canada thistle, the most common invasive thistle in Ohio, is a native of southeastern Eurasia that immigrated to North America in the early 1700's, probably as a contaminant in crop seed. In Ohio, Canada thistle has been identified in 58 of the 88 counties in Ohio. Ohio Department of Agriculture has designated it as a prohibitive noxious weed, which requires property owners to prevent it from going to seed and spreading from their properties. Canada thistle can be distinguished from other spiny thistles by its perennial growth habit, shorter stature, and small lavender flower heads that arise singly or in groups of 2 to 5 at the ends of stems and axillary branches. One stem of flowers can produce over 1000 windborne seeds that can start new plants many miles away. The windborne seeds are an issue, but Canada thistle is more known for forming dense patches of plants (some people call a creeping carpet) from its aggressive and expansive rhizomes (underground stems) that can stretch out horizontally for three to four feet and reach depths of three feet. Since rhizomes are stem tissue, they can form new plants even from an inch piece that has been chopped off and left in the soil. With a root system like that of the Canada thistle, it makes it very difficult to eradicate. According to Penn State Extension, you have to exhaust the root system through repeated elimination of its photosynthesis capability. To do this you must cut the plant every two to three weeks during the growing season. Use sharp garden shears or a string trimmer close to the ground. The goal is to deplete the root reserves by not letting them photosynthesize, allowing regrowth and carbohydrate use, but not allowing root reserves to build up again. Another method is to smother the area with cardboard, landscape fabric and 5 to 6 inches of chipped wood mulch. This blocks the sunlight and weakens the plant. In the summer, solarization can work very well. Cover the area with clear plastic for four to six weeks during hot weather. This method kills the roots by heating the soil to 120 to 140 degrees F. Herbicidal control If you should choose to enhance the mechanical process, use a systemic herbicide with the active ingredient being glyphosate, clopyralid or aminopyralid which are often used for lawn settings. When using any chemical, read the product label to be sure the active chemical will target the plant, use the proper sprayer equipment and exact mixing directions. Additionally, wear protective clothes, gloves and eyewear. Do not spray on a windy day, it's best to spray in the morning before the heat of the day. The best time to apply herbicide is in the late summer to early fall when the plant is storing energy in the roots. Apply directly to actively growing foliage. Avoid overspray and drift of product by screening nearby plants with a sheet of cardboard. Another way to avoid overspray is to cut back the thistle plant and spray the lower remaining leaves and stem. Be sure that the product has dried before removing the screen from desirable plants. Persistence is the key to removing and eliminating Canada thistle. Total eradication takes time, maybe two, three, or more years with consistent effort. Being timely and persistent will rob the root system of its energy reserves. Before long you will see that the Canada thistle is no longer sprouting in your garden and you can celebrate your victory. (https://extension.psu.edu/removing-canada-thistle-from-the-home-garden Becky Sherrard is an Ohio State University Extension Hancock County Master Gardener Volunteer. Find us on Facebook at "Hancock County Master Gardener Volunteers."