The introduced thistles arrived as weed seeds in the United States accidentally either in shipments of crop seeds or in ballast in the holds of ships. They have become major problems in agricultural landscapes and 22 states have designated them as noxious weeds. If you live in a state where these thistles are considered noxious weeds, you may be obligated to eliminate them from your property. Thistles do attract pollinators and birds, but the spiny leaves and stems keep grazers away. Some thistles release chemicals into the soil that inhibit the growth of other plants.
Considered a sign of a neglected landscape, Canada thistle is an opportunist that can colonize field and road edges, wet and dry sites, pastures and your backyard. It is one of the few thistles that spreads through an extensive root system, sending up a dense growth of stems. Canada thistle grows throughout most of the United States excepting a few of the southern-most states. Despite its name, Canada thistle came from Europe.
There are several other species of problem thistles including the very large musk or nodding thistle (shown in picture) and the plumeless thistle. These thistles can grow up to eight feet tall and have large purple flower heads. It is important to learn to identify thistles because there are also native thistles, some of which are very rare.
Because of their tough roots thistles can be difficult to control. Canada thistle is a perennial plant. It can be controlled by repeated mowing or cutting to exhaust the root system, but this can take up to four years. Small patches can also be covered in black plastic during the summer to kill the roots with heat. Canada thistle can be controlled with herbicide most effectively by cutting the plant and spraying re-growth or by spraying rosettes in late summer to early fall. Most of the large thistles grow as annuals or biennials. Plants that are going to flower can be mowed during the growing season to cut off the flowering stems. They may need to be cut several times. Seedlings and rosettes can be sprayed with herbicide containing 2,4-D or a combination of 2,4-D and glyphosate in spring to kill them. Roundup® Weed & Grass Killer Ready-to-Use Plus is also effective; just be careful to protect desirable plants, because the product will kill them, too.
Thistles tend to take hold on disturbed soils. To keep thistles from establishing, minimize soil disturbance and cover bare soils with mulch. Thistle seedlings need sunlight to grow, so establishing a dense community of plants that you do want will help prevent thistles from establishing. Thistle seedlings have bristles or spines on their leaves, so as soon as you notice a seedling, pull it out before the roots are well established.