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Issue No. 6, Article 6/April 30, 2004

Thistle Identification and Management

Thistles can be a pest and difficult to remove, whether they are found in your yard, garden, pasture, or field. Before determining the proper control measure for your thistle population, you must identify the species of thistle(s) you have. Table 5 lists some thistles common to Illinois, their life cycles, and some key characteristics for identification.

The biennial thistles, such as musk, bull, tall, and plumeless thistle, germinate in the summer and fall and then overwinter as rosettes. Early the next spring, the plants resume growth and bolt, producing numerous flower heads. Occasionally, biennial thistles will act as annuals or short-lived perennials. Biennial thistles reproduce by seed, so controlling these thistles before seed set is essential to successful management. Canada thistle, a perennial, reproduces by seed and rhizomes, thus making it more difficult to control than the biennial thistles.

Musk thistle.

Bull thistle.

Canada thistle.

Management of thistles can be achieved through a combination of tactics, such as mechanical, chemical, and biological control.

Mowing can be effective if combined with herbicide treatments in pastures. However, mowing alone is generally not effective unless conducted at 1-month intervals over several growing seasons, which can be time-consuming. The reasoning behind the multiple mowing is that basal and root buds on some thistles will break dormancy once the main flower stalk is removed, causing more thistles to appear. Biennial thistles need to be cut as close to the ground as possible to prevent regrowth. If biennial thistles are in the yard or garden, it may be best to dig them out or till them up.

Herbicides should be applied in the fall and early spring for optimum control of thistles. Biennial thistles should be controlled while in the rosette stage. Once the flower stalk has begun to elongate, the thistle is much more difficult to kill and is tolerant to most herbicide applications. The most effective herbicides for perennial thistle control are systemic and will have the most activity when applied at the bud to early-flower stage. Broadleaf herbicides for the lawn containing 2,4-D and MCPP can control thistles. Options for chemical control in pastures are included in Table 6.

Some things to keep in mind when controlling thistles in pastures: (1) control small patches before they spread, (2) control thistles in fence lines and alleyways, (3) establish a long-term management plan using different methods of control, (4) do not overgraze, and (5) reseed disturbed areas with desired plants. It is important to avoid spreading thistle seed by quarantining animals before moving them from an infested area to a noninfested one and by cleaning equipment such as mowers to prevent transportation of seed.

Thistles, especially Canada thistle, can be quite a problem during wheat harvest. Table 7 lists some herbicides that will control or suppress Canada thistle in wheat, allowing for an easier harvest. Stinger is the best choice for thistle control, but it is the most expensive. Harmony Extra and Express can be tank-mixed with 2,4-D for increased effectiveness. Consult product labels for additional information pertaining to rates, additives, and crop development stage before making an application.

Table 8 lists herbicide options for control of thistles in corn and soybean. Many herbicides will control only aboveground parts and suppress growth. Sequential applications are necessary for more complete thistle control.

There are biological control options for certain thistle species, such as the musk thistle seed head weevil and the Canada thistle flower head weevil. These insects are can be used to reduce small thistle populations; however, as with most biological control agents, it takes time for the agents to become established. The effectiveness of these agents can be compromised if chemical control is used after the plants have bloomed. Spraying thistles after bloom interferes with natural cycles; enough thistles have to survive to support the insect population. Later in the season, it may be worthwhile to examine the underside of the flower head for weevils and then continue to monitor effectiveness. More information on the use of biological control agents for thistles can be found at www.bio-control.com.

Rosette weevil.

By integrating some of the aforementioned management methods, along with diligence and perseverance, you can control your thistle populations.--Dawn Nordby and Aaron Hager

Aaron Hager
Dawn Refsell

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