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Late spring is a good time to control thistles in pasture

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By David Harrison

Most farmers don’t think much about thistles until they see the plants begin to send up a stalk and then a seed head in late spring. However, now is a good time to inspect pastures for the rosette stage of the plant and take action if needed.

Thistles are one of the most troublesome weed problems in pastures and hayfields. Thistle plants can interfere with grazing, limit forage availability and become a major problem for hay production.

Musk thistle, or nodding thistle, is the most common type of thistle locally. The primary growth period is generally in the spring through the early summer. However, most seed germinate in the fall and form a rosette, which grows close to the ground, often growing unnoticed until spring. The leaf surface is waxy in appearance and contains spines along the leaf margins. Flower stalks develop in the spring followed by bright purple to reddish flowers, which bloom in late May to early June. The seed are easily carried by wind and spread to other areas, not a very neighborly thing to do.

The most important step in long-term control of musk thistle is to prevent flowering, and the production and spread of new seed. This can be accomplished by using various mechanical, biological or chemical control methods.

For mechanical control, mowing, clipping pastures or even hand-grubbing can be used. These control methods should be started before flowers begin to open. Some regrowth and production of flowers can occur after mowing, but seed production will be notably less than if a mechanical control method had not been used. Thistle plants mowed or removed by hand after flowers have bloomed contain enough energy reserves to still produce viable seed.

Some reduction in musk thistle occurs naturally through feeding of two insects, the thistle-head weevil and the thistle rosette weevil. While these may reduce the problem in some individual plants, they are not much help for the total problem.

Broadleaf herbicides labeled for use in pastures can be applied in grass pastures and non-cropland areas for control of thistle rosettes. However, for herbicides to be effective the timing of the application is critical. Best results can be obtained if herbicides are applied to plants that are in the early rosette stage of growth and actively growing.

Therefore, the best times for herbicide application is in the early spring (March and April) or fall (October and early November). These herbicides will also usually kill any clover in the field, and are therefore only labeled for grass pastures.

Herbicides which can be used in pastures include 2, 4-D, Banvel, Crossbow, Redeem and Weedmaster.

For spring herbicide applications apply when air temperatures are above 55 degrees for two to three days. Complete spray coverage of the plant is also important. When herbicides are applied after flower stalks elongate, control will be less effective.

When using herbicides for control, consult the waiting period on the product label for livestock grazing restrictions following herbicide application. Avoid spraying near crops such as tobacco, vegetables or ornamental plantings. Also, avoid spray drift by not spraying on windy days or days with extremely high temperature and high humidity.

The Kentucky Department of Agriculture has a thistle control demonstration program that sprays 10 acres of pasture for free (you supply tractor and driver). The date is April 22.

Call the Extension Service office if you have an interest in the program.

For more information on musk thistle, call or drop by the Extension Office and ask for publication AGR 20, Nodding Thistle and It’s Control in Grass Pastures.