Ag Notes: Control thistles by preventing seed production

April 27, 2004|JERRY LITTLE

Thistles are one of the most troublesome weed problems in central Kentucky pastures and hayfields.

Thistle plants can interfere with livestock grazing and limit the amount of available forage. The spring and early summer months are when thistles present a major problem for land owners and livestock producers who graze cattle or produce hay.

Musk thistle, also called nodding thistle, is the most common type of thistle plant found in Kentucky. It is considered a noxious weed because of its ability to reproduce rapidly and limit pasture production. Musk thistle only reproduces by seed. The major control effort is to prevent or limit new seed production.

Seed typically germinate in the fall, and young thistle plants form a rosette. The most active vegetative growth period of the plant is in the spring throughout the early summer months.


The leaf surface is waxy in appearance and contains spines along the leaf margins. Flowers emerge from the rosettes and are followed by bright purple to reddish flowers, which bloom in late May to early June. New seed develop within the flower heads and at maturity are easily carried by the wind and spread to other areas.

The most important step in a long-term control program for musk thistle is to prevent flowering, and the production and spread of new thistle seed.

Mechanical control consists of mowing, clipping pastures, or even hand-grubbing individual plants.

These control methods should be initiated before flowers start to open. Some regrowth and production of flowers can occur after mowing, but seed production will be notably less than if a timely mechanical control method had not been used.

Thistle plants mowed or removed by hand after flowers have bloomed contain enough energy reserves that these plants will still produce viable seed.

Broadleaf herbicides labeled for use can be applied in grass pastures and non-cropland areas for control of musk thistle rosettes. For herbicides to be effective, the timing of the application is critical. Best results can be obtained if herbicides are applied to thistle plants that are in the early rosette stage of growth and actively growing. The best times for herbicide application are in the early spring, or in the fall following new seed germination.

When thistles are in the rosette stage, they are more susceptible to herbicide applications.

Herbicides that can be used in pastures include 2,4-D, Banvel, Crossbow, Redeem R&P, 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 important.

When herbicides are applied after flowering, control will be less and inconsistent. When using herbicides for control, consult the waiting period on the product label for livestock grazing restrictions following a herbicide application.

Avoid spraying near crops such as tobacco, vegetables, or ornamental plantings. Avoid drift by not spraying on windy days or days with extremely high temperature and high humidity.

Jerry Little is Boyle County extension agent for agriculture and natural resources.

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