Farm Talk

Crops

November 12, 2013

Good time to control rosette weeds

Parsons, Kansas — Fall is one of the best times to control rosette-forming weeds and other winter annuals, says Sarah Kenyon, an agronomy specialist with University of Missouri Extension.

According to Kenyon, it is important to make sure spraying occurs on days where the high temperature is a minimum of 60 degrees Fahrenheit, there is no hard freezing at night, and plenty of sunshine.

“Weeds that form a rosette typically germinate in the fall or early spring,” said Kenyon. “Because of their low, spreading growth, these weeds can smother the grass and create open spaces in the pasture.”

Plants that form a rosette include: thistles, dandelion, Queen Anne's lace, chicory, spotted knapweed, red sorrel, poison hemlock, and others. In addition to decreasing forage quality, weeds like Queen Annes's lace, chicory, and dock can gum up fescue seed equipment and create more trash in the gleaned seed.

The most common thistles that occur in southern Missouri are muck, bull, and tall thistles. Control is best achieved when sprayed in the rosette stage, from October through April. Use products that contain 2,4-D, have dicamba as the active ingredient, Grazon P+D, Chaparral, GrazonNext HL, or Tordon 22K.

If mowing, mow within two days after the terminal flower head blooms, then repeat 3 to 4 weeks later.

For musk thistle, do not spray during flowering; the musk thistle flower head weevil will provide the best control at that time. Thistle weevils are a help in the control process and cannot be relied on for full control.

Poison hemlock is poisonous to both humans and livestock. It can also be mistaken for Queen Anne's lace. Poison hemlock has purple/red spots on the stem and has an odor. Use Grazon P+D products, Tordon 22K, or Remedy Ultra before the plants bolt in the spring. The best control of poison hemlock is achieved during rosette stage in fall or spring.

Spotted knapweed can also be controlled with fall spraying according to Kenyon. Use Milestone, Grazon P+D, Chaparral, or Tordon 22K when spotted knapweed is in the rosette or early bud stage. Control is more effective when the plant is less than a foot tall.

“Spotted knapweed has been rapidly taking over many pastures and hayfields in southern Missouri. This is because the roots excrete a toxin that kills neighboring plants,” said Kenyon.

Other weeds to consider for fall spraying are henbit and chickweed. These plants germinate in the fall, grow through the winter, and produce seed in the spring.

“They are most common in dormant warm-season grass pastures or in new seeding, but can also be problematic in established pastures,” said Kenyon.

Banvil or Clarity mixed with 2,4-D, or Grazon P+D can be used in grass pastures. Roundup or Gramoxone can be used on warm-season grasses after the crop is in winter dormancy.

“Make sure to read the herbicide label carefully before applying, because many herbicides can damage newly germinated grass seedlings. New seedlings need to have a sufficient root system before herbicides can be applied,” said Kenyon. £

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