Purdue Extension Service

Vanderburgh County, Indiana

Winter Annual Weeds Cause Problems for Farmers and Gardeners

By Larry Caplan, Extension Horticulture Educator, Vanderburgh County, IN

For the Evansville Courier and Press, January 13, 2008

The unusually warm weather of last week has sped up the growth of several winter annual weeds.

Winter annual weeds are plants that normally germinate in the late fall or winter, grow throughout the winter, and then bloom and set seed in the early spring. They usually die before the weather gets warm. Some of the more common winter annual weeds for our area include:

•  Common chickweed: a low growing plant with white flowers that forms dense mats.
•  Purple deadnettle: a member of the mint family, it has opposite leaves on square stems that are purplish near the top of the plant; the leaves have short petioles; the flowers are purple, about 1/4 inch long, and are found at the top of the plant.
•  Henbit: a very similar plant to purple deadnettle; it has opposite leaves with no petioles; the stems are square and green; the flowers are pinkish/purple, and about 2/3 inch long.
•  Cressleaf groundsel: once known as Senecio but now in the genus Packera, this plant produces fields of yellow so dense, most people wonder if our farmers are planting mustard. However, mustard blooms only have 4 petals, while groundsel has 7 to 12 yellow petals growing around a central disk, like a daisy bloom.

While winter annual weeds don't directly compete with corn, soybean, or garden crops, they can cause a number of other problems, including:

•  The dense mat of chickweed and other winter annuals can slow soil drying and warming and interfere with planting and tillage in the spring.
•  Purple deadnettle, henbit, and several other winter annuals serve as alternate hosts for soybean cyst nematode, a microscopic worm that is considered one of the worst pests to soybean crops.
•  Cressleaf groundsel can be toxic to cattle and horses.
•  Winter annuals can harbor insects in the spring, which later infest corn or soybeans.

The best way to control these weeds is to apply herbicides to farm fields or gardens in the fall. In soybean fields, herbicides should be applied right after the combine leaves the field or as early in the fall as feasible. This helps prevent weeds from getting started, and disrupts the soybean cyst nematode life cycle. Glyphosate (Roundup) or a tank mix of glyphosate and 2,4-D are very effective and comparatively inexpensive.

Residual herbicides should be applied as pre-plant or post-emergent treatments. A spring burn-down (mid- to late-March in southern Indiana) will be the best strategy for reducing nematode populations.

Home gardeners can use glyphosate if their gardens are heavily infested. Be sure to avoid the extended season versions of Roundup, as this will prevent you from planting your garden. Both gardeners and farmers should read and follow label directions carefully before using any pesticide.

For more information on winter weed control, contact the Purdue Extension Service at (812) 435-5287.