Fall Control of Winter Annual Weeds

Kelly Jackson
Christian County Extension Office

Fall Control of Winter Annual Weeds

Here’s a
little known fact - the purple-blooming weeds like henbit and deadnettle that are so easily spotted in the spring are actually growing in your lawn right now. As gardeners, we typically associate spring with new growth and seed germination but for one class of weeds, called winter annuals, growth begins in October and November.

Winter annual weeds emerge from seeds in the fall and begin to develop. They will go dormant or semi-dormant during the coldest months of winter and flower
the next spring. After they flower, winter annuals set seed and then die as temperatures get warmer. Those seeds remain in the soil until the following fall. As previously mentioned, purple-blooming weeds like henbit and deadnettle are the most noticeable winter annuals. At first glance, these two weeds may be difficult to tell apart. Both have pale violet to purple flowers and square stems. Leaf arrangement is key to identification. Henbit has small round leaves with a scalloped edge held upright or horizontal to the stem; deadnettle has similar sized leaves held down (think Christmas tree), its stem also has a purplish cast. Common chickweed is another winter annual.  It is a low spreading plant with a creeping habit, its leaves are small and oval with pointed tips, and its blooms are small, white star-shaped flowers. Of course, this time of year, seedlings of all these plants are very small, less than the size of a quarter, and have not taken on their mature characteristics.

Controlling weeds in the immature seedling stage is more effective than waiting until next spring as mature plants have a greater opportunity to set seed before they are killed, perpetuating the problem. Herbicides effective against winter annuals include 2,4-D, MCPP, and dicamba.  A combination of these three chemicals found in Ortho Weed-B-Gon, Spectracide Lawn Weed Killer, and others is a homeowners best bet for both cool-season and warm-season lawns. Follow directions on the herbicide label for application rates and timing. These products can be used provided temperatures remain in the sixties and there is no rain in the forecast 24-hours after application. Because of cooler fall temperature, there is also a lower risk of drift from
these chemical which can potentially cause damage to non-target broadleaf plants (i.e., trees, shrubs, perennials). Recently seeded lawns should not be treated until the grass has received at least two mowings.

It is important to note that a healthy lawn does not require frequent herbicide treatments for winter annuals or other weeds. If a lawn is being maintained at the correct mowing height, correctly fertilized and limed (based on a soil test), properly irrigated, treated for insects and diseases when necessary and does not experience problems associated with drought or winter injury the dense, healthy growth of the turf itself will restrict weed growth. However, any one of these problems can weaken turf and make it more susceptible to weeds.  Failure to control winter annual weeds usually leads to an increase in summer weed problems. Finally, if your lawn is more than 50% weeds (i.e., common bermudagrass, crabgrass, broadleaf weeds), a complete renovation next spring or fall, rather than a single herbicide treatment, is required.
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