Year-round Yard: Applying post-emergence herbicides helps control winter weeds

Published 12:00 am Saturday, February 3, 2007

By Darrell Blackwelder

Special to the Salisbury Post

Cold weather earlier this week has slowed down weed growth in our lawns. However, they will continue to grow when it gets warmer.

The unseasonably warm weather experienced in early January has many cool-season annual weeds growing like it’s spring. After reviewing archived garden columns written over the past four years, it seems our area has a history of unseasonably warm weather followed by brief periods unseasonably cold weather.

Winter annuals such as chickweed, henbit and annual bluegrass seem to pop out when they are least expected. Controlling winter annuals with pre-emergence herbicides before they germinate is tricky.

Unlike summer annuals such as crabgrass, which germinates when the soil warms in the spring, winter weeds germinate when soil temperatures drop in September. Therefore, applications of pre-emergence herbicides to prevent germination of these weeds must be applied in mid-September. Most don’t apply pre-emergence herbicides to lawns with good reason. Pre-emergence herbicides, as a rule, are not applied when lawns are over-seeded. Most lawns are over-seeded in the fall. eliminating the chance to reduce winter weed germination.

Commercial lawn care companies and home owners are now scrambling to apply post-emergence herbicides to control winter weeds that seem to be out of control. It is important to control the weeds now because weeds such as chickweed and henbit are almost impossible to control in bloom. Some of the weeds are approaching this stage, so it is important to apply post-emergence herbicides soon.

Cold weather, below 40 degrees, impedes the effectiveness of post-emergence herbicides such as 2, 4-D and others. Watch the weather forecast and apply when day temperatures are approaching 50 degrees or more for a few days.

It’s best to apply blended herbicides since many weeds are difficult to kill with one particular herbicide. Weed specialists have learned that blended herbicides, 2,4-D, banvel and MCPP in one solution, provide much better control of both winter and summer broadleaf weeds. These are sold as various trade names and are readily available at local garden centers. These herbicides can be purchased as pre-mixed, concentrates or hose-on type applicators. I prefer the hose-on type applicator because of the ease of use and container disposal after use.

Winter weed populations can be reduced with a healthy lawn. Healthy, vigorous lawns shade the soil, preventing seed germination in the fall. This is another issue that takes a bit of skill, but is not impossible. Those with healthy lawns that don’t want to use herbicides often overlook a few winter weeds in the spring. When June arrives, winter weeds are gone.

Darrell Blackwelder is an agricultural agent in charge of horticulture with the North Carolina Cooperative Extension Service in Rowan County. For archived garden columns or other information, visit the Rowan County Master Gardener Web site at, e-mail