Photo courtesy of Paige Burns Clark
                                Pictured here are crabgrass seedlings, which sprout up from seeds in early spring and can be prevented using preemergence herbicides, or PREs.

Photo courtesy of Paige Burns Clark

Pictured here are crabgrass seedlings, which sprout up from seeds in early spring and can be prevented using preemergence herbicides, or PREs.

Things may be miserable, cold, and rainy now, but believe it or not, spring is only a few weeks away. Now is the time to think about the warm season weed seeds that are lying dormant in your lawn, just waiting for warmer days to pop out and take over. If vanquishing weeds in your lawn is on your priority list, now is the time to take stock and plan your attack.

There are basically two kinds of herbicides: preemergence and postemergence herbicides. Preemergence herbicides (called PRE, for short) are useful for preventing weed seeds from growing, stopping them before they become a problem. PREs do not affect weeds, such as dandelions, which are already present in your lawn. PREs are useful for annual weeds such as crabgrass, which sprout up from seeds every spring. Timing of their application is critically important. The PRE must be applied before the weed seeds germinate.

For warm season weeds, the PRE needs to get out early, and that’s where things get tricky. Crabgrass germinates when soil temperatures warm up to around 53-55 degrees, at 2-4 inches in depth, for several consecutive days. Typically, PRE for crabgrass is prescribed to go out about the first week of March in our area, which is historically when soil reaches that temperature. However, for the past several years, weed scientists have noted that warmer winters are pushing up the germination period of crabgrass up by a week or so. They are now recommending that preemergent herbicides be applied two weeks earlier – in other words, about now.

One of the more delightful aspects of the weather these days is how unreliable it is. While we have had warm winters overall recently, this year January has felt cold. According to National Oceanic and Atmospheric Information (NOAA), North Carolina’s January temperatures have been about average this year, although rainfall has been higher than normal. Current data from the NC State University Turfgrass Field Lab, however, shows soil temperatures are slightly cooler than they were in January last year, by almost six degrees. Also, as you’ve probably noticed, it’s rained. A lot. Putting out a preemergence herbicide on a lawn saturated with water and more rain coming down is not effective.

So, what is a Lawn Warrior to do?

Have a plan to use a preemergence herbicide at the proper time, and watch the weather. Most preemergence herbicides are granulars, and require some rain or irrigation to activate. Clemson University helpfully advises to apply PREs when the average daily air temperature is between 65 and 70 degrees for four consecutive days, which should get the soil temperature to where it needs to be. If you’re nervous about missing the application window, NC State University weed specialists suggest that putting a preemergence herbicide out too early (ie., before the soil is warm enough), doesn’t cause too much of a problem. Soil microbes break down the herbicide, but they won’t be active if the soil is cool either, so you won’t lose much efficacy by putting it out too early. However, you will want to wait for things to dry out a bit before applying your PRE.

What to look for in a preemergence herbicide? Look for the active ingredients benefin, oryzalin, and pendimethalin, among others. Annual grasses and some broadleaf weeds can be suppressed with these materials. Read labels carefully, and apply according to label directions, knowing your specific turfgrass (bermudagrass, zoyiagrass, or centipedegrass) and your targeted weed(s). Application rates may vary depending on the type of turfgrass you have, because turfgrass can be damaged by herbicides in some cases. Preemergence herbicides typically have an eight to 10 week effectiveness period, so reapplying in 60 days may be a consideration if you had good control with the first application.

Cool season weeds, such as henbit and chickweed, will be maturing soon and flowering. They will not be controlled with either a pre- or postemergence herbicide, once they begin flowering. Consider making a plan to control them using a fall applied preemergence herbicide next year.

As any warrior does, have a plan B for a possible failure of a preemergence herbicide. Postemergence herbicides are used after the weed has germinated and is actively growing. Keep in mind that PREs (and postemergence herbicides) should only be used on established lawns.

Finally, as with any weed “problem”, ask yourself this: is this weed truly a problem? Chickweed, henbit, dandelion, and many other broadleaf weeds are important pollinator plants for honeybees and native bees, especially in the early spring when there is little else flowering. Consider leaving some weeds for the bees!

If you have questions about your lawn turfgrass, herbicides, or weed identification, contact NC Cooperative Extension, Richmond County Center, at 910-997-8255.

Paige Burns Clark is director of the Richmond County Cooperative Extension and its horticulture agent.