Fall Pre-Emergent Weed Control

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Remember the chickweed and henbit that overtook your gardens this spring and made a mess in your yard? Theses winter annual weeds are lurking in your yard, just waiting to grow this fall and surge out of the ground about the last week of February and start flowering the first of March.

One of the fall gardening chores that must be undertaken soon is the application of a fall pre-emergent herbicide to prevent winter weeds in home lawns. Applications must be made prior to weed emergence or poor control will result. Recommended dates for fall pre-emergence herbicide applications to prevent annual winter weeds are the third week of August here in Central North Carolina.

Winter annual weeds begin to emerge from seed when warm season turfgrass begins to enter dormancy in the fall. Typical winter annual weeds in this area include common chickweed, annual bluegrass or poa annua, wild garlic and onion, several thistles, wild mustard, Shepherd’s Purse, Carolina Geranium, henbit, black medic and many more that plague North Carolina gardeners during the cooler months. These weeds emerge in the fall when temperatures begin to cool from seeds that have been dormant since late spring. Annual winter weeds grow slowly throughout the winter and begin rapid growth when temperatures rise in the late winter.

Winter weeds are unsightly and heavy infestations can be extremely damaging to warm-season turfgrasses during spring green-up. Weeds compete for sunlight, soil moisture and plant nutrients during this time period. When winter weeds die, the warm-season turfgrasses can be severely stunted or have large bare spots that can easily be infested by summer annual weeds such as crabgrass.

Now that we know who they are, how do we prevent them from making a mess in our lawns? Preventing the weed seeds from germinating is the key. Part of the problem is that all seeds don’t germinate at the same time. That means that we have to get our pre-emergent herbicide into the ground in a timely basis. September 15th is an easy date to remember and offers the best timing for a fall application. It also means that a second application around December 1st may be necessary to catch the late germinators. Contrary to popular belief, pre-emergent weed killers don’t destroy weeds and their seeds. They simply stop them from growing. Some seeds are known to last fifty years, so if the herbicide isn’t applied each year, the weed will grow.

Henbit is a winter annual broadleaf weed that is difficult to control. In cool season turf, fall applications of two, three and four way broadleaf herbicides will provide fair to good control, with control dropping off if spring applied. In warm season turf, excellent control is achieved with various sulfonylurea herbicides applied in fall or early spring.

There are many pre-emergent products available. Some of the more common are benefin (Balan, Crabgrass Preventer, etc.), pendimethalin (Halts, Lesco Pre-M Plus), dithiopyr (Dimension, StaGreen Crab Ex, Crabgrass Preventer, Vigoro Crabgrass Preventer and prodiamine (Barricade). Read the label carefully before applying since manufacturers often vary in their application requirements.

In lawns, I really like using Dimension or Barricade as a persistent pre- emergent for most broadleaves and annual grasses. I would tank mix it with dicamba if you are after henbit, which could be already emerged in lawns that were watered last fall. If you want to go a cheaper route, you can use pendimethalin. It won’t last as long and you may end up using it twice.

The trimec combinations of 2,4-D, (MCPA or MCPP) and dicamba work well over the top. Just realize that it is the dicamba part that gets the henbit. The other two ingredients will get other weeds. Most of these come in fancy names like Ortho Weed- B-Gone or Scotts Lawn Weed Control.

In cooler weather, I prefer liquids as opposed to granular products, although both can be effective. Sometimes it is hard to find Barricade without fertilizer in combination but it can be done. The active ingredient for Barricade is prodiamine. It seems that Dimension (dithiopyris) is marketed more to commercial applicators. Another thing I like about the Barricade and Dimension, in addition to their longevity is that they can be used on centipede grass and other warm season grasses. Many contact herbicides are a no-no on warm season grasses that are greening up and centipede is especially tender.

If you miss the window for applying a fall pre-emergent herbicide you can use post-emergence herbicides (chemicals that are applied to the plant itself after germination from seed) on an as needed basis.

Here are the basics to a successful pre-emergent program:

–Timing. Most pre-emergence herbicides will not control weeds that have germinated prior to application. Therefore, try to apply these herbicides several weeks before germination. If they are applied too soon before germination, the herbicide may lose its effectiveness. For the fall a good rule of thumb is to apply a pre-emergent the third week of August to mid-September. Specific dates for applications of pre-emergence herbicides are difficult to give due to varying environmental conditions for each location and year.

–Turf preparation. To ensure the pre-emergence herbicide of getting into the soil where weed seed is located, remove excessive layers (thicker than 0.5 inch) of thatch, and also remove debris such as leaves and grass clipping before you apply the herbicide.

–Amount. Always read the label and apply the recommended amount on your lawn. Check the label to see that the herbicide is safe for use on your lawn.

–Coverage. Achieve a complete, uniform coverage by dividing the recommended amount of granular herbicide into two equal portions and spreading each in opposite directions. For adequate coverage, make spray applications at approximately three quarts per 1,000 feet.

–Activation. Water the pre-emergence herbicide area if 0.5 inch of rain does not occur within 24 to 48 hours following application. All pre-emergence herbicides are soil applied and must be “washed” into the soil where weed seeds are located.

–Second application. A second application may be required for season-long control. This will depend on the particular herbicide and environmental conditions, but pre-emergence herbicides generally remain effective for 60 to 110 days.

Minda Daughtry is Horticulture Agent for North Carolina Cooperative Extension in Lee County.