Sarah Denkler

Horticulture Educator

Salute to Agriculture

March 15, 2009

Weed Control

I was hoping to get this information out before you needed it but based on the bloom of the forsythia.  Because that bloom is upon us, this is the week to apply your weed control.

To control summer annual grass weeds and other broadleaf weeds you may want to consider and application of pre-emergent.  The first of the year’s application for control of weeds is in the form of a pre-emergent.  For our area this should normally be applied before the first part of April or before the forsythia blooms drop.  The forsythia bloom is an indicator of ground temperature, which is the same as the temperature necessary for weed seeds to germinate.  These applications are for the control of summer annual grasses and broadleaf weeds.

After applying pre-emergent it must be watered in to activate.  This will also insure that the sun will not break down the material and that it will penetrate to the soil where germination occurs.  Applying before a light rain is a great way to see this happens.  If there is no rain is in the forecast then water the application in with at least a half inch of water.

If you have St. Augustine, make sure that your chosen pre-emergent is appropriately labeled for use.  Most pre-emergent that is available for homeowners contains fertilizer.  It is better to look for a fertilizer free, pre-emergent and avoid the waste of the fertilizer that is included.  The best time to apply fertilizer is a few weeks later, after yards start to green up.  Once the grass turns green, apply a fertilizer containing nitrogen to help invigorate your lawn.  A healthy stand of turf is the best way to keep weeds at bay.

Remember not to use a pre-emergent if you are planning to seed your yard since it stops germination of turf seed as well as germination of grassy weeds.

The predominant summer annual grasses are crabgrass, yellow foxtail, barnyard grass and goosegrass, sometimes referred to as silver crabgrass or hard crabgrass.  Many broadleaf weeds will be controlled with the application of pre-emergent to control summer annual grasses.

The information below shows common herbicide labels for control of summer annual grasses and miscellaneous weeds.  This recommendation was taken from the MU Extension Publication G6750, “Home Lawn Weed Control” by Brad S. Fresenburg in the Division of Plant Sciences.  Recommendations are given by trade name then common name.

For control of Summer annuals like Crabgrass, Barnyard grass, Goosegrass, Fall panicum, and Foxtail apply one of the following pre-emergent herbicide – Balan (Benefin), Barricade (prodiamine), Dimension (dithiopyr), Pendulum or Pre-M or Weedgrass Control (pendimethalin), Ronstar (oxidiazon), Team (benefin + trifluralin) or Tupersan (siduron).  Annual bluegrass does not compete with dense, well-maintained lawns.   The following may also be used as a post-emergent herbicide - Acclaim Extra (fenoxaprop-ethyl), Dimension (dithiopyr) or Drive (quinclorac).  Pre-emergence herbicides should be watered in for best results. Post-emergence herbicides should be applied when annual grasses are in a 1 to 3 leaf stage for best control. Make sure the desirable grass species is not under stress and note any temperature restrictions on the label. 

Inevitably, some of the summer weeds will germinate so a Post-emergence application can be used to control these outlaws.   This can be done as a spot application if you have an area that is particularly hard to control.  The best time is in the morning when the dew is still on the leaves.  The dew can aid in absorption of the post-emergent.  Post-emergent herbicides can be a better option for perennial sedge control since these weeds generally occur in patches. This lends them to spot-treatment of a post-emergent product, thus utilizing less overall herbicide.

To control the broadleaf weeds (winter annuals) that show up in early spring, apply a pre-emergent in October.  You can also use roundup in January to control weeds in dormant bermuda or zoyzia lawns.

There can be another option for those who try to avoid chemical applications.  Organic control can be obtained by using corn gluten.  This product, when applied in late March, will prevent germination of seeds for a short time.  As it breaks down it will also provide an application of Nitrogen to already existing plants.  Although the window of weed control is short and restricted to seed germination, a healthy stand of turf will aid in choking out weeds and the two methods in combination can, over time, be an effective form of weed control.

Helpful Publication:  Fresenburg, Brad S.  “Home Lawn Weed Control.”  Publication G6750.  MU Extension Publication.  October 2006.

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Sarah Denkler, Horticulture Educator University of Missouri Extension P.O. Box 160, Kennett, MO 63857 573-888-4722   Denklers@missouri.edu