Master Gardner: Brown lawns that aren't dead may still need pre-emergent herbicides
BY JOE WOLKING Ask a Master Gardener
Saturday, August 18, 2012
8/18/12 at 4:35 AM
Q. My lawn is brown from the lack of rain, is it dead? Should I still use a pre-emergent herbicide? K.F., Tulsa
A. Many of the lawns in our area are brown due to heat and drought stress. Bermuda and zoysia lawns enter dormancy and turn brown in these conditions if they're not watered. They will be green again when it rains and the weather cools.
Most fescue lawns, especially those in full sun, are brown-green even if watered. They may also survive and turn green when the weather cools. Fescue lawns that are not irrigated and are totally brown may very well be dead and will need reseeding. Reseeding fescue is best done from mid-September to mid-October; call Master Gardeners at 918-746-3701 for more information on reseeding.
Pre-emergent herbicides are used to prevent weed growth. They are used twice yearly: in early spring to prevent crabgrass and summer weeds and from mid-August to mid-September to prevent annual winter weeds.
Three effective pre-emergents popular among homeowners are dithiopyr (Dimension), pendimethalin (Halts) and prodiamine (Barricade). Many brands use these active ingredients, and they should be available in local garden centers.
Most of these pre-emergents are marketed for use in the spring, to prevent crabgrass, but also fight fall-germinating weeds such as henbit and Poa annua (annual bluegrass). Henbit is the bright purple weed commonly seen in many lawns and roadsides in the spring.
These herbicides don't prevent seeds from germinating, but they work by preventing new root growth in sprouting weeds. The downside to a pre-emergent is it may prevent new growth of the above-ground stems of Bermuda and zoysia grasses. In healthy grass this is insignificant, but it may hamper full recovery of heat- and drought-stressed Bermuda and zoysia after the temperature cools and water is available. OSU recommends caution in this situation, and most labels of these products state that they should not be applied to heat-stressed turf.
Licensed pesticide applicators with access to professional-grade chemicals have several other options that allow flexibility of pre-emergent usage in these situations.
A "plan B" for weed control in Bermuda lawns not treated with pre-emergent herbicides this summer is to use glyphosate (found in Roundup and other brands), on green weeds during winter dormancy, normally in January through early March. Glyphosate cannot be used on zoysia or fescue lawns at any time.
Do not apply pre-emergents to fescue lawns that will need reseeding in September and October. Any pre-emergent applied now will prevent fescue seeds from germinating.
If you have a garden-related question you would like the Master Gardeners to answer in a future column, call 918-746-3701.
Original Print Headline: Brown lawns can still get fall treatment
August gardening tips
During a drought and heat wave is a bad time to use post-emergent broadleaf weed killers such as those containing 2,4-D. Weeds need to be growing to be susceptible to these chemicals. Weeds will start to grow when it cools.
Now is a good time to submit a soil sample to the OSU Extension office for testing. Do this before reseeding fescue or creating a garden bed this fall. Call the Master Gardener office at 918-746-3701 for instructions.
When planting fall vegetable seeds, soak them overnight prior to planting. Once planted, mulch to conserve moisture and reduce ground temperature. Provide shade during initial establishment.
Henbit, commonly seen in many lawns and roadsides in the spring, is a winter weed that can be prevented using fall pre-emergent herbicide. BILL SEVIER/Courtesy