Applying non-selective herbicides to dormant lawns (1-11-10)

Each winter I get several calls requesting information on applying nonselective herbicides to dormant lawns.  Glyphosate (Roundup) herbicide applied at the labeled rate of up to 16 ounces per acre to dormant bermudagrass can be an effective method of controlling many winter broadleaf and grassy weeds. The efficacy of the herbicide is much greater when temperatures rise above 60 degrees, so it becomes a challenge to the applicator to target the application when the turf is dormant, but yet have temperatures warm enough for the herbicide to be most effective. 

The greatest challenge and fear of applying glyphosate or other non-selective herbicides to dormant turf here in Mississippi is determining if the turf is truly dormant or not.  With the winter fluctuating temperatures we often experience our turf may never go completely dormant.  The extended period of sub-freezing temperatures of this past week probably has put warm-season turf species lawns in as dormant condition as they will be this winter.  Research data indicates that common bermudagrass is slightly more tolerant to glyphosate than hybrid “Tifway” bermudagrass and at the labeled rates of 16 ounce per acre or less will not kill semi-dormant bermudagrass but may delay spring greenup.

Therefore, my suggestions to anyone planning to apply glyphosate or other non-selective herbicides to their lawn is to carefully examine the turf for stage of dormancy; read the label completely to be certain the herbicide can be applied to the turf species you have; accept the possibility of delay in spring greenup; and assess other weed control options as well.

Published January 11, 2010

Dr. Wayne Wells is an Extension Professor and Turfgrass Specialist. His mailing address is Department of Plant and Soil Sciences, Mail Stop 9555, Mississippi State, MS 39762. wwells@ext.msstate.edu