The Landscape Alerts

Rank growth of winter annual weeds occurs at the same time that warm-season turfgrasses begin to green-up, or emerge from winter dormancy. These weeds compete with the turfgrass for soil moisture, nutrients, growing space, but most importantly for sunlight. Warm season turfgrasses, with the exception of St. Augustinegrass, are not highly shade tolerant. Thick mats of winter weeds will shade turfgrass at a time that root carbohydrates are being exhausted during green-up. Additionally, thick mats of winter weeds that shade and weaken turfgrasses may allow summer annuals such as crabgrass and goosegrass to establish in late spring.

Research has shown that the preemergence herbicides, such as Balan, TeamPro, Surflan, XL, Ronstar and many others, that are used for summer annual grass control do not significantly affect the spring green-up of labeled warm-season turfgrasses. Possible exceptions could be with Dimension (dithiopyr) and Pennant (metolachlor). There have been some significant delays in the green-up of hybrid bermudagrass with these herbicides applied during dormancy but turf usually outgrows injury by early summer. Injury may be worse with higher rates of herbicides or if conditions are unfavorable for bermudagrass growth in late spring.

Postemergence herbicides have greater potential to delay spring greenup of turfgrasses than preemergence herbicides. Most postemergence herbicides will slightly delay the early spring growth of warm-season turfgrasses which may range from a few days to a few weeks. Delays in spring green-up often occur for a longer period of time from high herbicide rates or when spray patterns overlap. Usually, the turfgrass will completely recover within two to six weeks with proper cultural management. With the exception of Image, most postemergence herbicides can safely be used during green-up. Some slight delay in green-up may be noted; however, this effect can be lessened by:

1. Using the lowest recommended rate
2. Insuring that the application equipment is properly calibrated
3. Using spot treatments
4. Following all recommended cultural practices, fertility, irrigation, etc. to promote rapid spring growth

The use of postemergence herbicides should be avoided during the spring green-up of turfgrasses that have been poorly managed, or that are experiencing winter injury problems. Properly maintained, healthy, vigorous turfgrasses are more tolerant to postemergence herbicides than turfgrasses that have not been properly maintained or are suffering from winter injury.

Some postemergence herbicides state on the label “Do not apply during spring green-up.” Obviously, these products should not be used at this time of year. However, the use of other postemergence herbicides is warranted if there is a severe weed infestation on properly maintained turfgrasses. The slight delay in green-up from the use of these herbicides more than compensates for the competitive effects that dense mats of weeds would exert on the turfgrass.

St. Augustine-College of Agricultural and Environmental Sciences
 

Bermuda -  Department of Crop Science, College of Agriculture & Life Sciences, North Carolina State University