Warm-season weeds

Four of the most troublesome weeds in warm-season turf are (from top) blue violet, goosegrass, common bermudagrass and common lespedeza. Photos by Shawn Askew

Teresa Carson

In early July, turfgrass researchers from some of the warmer states in the U.S. gathered in Newport, R.I., to escape the heat and to talk about weeds: weeds in warm-season grasses, problem weeds, emerging weeds.

Five members of the group, representing Alabama, Arkansas, Georgia, Hawaii and Virginia, presented their lists of troublesome weeds. Each list betrayed some regional differences, but two plants, common bermudagrass and goosegrass, were in everyone’s top 10. Four more weeds appeared on four of the five lists: annual bluegrass, crabgrass, dallisgrass and common lespedeza. Rounding out the combined top-10 listing are four plants that were on three of the five original lists: prostrate spurge, sedges, Virginia buttonweed and wild violet.

But what makes these weeds (and others) troublesome or problem weeds? Scientists at the meeting cited several issues: weed resistance to herbicides; large weed seedbanks in the soil; cultural practices; contamination of seed, sod or soil; and a lack of broad-spectrum products for weed control.

Weeds can develop resistance to herbicides, and cases of weed resistance are becoming more common (www.wssa.net/Weeds/Resistance/index.htm). Numerous reports of resistance have been documented in the U.S. and throughout the world for both annual bluegrass (ethofumesate, prodimaine, pendimethalin, simazine, atrazine) and goosegrass (pendimethalin, tifluralin).

Cultural practices like low mowing may favor a weed over the desired turfgrass, and failing to aerify promotes weeds like annual bluegrass, which thrives in compacted soil. In addition, poor water management (for example, a broken water head) can lead to wetter areas that are hospitable to a weed but not to the turf.

Invasive or especially aggressive weeds can hitch a ride in nursery ornamentals and make their way into the turf. Weed seeds can also be present in turfgrass seed (tiny annual bluegrass seeds are a frequent offender), and the weeds themselves can be found in sod.

Researchers at the gathering indicated that the lack of broad-spectrum herbicides makes weed control more difficult. The companies that develop herbicides for turfgrass are actively seeking new products in the form of combinations of existing herbicides as well as new molecules. Recent introductions include Velocity, Certainty, LockUp and Sapphire. Some of the major players in research and development have indicated that they expect to have new products available in the next two years or so.

For more information about weeds in warm-season turf, see Turf Weeds at Virginia Tech (www.ppws.vt.edu/~saskew/turfweeds/), “Weeds in Southern Turfgrasses” by Tim R. Murphy et al. (available from www.caes.uga.edu/publications/for_sale.html), University of California pest management guidelines (www.ipm.ucdavis.edu/PMG/r785700999.html) and “Color Atlas of Turfgrass Weeds” by L.B. McCarty et al. (www.wiley.com/WileyCDA/WileyTitle/productCd-0470189517.html).

Teresa Carson is GCM’s science editor.