Boosting Crabgrass Control
As spring approaches, the majority of turf managers budget pre-emergence herbicide applications as the first big expense of the year for control of summer annual weeds such as crabgrass and goosegrass. The use of pre-emergence herbicides has traditionally been the basis of turf chemical programs; however, applying these materials never guarantees complete weed control. Erratic levels of control are often attributed to seasonal variations or cultural mismanagement that reduces herbicide efficacy.
Split pre-emergence herbicide applications are more commonly implemented in the Northeastern United States because of weather fluctuations, more consistent crabgrass control and increased turfgrass safety. For example, instead of applying pendimethalin at 3 pounds active ingredient (a.i.) per acre in April, it may be more effective to apply 1.5 pounds a.i. per acre in late April with a reapplication of 1.5 pounds in June. This application regime is safer for turfgrass rooting but also takes into account potential seasonal weather fluctuations that may influence weed germination.
To be effective, pre-emergence herbicides must receive irrigation or rain to be concentrated in the surface layer of the soil. Adequate moisture is especially critical when the herbicides are sprayed rather than applied as granules to move the herbicide off the foliage. If these materials do not reach the soil, crabgrass has greater opportunities to emerge and infest turfgrass areas. For non-irrigated turfgrasses, April applications likely receive adequate rainfall to move the herbicide into the soil. The key to effective summer weed control in non-irrigated turf is the sequential application in June. Typically, there is less rainfall in June and soil temperatures are more conducive to crabgrass germination.
A common trend in lawn maintenance is clipping collection. Clippings left on the turf create unsightly clumps that reduce aesthetic qualities of a well-maintained lawn. However, turfgrass leaf tissue may contain a considerable amount of herbicide residue on sites that do not receive irrigation or sufficient rainfall. It is no surprise that these turfgrass areas, including lawns, roadsides and commercial landscapes, experience the greatest crabgrass problems during summer months. These problems may be potentially alleviated by returning clippings when irrigation water or rainfall is limited for pre-emergence herbicides.
Two experiments were conducted at the New Jersey Experimental Greenhouse Research Complex, New Brunswick, N.J. Greenhouse day/night temperatures were set at approximately 74°/60°F to simulate typical temperatures for pre-emergence herbicide applications. ‘Applaud’ perennial ryegrass was seeded at 8 pounds per 1,000 square feet in pots with a 16-inch-squared surface area and 4-inch depth. Soil medium consisted of 80 percent Canadian Sphagnum peat moss and a 20 percent mixture of perlite, vermiculite, dolomitic limestone and calcitic limestone. Ryegrass was irrigated to field capacity 3 days per week and mowed weekly with grass shears (Black and Decker, Towson, Md.). Turf was established approximately 3 weeks before herbicide treatments.
Herbicides applied included Betasan (bensulide, 4LF) at 0, 6, 12 and 24 pounds a.i. per acre, Dimension (dithiopyr, 1EC) at 0, 0.125, 0.25 and 0.5 pounds a.i. per acre, and Pendulum (pendimethalin, 3.3EC) at 0, 1, 2 and 4 pounds a.i. per acre. Herbicides were applied to separate pots with a greenhouse spray chamber delivering 40 gallons per acre with an 8002E nozzle. Half of the pots received irrigation immediately after treatments and were watered every other day for one week. The other half of the ryegrass pots was not irrigated.
Clippings were harvested after 7 days from all pots at an approximate 3-inch height of cut. Ryegrass clippings were placed in pots where crabgrass had been seeded at approximately 3 pounds per 1,000 square feet. Pots seeded with crabgrass had the same dimensions of pots used for ryegrass with the same soil medium. Clippings were uniformly dispersed at the soil surface and pots were irrigated to field capacity 5 days per week. Crabgrass germination was determined by counting plants per pot 10, 20 and 30 days after seeding.
Over the two experiments, we found pots receiving clippings from irrigated ryegrass had 32 to 36 percent greater crabgrass germination than pots with non-irrigated turf clippings (see photo, on page 20). Greater germination of the crabgrass likely occurred from less herbicide remaining on the ryegrass clippings as a result of routine irrigation. Since turf clippings have the potential to be an herbicide carrier, clippings should be returned whenever there is insufficient rainfall or lack of irrigation. This concept is especially applicable for less intensively managed turf areas such as industrial sites and home lawns.
Also exemplified was the importance of irrigation in reducing retention of liquid pre-emergence herbicide formulations from turfgrass leaf tissues. When clipping collection is required, sufficient irrigation following pre-emergence herbicide applications may be a critical practice that affects weed control. Routine irrigations allow pre-emergence herbicide to move to the soil, thereby increasing the potential for the compound to be effective against crabgrass emergence.
Differences among herbicide rates were not detected. However, pots receiving clippings treated with Betasan (bensulide) had greater crabgrass germination than Dimension (dithiopyr) and Pendulum (pendimethalin) treatments. Herbicide rate effects may have been mitigated from the sample size, approximately ⅓ of the leaf blade, harvested per pot.
Since pre-emergence herbicide applications are the basis of your chemical programs, cultural management strategies that influence herbicide efficacy have economical importance. For example, reduced effectiveness of pre-emergence herbicide applications could warrant increased use of post-emergence herbicides that would increase labor, herbicide costs and may also increase turf injury potential. On non-irrigated sites, turf managers may want to consider two options to reduce potential loss of pre-emergence herbicides applied in June. First, return clippings to the site until rainfall is adequate to wash the herbicide off the foliage, or consider the use of a granular herbicide for the June application.
Another perspective on turfgrass clipping return is fertility. Grass clippings contain a considerable amount of nitrogen, phosphorous, potassium and other essential plant nutrients. Returning or recycling clippings to the soil serves as a secondary source of fertility and is of particular importance to less intensively managed turf areas. Overall, practitioners managing home lawns and commercial turf sites should return clippings whenever possible, especially during periods of drought or when irrigation is limiting.
Patrick McCullough is a program associate and Steve Hart is an associate professor at Rutgers University (New Brunswick, N.J.).
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