February 1999

Spring Weed Control: Pre-Emergence Herbicides

David M. Kopec, Extension Turfgrass Specialist

Late February or early March is the season to have your spring pre-emergence program executed. Pre-emergence control is the best and easiest was to control summer annual grassy and broadleaf weed emergence.

On golf courses, parks, sports turfs and commercial turf settings, the spring grassy weeds that germinate (summer annuals) include crabgrass (Digitaria spp.) and southwest cupgrass (Erichloa gracilis). We actually have more southwestern cupgrass than crabgrass in the low elevations of Arizona. During the seedling stages, southwestern cupgrass resembles large crabgrass but has bright (light color) green leaves and the leaves have no hair (unlike large crabgrass). Other problem grasses can include goosegrass (Elusine indica) and to some extent stinkgrass (Eragrostis cilianensis).

Goosegrass is more of a problem in shaded areas which are also compacted. It also seems to grab a foot hold on steeper tee and green banks, especially on south facing slopes. Stinkgrass grows on the edge of the fairway where run-off occurs and the muddier sides of cart paths. It does not tolerate frequent mowings as the other grassy weeds previously mentioned do.

Summer annual weeds include spurge and prostrate pigweed. Spurge will tolerate almost any mowing regime, while prostrate pigweed tolerates higher mowing heights in weaker turfs and can be a problem on roughs..

Most of the grassy weeds are easily controlled with dinitroanalin herbicides such as, balan, treflan, surflan, pendimethalin, prodiamine, etc. Other products include Dimension (dithiopyr) , which is a also another grassy weed control agent. Prodiamine (BARRICADE) will also control spurge quite effectively at the 1x label rate.

Make sure that the host grass (your turf) can tolerate the herbicide. Surflan is not good for perennial ryegrass, while Treflan is. Barricade is safe for most grasses, as is Balan and pendimethalin. Betasan (bensulide) is registered for use on creeping bentgrass and so is REGALSTAR, a special mixture of Ronstar and betasan, formulated at reduced rates. DIMENSION works very well for cupgrass/crabgrass and often provides season long control with one application. DIMENSION also provides post emergence control of cupgrass/crabgrass when the plants are first visible and are at the 2-3 leaf stage with no more than two tillers. RONSTAR works very well for goosegrass, which germinates about 3-4 weeks later than crapgrass/goosegrass.

As mentioned, BARRICADE works very well for spurge pre-emergence control as well. Pendimethalin also works, but from experience, timing seems to be more critical.

Note the product label for recommendations for the time interval between actual application and the must "water-in" period. Also, check and see if a split or repeat application option is recommended.

A pesky and fast growing summer weed is khakiweed or chaffweed. The scientific name is Alternanthera pungens. It's a low growing broadleaf with short semi-rounded leaves. The leaves are dark in color and very shiny. The mid-vein is easy to see. This beast germinates from seed and probably grows back from underground root stocks/pud pieces. Gallery (isoxaben) is a pre-emergence herbicide for broadleafs and it should work on khakiweed seeds. I do not know if other dinitroanalins or other "grassy" weed herbicides would be effective at label rates as a pre-emergence. Post emergence control will be discussed in an upcoming issue.

No recommendations are made or implied. Products are listed based on popular use and appear as examples only. Similar trade names may exist for similar products. Exclusion of similar products is not intended. Always read and follow label directions. 

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