Crabgrass is running rampant this year as the hot summer and more than adequate rainfall has helped to push this beast of a weed along. Crabgrass is a warm-season annual grass meaning that it thrives under hot and moist conditions. The high rainfall during germination (late April and early May) coupled with the wet and warm conditions this summer have provided perfect growing conditions for this weed. As mentioned in a recent turf tip (Summer Stress: Part I: Too hot: Why some turfgrass species look poor in summer), cool-season grasses are not growing well right now and in some cases weather, disease, and insect pressure is providing more opportunity for crabgrass to invade lawns by allowing crabgrass to either germinate or fill-in thin areas of turf due to lack of competition from the desirable turfgrass.
There are many species of crabgrass, but the two most common species in lawns are smooth crabgrass (Digitaria ischaemum) and large or hairy crabgrass (Digitaria sanguinalis). Both are spreading summer annual grasses with a rolled vernation. Large crabgrass will have a hairy stem and leaves (A) and smooth crabgrass will lack the hairs except for a few hairs at the collar region near the membranous ligule (B). Otherwise, both are fairly similar in appearance. Both are opportunistic weeds and will germinate in bare soil areas (C) or thin turf (D). When plants begin to tiller they often form clumps (E) although they are stoloniferous and can spread.
Crabgrass is especially problematic when:
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Turf is mowed too low.
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Turf is underfertilized resulting in a weak, uncompetitive turf.
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Turf is thinned due to increased disease and insect pressure.
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Turf is thinned due to unfavorable growing conditions (hot, humid weather).
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A turf species is planted that is less competitive in the summer months (i.e. perennial ryegrass).
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Next to sidewalks (F) and other areas with thin turf or poor shallow soil.
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Winter annual weeds such as annual bluegrass die and provide space for crabgrass growth.
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Excessive rainfall and high temperatures cause more rapid degradation of preemergence herbicides.
What should you do now:
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Use proper cultural practices such as a high mowing height and proper fertilization (primarily fall and spring and little fertilization in the summer). Aggressive fertilization with a minimum of 1.0 lb N/1000 sq ft in September and 1.0 lbs N/1000 sq ft in November will improve recovery in thin areas. Consider applying 1.0 lb N/1000 in September, October, and November to maximize recovery on very thin turf. Usually a product with some slow and some quick-release nitrogen for the application in September. Applications in October and November can be quick-release nitrogen only.
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Post-emergence control of crabgrass is not as effective this time of year due to the size of the crabgrass plants. If you want to attempt post-emergent control use Tenacity (mesotrione), Acclaim Extra (fenoxaprop), MSMA, Drive (quinclorac), Q4 Plus (quinclorac + sulfentrazone + 2,4-D + dicamba), or Solitare (quinclorac + sulfentrazone).
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Crabgrass will die on its own soon since it is a summer annual. Crabgrass growth will slow in September during the cooler nights and then die with the first hard frost in October.
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Plan on using a preemergent herbicide in these areas in the future.
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Consider using a split or sequential application strategy in the future. Rather than applying only one application of a preemergence herbicide in the spring, apply a three-quarter or half-rate at the normal early spring application timing and apply a second application at a half-rate in late May or early June. This strategy has worked successfully for many.
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If areas are thinned by crabgrass seed Kentucky bluegrass or tall fescue into lawns rather than perennial ryegrass to reduce future crabgrass pressure in summer. Seeding between August 15 and September 15 is optimum seeding time in the northern half of Indiana, and September 1 to September 30 is optimum in the southern half of Indiana.