The tremendous crabgrass pressure mentioned in earlier Turf Tips deserves more attention and the most common question is “Why is it so bad this year?”. The obvious answer to this is the above average heat and humidity during July increased disease pressure and thinned or weakened turf. Crabgrass germinated in these new gaps and quickly matured in the heat and thus we now have a bumper crop of crabgrass in almost all turf areas. Additionally, excess heat and saturated soils (in some cases) likely increased breakdown of dinitroaniline preemergence herbicides pendimethalin and prodiamine. However, the heat and drought of last August and September also comes into play. Many damaged and thinned turfs were overseeded with perennial ryegrass last fall and/or invaded by annual bluegrass during the moderate winter. Both of these species are extremely susceptible to summer heat and diseases, further adding to this summer’s thinning and current crabgrass pressure.
What to do now:
- • Selective herbicides are now ineffective because of the current size of crabgrass.
• Cool temperatures and frost will be most effective for controlling areas that have minimal crabgrass populations.
• Aggressive fertilization with a minimum of 1.0 lb N/1000 sq ft in September and 1.25 lbs N/1000 sq ft in November will improve recovery. Consider applying 1.0 lb N/1000 in September, October, and November to maximize recovery on very thin turf.
• Areas with 50% or more cover will likely need reseeding.
• Low mowing followed by power raking to destroy crabgrass cover and improve seed-soil contact may work best.
• An application of glyphosate or glyphosate+diquat 2-7 days prior to low mowing or power raking will also improve seeding success, but will kill any desired turf.
• Aerification in combination with the previous practices will also improve seed-soil contact, but aerifying alone in a significant stand of crabgrass will likely not allow adequate seed-soil contact.
• Avoid using perennial ryegrass at more than 10% in any seed mix, and preferably avoid using perennial ryegrass at all on lawns and golf courses. Unfortunately, perennial ryegrass is our only choice for overseeding athletic fields during the season.