Crabgrass is out in full force in 2016! – Turfgrass Science at Purdue University

Crabgrass is out in full force in 2016!

Why is crabgrass so bad this year?
Several factors contribute to a bad year for crabgrass.

  • Crabgrass is a warm-season grass meaning that it thrives in hot conditions with optimum growth in temperatures ranging from 88 to 95 °F. However, our primary lawn species in Indiana are cool-season grasses and they prefer cooler conditions and have optimum growth in temperatures ranging from 68 to 77 °F. Therefore, our warmer than average summer resulted in an environment that encouraged crabgrass growth and decreased the plant competition from our existing cool-season grass lawns. NOTE: One minor exception to this is that tall fescue lawns – the cool-season turfgrass species with the best heat tolerance – had less crabgrass than Kentucky bluegrass and perennial ryegrass lawns.
  • The preemergence herbicides (crabgrass preventers) that are applied in spring to prevent the emergence of crabgrass seedlings work only as long as they remain in the soil. Generally, these products last in the soil about 60-120 days depending on a number of factors (rate applied, ingredient, turf cover, temperature, moisture, etc.). They breakdown in the soil overtime due to microbial activity.  When soil temperatures are higher, as they were this year, the preemergence herbicides breakdown more quickly. Additionally, the adequate rainfall that we had this year kept the soil moist and the microbial populations high, which also assisted in the herbicide’s breakdown. As such, a hot, humid summer results in the herbicide lasting a shorter length of time and the crabgrass “breaking through”. NOTE: When we have wet springs, some preemergence herbicides breakdown more quickly in the saturated soils due to anaerobic breakdown in saturated soils. It is a myth that preemergence herbicides leach through soils. They do not. Instead they are strongly bound (adsorbed) to the soil (at the surface where the crabgrass seed lays).
  • This was a good summer for turf diseases. Read the recent update from Dr. Rick Latin on disease in 2016. Crabgrass is susceptible to some diseases including leaf spot, but is generally not susceptible to the lawn diseases that cause our turf to decline in hot, humid summers. As such, lawns suffered and crabgrass did not.
  • Many homeowners mow too low and as a result, crabgrass increases. Lawns mown at 3.0 inches or higher have far fewer crabgrass problems than lawns mown short. Therefore, raise that mower so that you’ll have less crabgrass in the future. BONUS: Lawns mown at a higher height need mown less frequently, have deeper roots, are healthier, and need less watering.
  • Lastly, crabgrass plants get pretty big by the end of the summer. One plant can cover an area as big as a pie plate. This makes the lawn look poor even when a preemergence herbicide was applied. Keep in mind that there are thousands of crabgrass seeds in the soil (millions in some lawns). Even when a preemergence herbicide controls 95% of the crabgrass plants in your lawn, it may still not appear to have worked that well in a year like this.

What to do next?
The good news is that the end is near. Crabgrass will get zapped with the first hard frost this fall and you can begin now to battle against it and keep it from being problematic next year. This fall you’ll want to

  • Fertilize your lawn (at least once, twice is better). Read more here. Fertilizing the lawn in the fall months will help increase turf density and allow the turf to be much more competitive with crabgrass the following year.
  • Seed thin areas of your lawn as soon as possible. Read more here.
  • Adjust your mowing height so that your lawn is cut at 3.0 to 4.0 inches tall to help prevent future crabgrass problems.

Should I attempt to treat the crabgrass now in September?
NO! This is a summer annual plant, meaning that it will die naturally soon with the first frost. Additionally, large crabgrass plants are very difficult to control with herbicides. Two herbicide applications are needed to kill large, tillered-plants. As such, it makes no sense to spray expensive, marginally effective applications on a plant that is about to die naturally.

More information about crabgrass:

Aaron Patton
Follow @PurdueTurfDoc


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