August 2010

Indiana lawns have a bumper crop of crabgrass

Many lawns have crabgrass this year because of the rain, hot weather and other factors. (C) Jo Ellen Meyers Sharp

Many lawns have crabgrass this year because of the rain, hot weather and other factors. (C) Jo Ellen Meyers Sharp

Crabgrass is sprouting all over the lawn. And, it’s probably worse this year than in years past, even though you applied a pre-emergent herbicide this spring to keep the weed from sprouting.

There are several factors that caused this bumper crop, not the least of which is the weather. It did nothing but rain in central Indiana during crabgrass’ key germination period in late April and early May. After that, the temps turned hot and stayed there, nourishing tiny seedlings into full-grown weeds.

The weather also contributed to several grass fungus diseases, such as pythium blight and red thread. Like most weeds, crabgrass (Digitaria) is opportunistic and will take advantage of thin, weakened spots in the lawn.

Timing of herbicide applications is also a contributor. Many consumers apply lawn fertilizers with pre-emergent herbicides in March, which is too early. Most pre-emergent herbicides are effective in keeping crabgrass seeds from germinating for about eight weeks. Waiting until April and May to apply the pre-emergent extends protection well into summer.

It’s too late to apply a post-emergent herbicide on crabgrass now, because the weeds are so large and the weed killer is much less effective. Hand pull or hoe out crabgrass from garden beds.

Fortunately, crabgrass is an annual weed, which will be killed by cold temperatures. Draw a sketch of the landscape and indicate which areas were infested so you’ll know where to apply the pre-emergent herbicides next spring.

Other tips:

  • Mow grass to 3 ½ high.
  • Apply two-thirds of the annual lawn fertilizer in fall, preferably in September and November.
  • Irrigate the lawn during extremely hot periods with no rain to keep the grass from going dormant. A dormant lawn is an invitation for weeds to move in.

The Hoosier Gardener talks about crabgrass and offers tips for controlling it Aug. 25 on Indianapolis’ Fox59 Morning News.

For more information, download Purdue University’s Control of Crabgrass in Home Lawns.