Knowledge To Grow – Managing Crabgrass

Crabgrass, a summer annual, appears in the lawn as a light green, weedy plant and can be eliminated with or without chemicals.  One of the best non-chemical lawn care practices to prevent the invasion of crabgrass or any other weed is to mow at the correct height.  Setting your mower to a height of 2 ½ “to 3” can have a big impact as it helps to create a dense, thick lawn.  Closely mowed grass leaves an open invitation for weed seeds to germinate as it “opens up” the lawn.  A thick stand of grass will shade the soil and therefore discourage crabgrass germination.

Adequate and timely fertilization can further reduce weed competition by increasing turf-grass vigor.  Open and weak turf-grass areas promote crabgrass infestations because of higher soil temperatures which promote germination and decrease competition from cool season grasses.  It is recommended to apply 2 to 4 pounds of nitrogen per 1000 ft. squared each year.  Apply 60-100% of the nitrogen in two applications in the fall, one in September and one in November after the last mowing. A summer application of nitrogen will just feed and strengthen the crabgrass you have.

Crabgrass seeds germinate when soil temperatures are approximately 60 degrees F for 3-5 days at the ¼” level.  You may like to refer to the Growing Degree Days chart by clicking here (insert your zip code of different than 46746).  It will help you determine the best time to apply herbicides.

The ideal growing conditions for crabgrass are light, frequent watering and areas of bare, warm soil.  When irrigating, do so deeply to wet the soil to the depth of the roots.  Do not water again until you notice drought stress.  Apply grass seed to bare areas.

Weed killers, also known as herbicides, are available to manage those annual weeds that may plague your lawn.  There are pre-emergence herbicides that prevent the emergence of annual weeds such as crabgrass.  This product should be applied before the crabgrass emerges from the soil.  For our area it is recommended that this type of herbicide be applied between late April and early May.  It just depends on the type of weather we are having.  Herbicides may differ so always read, understand and follow the label for suggestions on when to apply and the application rate of what you have chosen to use.  Keep in mind when applying that some pre-emergence herbicides will also damage emerging desirable grass seed.

There are post-emergence herbicides available and need to be applied when the crabgrass plants are very small.  Usually by the time you notice the crabgrass it is too mature for the post-emergence herbicide to work.  There is more difficulty in using these products than in using the pre-emergence ones.  It is extremely important to follow the label instructions.  Consider herbicidal control only if necessary.

As always, Happy Gardening!

More information about gardening and related subjects is available online. The Purdue University Cooperative Extension Service can be reached at 499-6334 in LaGrange Co., 636-2111 in Noble Co., 925-2562 in DeKalb Co. and 668-1000 in Steuben Co.

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