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Knowledge to Grow by Karen Weiland - Crabgrass

Crabgrass, a summer annual, appears in the lawn as a light green 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 2½-3 inches can have a big impact as it helps to create a dense, thick lawn. It has been estimated that using the proper mowing practice can eliminate about 80 percent of weedy species in the 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 2-4 pounds of nitrogen per 1,000 ft. squared each year. Apply 60-100 percent 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.

Weed killers, also known as herbicides, are available to manage those annual weeds that may plague our lawns. There are pre-emergence herbicides that prevent the emergence of annual weeds such as crabgrass. This product needs be applied before germination takes place. Application in a timely manner allows the herbicide to form a barrier before the crabgrass seedlings try to emerge. These products do not eliminate already existing plants. I would suggest you refer to the Growing Degree Days chart at (insert your zip code if different than 46746). This chart will help you determine the best time to apply herbicides. This year the week of March 21 is in the optimum time frame to be applying a pre-emergence herbicide for crabgrass. 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. Do not use pre-emergence herbicides on newly seeded areas or areas to be seeded.

Quite often pre-emergence herbicides are combined with fertilizers as a weed and feed product. It is generally recommended to purchase a pre-emergence herbicide/fertilizer product with slow release nitrogen in it.

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.

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!

The Purdue University Cooperative Extension Service can be reached at 499-6334 in LaGrange County.