Both large (Digitaria sanguinalis) and smooth (Digitaria ischaemum) crabgrass species invade home lawns. Both species are annuals and grow most vigorously following germination in the late spring through mid summer. Another common weedy grass, quackgrass (Agropyron repens) is a perennial that lives from year to year as the same plant.

Since crabgrass is an annual, it germinates and grows from seed each year. It produces several flowering stems from a single plant and produces many seeds from each of those stems. The plants have no frost tolerance and are easily killed when the first fall frost arrives. Prior to being destroyed, they will have produced thousands of seeds from each plant for next year's crabgrass crop.

Control Options: There are three approaches to crabgrass control – Prevention, Pre-emergence and Post-emergence.

Prevention – You will rarely find crabgrass in a lawn that is healthy and growing at a vigorous rate. Crabgrass invades those small openings in a thin, unhealthy lawn. Perhaps the single most important factor is to mow the grass at 2.5 inches or taller. This shades the bare soil spots and discourages crabgrass germination. Regular fertilization each year and irrigation during hot, dry periods will help also. If the soil is hard and compacted, core aeration during the spring or fall will help the desirable grass to grow better to the detriment of crabgrass. If you have a lot of crabgrass this year, it is often because your grass did not grow well the previous year for some reason.

Pre-emergence Control: Since crabgrass is an annual, it must germinate from seed each spring. Chemical herbicides called “pre-emergence” herbicides kill the new root that comes out of the seed. The key to this approach is timing. The crabgrass seed will not germinate until the soil temperature is about 50 degrees F.

When the yellow flowering forsythia bushes are in bloom in April is a good indicator that it is time to apply the pre-emergence product. During a cool spring, germination may be pushed back to May. Since the products have a short lifespan once you apply them, it is important that they not be applied until the soil is warm enough for germination.

Crabgrass preventers are excellent choices for the control of crabgrass in the spring. Other summer annual grassy weeds and some summer annual broadleaf weeds may also be controlled. Be sure to follow the label instructions for application rates.

Post-emergent Control: The pre-emergence herbicides will not kill plants that were allowed to germinate and begin to grow. So, in recent years, some herbicides have been developed that will kill the actual crabgrass plant while not bothering the Kentucky bluegrass, perennial ryegrass or fescue of your lawn.

If you missed the proper time for pre-emergence control and small crabgrass plants begin to appear in late spring or early summer, you might want to use a post-emergent herbicide.

Spot treatment is best for this approach. You need to apply the material to young plants before they have gone to seed for it to give effective control. If you treat later in the season, you will still kill the current year’s crabgrass plant but the seeds will already have fallen to the ground and it will be back next year. That is why the preventative and pre-emergence approaches are by far the most effective for homeowners.

Note: We have provided some general information and observations on this topic aimed at the home gardener. Before you take any serious action in your landscape, check with your state's land grant university's Cooperative Extension Service for the most current, appropriate, localized recommendations.


Copyright © 2000-