Undoubtedly you have noticed that crabgrass has invaded your lawn and is growing very well. You may be plotting against it, investigating herbicides. Perhaps you have been working on this problem for years. Crabgrass - and a weed-free lawn - may even have become an obsession. Crabgrass seeds can lie in the soil for years. This is why crabgrass suddenly appears after you turn over an unused garden bed or you renovate your lawn in spring or summer. The seeds are there, just waiting for the right amount of sunshine, heat and moisture to germinate and make you miserable.
Crabgrasses are coarse-bladed grasses with prostrate blades that spread at right angles to attached stems, reminiscent of the angular structure of a snowflake. They are lighter in color- and do not blend in with- finer bladed perennial turfgrass species and cultivars of Kentucky bluegrass (Poa pratensis), fine fescues (Festuca spp.) and rye (Lolium perenne). Two species of crabgrass, which can be found in the same lawn at the same time, are common in our area: large crabgrass (Digitaria sanguinalis) and smooth crabgrass (D. ischaemum).
Crabgrass is an annual; it spreads by seeds. At the first killing frost, crabgrass dies, turning brown against the green of more desirable species. However, it leaves behind in the soil seeds that will germinate the following late spring and summer, potentially filling in every available unoccupied space. Crabgrass seeds germinate every time you irrigate your lawn and after every rain. Compared with usual lawn species, crabgrass requires less water and fewer nutrients, and it spreads more efficiently in stressed areas, i.e. turf worn from foot traffic, compacted soils, dry soils, diseased and low nutrient lawns, and even areas under attack by white grubs! And because of its prostrate growth habit, crabgrass escapes the cutting action of the mower.
Fortunately, there is another way to look at this problem. Crabgrass can play a role in providing that inexpensive, green, maintenance - free lawn you have been striving for. It has the same utility as a fine home lawn. You can play on it, walk on it, complain about its appearance, not water or fertilize it and it will continue to grow with minimal care. Even if you cannot face the idea of a brown lawn after the first frost, you can still reduce the crabgrass population in your lawn without using herbicides.
Start by getting a soil test. Visit www.UMassGreenInfo.org for directions and costs. A soil test will determine the proper amount of lime to apply to correct soil pH (acidity) problems and allow you to select the right amount of organic fertilizer.
Be sure to water your lawn only one inch/week during the growing season (if there has not been sufficient rainfall); this encourages the growth of deeper roots. Lawns with deep roots have more resilience to environmental and biological stresses. Raise the mower blade height to at least two and a half inches; three is better. This will allow the grass to caste more shade on the soil below, thereby discouraging crabgrass (and other weeds) from germinating.
Throughout the growing season, remove crabgrass (and other weeds) by hand, which creates open spots where lawn grasses can spread. Then reseed your lawn between August 15-September 20, when nights are cool but the days, while still warm, are growing shorter, which inhibits weed germination and establishment. If you reseed your damaged lawn in spring or summer, the grass seed usually loses out to the quick germinating lawn weeds.
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