Local plant expert Arthur Lee Jacobson has written about crabgrass (Digitaria sanguinalis)in his book, Wild Plants of Greater Seattle (2008): "Crabgrass is difficult to get rid of because it seeds itself at an almost unbelievable rate; mowing simply makes if flower nearer to the ground. Control demands diligent weekly hoeing and pulling by hand, from July through at least September. Even a few specimens left to reseed ensure more seedlings next summer."

According to Ecologically Sound Lawn Care for the Pacific Northwest by David McDonald (Seattle Public Utilities, 1999), weed invasions are best prevented by making a habit of aerating and topdressing to correct soil compaction and build fertile soil. He recommends that you "overseed at summer's end with locally adapted grasses to fill bare areas with grass rather than weeds. Correct acidity or poor drainage. Mow higher (2-2 1/2 inches, or 1 inch on bentgrass), fertilize moderately with slow-release or natural products, water deeply and infrequently in the summer. Tolerate some broadleaf plants like clover and daisies. Hand weed or spot-spray problem weeds in spring or fall to stop them before they spread."

There is additional information on crabgrass from University of California, Davis Integrated Pest Management, excerpted here:
"Because crabgrass spreads and reproduces primarily by seed, any cultural operation that reduces seed production will decrease crabgrass. There are also a number of cultural operations that increase the vigor of turfgrass, thus decreasing the potential for a crabgrass invasion. These include using the proper mowing height for turf, applying fertilizer at the correct time of year, selecting the best turf species for your area, overseeding to keep the turfgrass thick, and properly irrigating turf.
Fertilization can also be used to increase turfgrass vigor and reduce the possibility of a crabgrass invasion. The best time to fertilize is when the turf is actively growing, which depends upon the turf species grown. Because crabgrass is not very competitive, a vigorously growing turf will crowd out crabgrass seedlings.
Selecting a turfgrass that is adapted to your local conditions will also help produce vigorous turf. (...) Irrigation timing and amount can also affect crabgrass germination and growth. Turf that is overwatered or has frequent (daily) light irrigations becomes weak and vulnerable to invasion by this weed. Irrigating infrequently (once a week) will improve turf vigor. Crabgrass is often found first in open areas where there is no turf, along sidewalks where the soil may be warmer, or around sprinkler heads where turf is mowed closer.
In the landscape, crabgrass can easily be controlled with mulching, hoeing, hand-pulling when the plants are young and before they seed, or with solarization.
In shrub beds, bedding plants, or around trees, mulching with wood products (wood chips, nuggets), composted yard waste, or synthetic landscape fabrics covered with a mulch will control the germination and establishment of crabgrass by blocking sunlight needed for its germination and growth. The depth of a mulch depends on the size of the particles: coarse mulch may need to be 3 to 6 inches deep to control all weeds, whereas a finer mulch may need to be only 2 to 3 inches deep.
Clear plastic mulching (solarization) is effective for eradicating crabgrass plants and seed if it is applied during periods of high solar radiation. (...) Before applying the plastic, closely mow the crabgrass, remove the clippings, and water the area well. It is not necessary to cultivate before solarization, but a shallow cultivation may improve control. Place clear, ultraviolet (UV)-protected polyethylene over the area for 4 to 6 weeks. Shade will reduce the effectiveness of solarization because it limits the amount of radiation. Solarization works most effectively when there is no slope in the land or if there is, the slope has a south or southwest exposure. Temperatures are not as high under plastic placed on a north-facing slope; consequently, control is not as effective. After solarization, do not cultivate the area deeper than 3 inches to avoid bringing weed seed into the upper soil layer."

King County has information on best practices for maintaining a healthy lawn.

Washington Toxics Coalition also has a helpful lawn care fact sheet that might be helpful to you.

Since the weed-and-feed approach to the problem was not effective (and chemical weed and feed should be avoided), I recommend trying some of the cultural controls discussed above (mow higher, only fertilize at appropriate times and don't use quick release fertilizer, water less often but more deeply, improve drainage by aerating, build soil by mulching). Solarization might be an option if the problem can't be addressed by hand-weeding combined with the other methods described.

Season All Season
Date 2009-04-02
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