Tricky Grass Weeds - Crabgrass and Other Annuals
Since annual weed grasses grow and behave as good grasses do, they are difficult to eradicate without harming your lawn grasses.
The annual sorts include crabgrass. These start from seed anew each spring, and the weakness in their life cycle is the seedling stage. Chemicals have been found which kill them at this time without injuring established grasses. These pre-emergence or preventive weed killers are applied to blanket the soil before the weeds germinate, and kill the seedlings as they appear.
Crabgrass is the most pestiferous annual grass in most lawns. Its seed lies dormant in the soil and only a small percentage of it sprouts in any given year. A big crop of it is almost sure to result if your lawn becomes weak. Seeds sprout when the soil warms to around 60 degrees. You will notice the seedlings first on south-facing slopes, as early as March near the coast or as late as May - June in the cooler mountainous area.
Smooth Crabgrass, photo by Clyde Elmore, UC Davis
The seedlings look innocent enough-- two small spear-shaped yellowish-green leaves. But with warm weather and ample water the plant expands rapidly, with its spreading stems rooting at the joints. By August a plant may be too big to be covered with a 10-gallon hat.
From middle to late summer it produces wiry "crowfoot"- shaped seed heads. Then before frost, the plant turns an unattractive yellow. Pulling up dead plants or scratching them out in autumn accomplishes little more than the threshing of the seed, thus planting it for next year.
New pre-emergent chemicals come out almost yearly. Good preventives remain effective for months, controlling later sprouting seeds as well as early. Rather than dictate a particular brand or type, visit a local reputable nursery and ask the advice of the staff.
Caution: Most effective preventives decimate newly sown lawn seed, so are best used on established lawns where the grass is old enough to be mowed. If you are planning to overseed a lawn that is looking sparse, do it well ahead of the time you apply a crabgrass preventive, or wait until autumn to do the new seeding.
The time to apply the preventive is before the first crabgrass seedling makes its appearance. But here is no sense in applying it much ahead of the sprouting season - which is about the time lilacs finish blooming - since it gradually loses its effectiveness. Most of the preventives are effective only a few months and in following years you will need to put on a booster treatment.
Spread the preventive uniformly over the lawn at the rate recommended on the product. The easiest way to do it is with granular materials dropped from the same sort of mechanical spreader you use to apply lawn fertilizer. You do not need to put any in shaded areas, for crabgrass will not grow in shade - as under trees.
If you miss out on killing annual grasses as they sprout, there are chemicals which will kill them selectively later. Sprays are ordinarily more effective for this, since the chemical is absorbed through the foliage, and it is easier to coat leaves with liquid than to make granular materials adhere. Ask your nursery for the best product to use. Usually 2 applications about a week apart are needed.