This is from ISU's WebPage, not the DM Register, but the article is almost
the same.


Application of Preemergence Herbicides

By Richard Jauron
Extension horticulturist
Iowa State University Extension
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Crabgrass, a common weed in many lawns, is an annual, warm-season grass.
Seeds begin to germinate when soil temperatures reach 55 to 60 degrees F.
Germination usually begins about mid-April in southern Iowa, early May in
northern parts of the state. Crabgrass continues to germinate over several
weeks from spring into summer.

While crabgrass germination begins in the spring, the weed doesn't become
highly visible in lawns until summer. Crabgrass is a low growing, spreading
plant with light blue-green foliage. The leaf blades are approximately 1/4
inch wide. Seedheads appear as several finger-like projections atop upright
stems. Crabgrass grows rapidly during warm summer weather. Growth slows with
the arrival of cooler temperatures in late summer. Plants are destroyed with
the first hard frost in the fall. However, before it dies, a single
crabgrass plant can produce up to 150,000 seeds.

The best way to prevent crabgrass infestations in lawns is to maintain a
thick, healthy lawn through proper mowing, irrigation and fertilization.
Crabgrass will have a difficult time germinating and surviving in a dense
turf. Gardeners who have had crabgrass problems in the past will need to
apply preemergence herbicides in the spring. Preemergence herbicides must be
applied before the seeds germinate and are not effective on emerged weeds.
Several preemergence herbicides will control crabgrass in established lawns.
These include benefin, bensulide, dithiopyr, and pendimethalin. Often these
herbicides are combined with a turf-type fertilizer. This allows the
gardener to apply a preemergence herbicide and fertilizer with one
application. If you intend to establish a new lawn from seed this spring,
the only preemergence herbicide that can be used is siduron (Tupersan). This
herbicide effectively controls crabgrass without affecting the germination
of the turfgrass seeds.

Gardeners also can control crabgrass by applying corn gluten meal. A corn
milling byproduct, corn gluten meal inhibits the root growth of crabgrass
seedlings. Unable to develop roots, the crabgrass seedlings die. In addition
to crabgrass, corn gluten meal offers preemergent control of dandelion,
plantain, lambsquarter and other weeds. Corn gluten meal is approximately 10
percent nitrogen by weight, thus making it a natural "weed and feed"
product. While corn gluten meal is not as widely sold as other preemergence
products, it is available at select garden centers and mail-order companies.
Additional information on corn gluten meal can be found at the Web site
http://www.gluten.iastate.edu/.

The keys to successful control of crabgrass in lawns are correct timing of
the preemergence herbicide application and proper application of the
material. Preemergence herbicides must be applied before the crabgrass
germinates. If the material is applied too early, the crabgrass that
germinates late in the season will not be controlled. If applied too late,
some crabgrass will have already germinated. Preemergence herbicides should
normally be applied in early to mid-April in southern Iowa, mid-April to May
1 in central Iowa, and late April to early May in northern areas of the
state.

Since the weather varies considerably from year to year in Iowa, gardeners
should make adjustments in the timing of the preemergence herbicide
application. This spring it would be wise to apply the preemergence
herbicide late in the normal time period because of the persistent snowcover
and cold temperatures in recent weeks. If you're still uncertain as to when
to apply the preemergence herbicide, Mother Nature does supply some helpful
clues. Crabgrass seed germination usually begins when the forsythia blossoms
start dropping or when redbud trees reach full bloom. Preemergence
herbicides should be applied prior to these occurrences.

To ensure the herbicide is applied properly, carefully read and follow the
label directions on the package. Also, make sure the spreader has been
correctly calibrated and is working properly.

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