Clear plastic mulching (solarization) is effective for eradicating crabgrass
plants and seed if it is applied during periods of high solar radiation. In
California's Central Valley, this means during June to August, whereas in
coastal areas the best time may be August to September or May to June when
fog or wind is most likely to be at a minimum. 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. (See the soil
solarization publication listed in Reference.)

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