Crabgrass is an annual grassy weed. Although it is hard to control
after you notice it to be a prolific problem, it is relatively easy
to prevent. All crabgrass preventers use the same strategy. They
set up a chemical barrier in the soil that either stops the seed
from germinating or kills the seed upon germination.
To get the most effective control, the chemical barrier needs to
be in place at the time of germination. When is that? Crabgrass
germinates when the soil temperatures reach 50° to 52°F
and maintain that temperature for two to three days.
The chemical barrier when initially applied to the turf (usually
in a granular form) needs time and moisture to set up. A slight
precipitation is required to move the chemical from the granule
into the soil. The soil then bonds to the chemical and surrounds
the ungerminated seed.