Pre-emergent and post-emergent herbicides are chemical treatments to prevent and eradicate crabgrass. Pre-emergent herbicides destroy crabgrass before it sprouts as it germinates, whereas post-emergent herbicides kill existing crabgrass.
Pre-emergent herbicides create a chemical barrier on the soil that crabgrass absorbs as it germinates, killing the plant. These herbicides must be applied in the spring or when the temperature of the soil warms to 60 degrees Fahrenheit and other plants have started to bloom.
Post-emergent herbicides should be applied in sunny weather so that rain cannot wash the herbicide away before the crabgrass absorbs it. These herbicides are best applied in warm weather after the morning dew to maximize absorption. Excessively dry soil may prevent absorption; water the soil the day before using the herbicide in dry conditions.