R Dyer, Bugwood.org
Smooth crabgrass is very similar to other crabgrass weeds. It germinates in soil when it reaches a consistent 55 degrees and branches out as it emerges from the ground. It's called smooth crabgrass because it lacks the hairs on the leaves and sheaths that large crabgrass has. It also has a seed head with two to six spiked branches.
When the temperatures rise and the soil is 55 degrees, it's time to apply a pre-emergent herbicide to prevent crabgrass from breaking through the dirt. Look for herbicides containing dithiopyr for the best results. This ingredient is also in some postemergent herbicides for killing and controlling weeds that are already established.