A pre-emergent weed control does not actually keep weed seed from germinating. In reality most pre-emergent weed controls kill the weed seed just after it emerges by killing the newly developing root system. Usually applied in a granular form, and applied 2-3 times per season. There are pre-emergent weed controls for both types of weed- broad leaf and grassy. Be sure to ALWAYS follow the instructions on any product you choose to use.