Pre-emergent herbicides prevent the germination of seeds by inhibiting a key enzyme preemergent herbicides are used to prevent crabgrass from appearing in summer lawns. Pre-emergent herbicides are applied to lawns in the spring and fall to prevent the germination of weed seeds. They will not affect plants. Pre-emergent herbicides must be applied before weeds emerge to have any real effect, since they work by preventing weed seeds from germinating in the first place. If you wait until after the weeds emerge, the herbicide won’t have any effect. Call us today and schedule your pre-emergent application!