A pre-emergent herbicide is an herbicide you spray on your lawn or garden to prevent the growth of weeds. Pre-emergent weed controls kill seeds before they can grow. Pre-emergent weed controls should be sprayed periodically throughout the growing season, because weed germination happens all the time.

When to Spray

Once you've planted your lawn, you'll want to wait until you've cut your lawn three or four times. This creates a nice canopy. You don't want the grass to be too long, though, because then the weed-control won't be able to get to the weed seed. Always remember to wet the area you sprayed afterwards. The area must be watered for the pre-emergent herbicides to work.

Mark your applications down on a calendar. Write down specifics, like how much land you covered, how much herbicide you sprayed, what kind of day it was and so on. Use that calendar to plan when to spray and how much to spray. This way, once you've had a few runs, you'll know what works for your lawn and you'll know how much herbicide you'll need to have on hand to keep your lawn weed-free.

Anytime you see weeds, not only should you kill those weeds with a post-emergent weed control, but you should spray the area with a pre-emergent, too. When you see weeds, it means that the area is already infected and the only way to stop the infestation is to stop the seeds. You can apply pre-emergent weed control periodically throughout the year to continuously prevent weed germination.