All About Pre-Emergents

 Herbicides that keep seeds from becoming plants are called “pre-emergents”.  Pre-mergents can be used in several different areas depending on the active ingredient. There are pre-emergents labeled for lawns, flowerbeds, and “non-crop” [1] areas.  Pre-emergents can save hours of labor and contribute to a healthier landscape.  Here is what you should know about these materials.

Click on the pictures of weeds for a larger image

When looking at preventing weeds, it is important to identify what types of weeds you are wishing to prevent. There are two main categories of weeds, broadleaf weeds and grass weeds. Below are some samples of each.


To prevent most of our grass weeds we apply "Pendimethlin" in January, April, July and November (if your lawn goes dormant we also apply in August). See our yearly calendar HERE for the whole schedule! 

Grass weeds have parallel veins


Poa Annua



To prevent the broadleaf weeds we apply "Gallery" in April, June and November.  See our yearly calendar HERE for the whole schedule!

Broadleaf weeds have a perpendicular vein


Bur Weed (Sticker Weed)



All pre-emergents need to be applied uniformly over the area treated.  Uneven application leads to pre-mature breakthrough of the weeds in the areas where herbicide is light.

Incorporation and Activation

All pre-emergents need to be watered in.  The amount of water is usually ½ inch.  The sooner water is applied the better.   Some pre-emergents need to be watered in within 24 hours or they lose potency.  Watering should be done within a week, either by sprinklers or rain. It is important that water is applied as uniform as possible as uneven watering can contribute weeds breaking through.

Range of Control

Once watered in pre-emergents can take up to two weeks to become active. Because of this, always apply pre-emergents two to three weeks before emergence of target weed.  For example, crabgrass typically germinates in mid-February. So your pre-emergent for crabgrass should be made by the last week of January.

 Just because you use one pre-emergent does not mean that you will get full weed prevention. Different pre-emergents will control different weeds.  For total weed control, several pre-emergents should be applied.


How long a pre-emergent barrier will last depends on the pre-emergent and how heavy you apply it. Our yearlong calendar has the months when to apply and the materials needed for the best control.  

  Root Decline

Many pre-emergents work by attacking the first root that comes out of a seed while others work.  This inhibits the plant’s ability to uptake nutrients and causing it to die.  Just as they attack the roots of weeds, pre-emergents can also attack roots of the existing vegetation, especially grasses. Using the wrong pre-emergent can even kill your lawn. 

Other pre-emergents work by attacking the first leaf out of the seed.  These materials are safe on roots but are very expensive.  

Regardless of what pre-emergent you apply, a proper fertilizer should be applied. See our yearlong Fertilizer/Pre-Emergent guide to know when to put down pre-emergents and what fertilizer to use.

[1] Non-crop areas are areas where you want nothing to grow.