What Is Pre-Emergent Herbicide?


Ensure a weed-free lawn by taking early steps to get rid of weeds before they start to grow. Pre-emergent herbicides are chemical substances applied to areas of land to prevent the emergence of new, germinating weeds. These toxic substances do not kill plants; they stop their seeds from starting new plants. Most often used in lawn gardening, pre-emergent herbicides must be applied with attention to safety and careful timing.


A pre-emergent herbicide is a tool used in gardening to prevent weeds from germinating. These herbicides usually come as grains or as a liquid. After the herbicide is applied to the desired area, gardeners use water supplied by rain or irrigation to activate the seed-killing herbicide.


Pre-emergent herbicide does not only work on weeds. When applied to a newly seeded lawn area, it stops grass from germinating as well. Pre-emergent herbicide will prevent growth of any new grass if seed and herbicide are applied to a lawn within too short a time frame. The only time a lawn gardener can use pre-emergent herbicide to fight weeds is in seasons when he does not plan to reseed the lawn.

Time Frame

Knowing the growing cycle of the target plant is key in applying pre-emergent herbicide effectively. To work, a pre-emergent herbicide must be applied before the target seed germinates, but not so early that the herbicide is washed away by rain before it can work. If the seed has already germinated, even underground, pre-emergent herbicide loses its effectiveness.

Expert Insight

The best time to apply pre-emergent herbicide depends on the growing season of the target weed. Many plants germinate when the soil reaches a specific temperature. Gardeners use a weed's germination temperature to determine the best time to apply pre-emergent herbicide. To prevent crab grass, gardeners use pre-emergent herbicides when the soil temperatures have been at 50 degrees F for three days or more because crabgrass seeds are known to germinate at approximately that temperature.


Types of pre-emergent herbicides include Benefin, Priodiamine, Surflan, Snapshot and Isoxaben.


Pre-emergent herbicides are toxic substances. Handle them carefully and keep them away from children and pets. If storage is necessary, keep them in a cool, locked, dry place away from other chemicals.

Keywords: kill lawn weeds, prevent unwanted weeds, invasive plants, crabgrass killer

About this Author

Terry Morgan is a freelancer who has been writing since 1992. Morgan has been published at Gardenguides.com, Travels.com and eHow, frequenting topics like technology, computer repair, gardening and music. Morgan holds an Associate of Arts with a journalism focus from Moorpark College and a Bachelor of Arts in music and technology from California State University San Marcos.