Understanding Weed KillersGrooming a gorgeous lawn requires persistence and patience as you work to eradicate weeds. Herbicides are a key weapon in the lawn weed control arsenal. Two types of herbicides are commonly used: pre-emergent and post-emergent. The names describe when the herbicide kills weeds. Pre-emergents kill weeds before sprouts emerge from soil. Post-emergents decimate actively growing weeds that have already emerged from soil.

Pre-Emergent Herbicides

These herbicides don’t stop weed germination, but rather interrupt the process before a sprout pushes through soil. A common example of a pre-emergent herbicide is a crabgrass preventer, which prevents crabgrass seeds from establishing new plants.

Secrets to Success

  • With pre-emergent herbicides, it’s vital to treat the entire lawn area. If you miss a spot, weeds can sprout there.
  • Most pre-emergents require watering in, even liquid forms applied using a hose-end sprayer. With liquid herbicides, the volume of water used to disperse the weed killer is not great enough to wash the material into soil, where weed seeds lie waiting to germinate. That’s why you have to water after application.
  • Use caution when applying pre-emergent herbicides to newly seeded lawns – or to areas you plan to seed. Read the label carefully. For most products, the label stipulates how many mowings, after seeding, to wait before application. The label also states how long to wait after application before sowing lawn seed.

Post-Emergent Herbicides

These herbicides tackle both perennial and annual broadleaf weeds (weeds with leaf shapes that don’t resemble grass), and some even control grassy weeds (weeds that resemble grass). Choose post-emergent herbicides to control existing weeds in a lawn. It’s an ideal herbicide for spot-treating lone offenders.

Secrets to Success

  • Young, actively growing weeds die most easily and create less of an eyesore than mature weeds, which might require repeat applications for complete kill.
  • Inspect the lawn frequently (while mowing is a good time) to look for new weeds that have germinated and require treatment.

Timing

For both pre- and post-emergent herbicides, timing is critical. While post-emergent herbicides kill weeds at any point in the growth cycle, you’ll have the best success spraying young, actively growing weeds. Mature weeds may require repeated applications for total kill.

With pre-emergent herbicides, you’ll want to apply the chemical prior to the time weed seeds start germinating. If you apply too early, these herbicides will have degraded and are useless when seeds start to germinate. Most pre-emergent crabgrass killers remain active in soil for six to eight weeks.

Weed seed germination occurs when soil reaches the correct temperature. The best way to determine the ideal time to apply pre-emergents is to contact your local extension agent or master gardeners, who have access to regional soil temperature data.

Other ways to gauge application time include using bioindicators, such as plants whose growth signals the correct time for application. For instance, in northern climates, spring crabgrass applications are often timed when forsythia is blooming, which frequently (but not always) occurs when soil temperatures are in the 50° F range. Another option is to time applications based on the calendar. For example, if you typically apply a pre-emergent herbicide in mid-April with success, then continue that routine.

Of course, you can avoid the issue of proper application timing altogether by purchasing a weed control product that combines both pre- and post-emergent herbicides. This type of product kills existing broadleaf weeds and keeps them from returning for as long as six months.