Weedkillers 101

— Written By Bill Hanlin and last updated by JoAnne Gryder

Many homeowners are in a quandary about when to apply weed killers, or herbicides, in the landscape. Some people get mixed results and may not have applied the weed killer at the correct time. Proper timing depends on the type of herbicide being used and can make all the difference in satisfactory results.

Herbicides are broadly classified into two different categories – pre emergent and post emergent. Pre emergent herbicides kill the weed before it has a chance to emerge from the soil. Post emergent herbicides kill weeds that have already sprouted.

Timing of pre emergent herbicides can be a little tricky because it is important that the active ingredient be close to the germinating weed seeds. If pre emergent weed killers are applied too early and we get a wet season, then the weed killer may leach below the area of germinating seeds. If the pre emergent weed killer is applied too late then the weed seeds may have already germinated and sprouted from the soil.

Most crab grass preventers are pre emergent herbicides and timing is critical for good control of crab grass. One general rule of thumb for most crab grass preventers is to apply the material when dogwoods bloom in the spring. Since there are different crab grass killers out there, read the herbicide label to make sure when to time applications.

Post emergent timing is not as critical as pre emergent, however most post emergent weed killers are more effective on younger weeds than older ones. Again, the label will tell you when to apply to get the most effective weed control.

One word of caution, herbicides are designed to kill plants and may also damage plants you want to keep. Weed killers that drift on to desirable plants can damage foliage and even kill the plant. Use extra caution when applying post emergent weed killers in particular since even small amounts contacting desirable plants can do a great deal of damage.