When Should I Apply Weed Killer?

Pre-Emergent Herbicide

• Pre-emergent herbicide prevents weed seeds from germinating in the yard. This weed killer type will have very little effect on existing weeds. Knowing what weeds infest your lawn will help in determining when to apply the pre-emergent herbicide. Typically, weed are broken up into two categories: cool and warm season. Cool-season weeds start growing at the end of summer and warm-season weeds pop up in the spring. Irrigate the lawn or choose a day that has afternoon rain when applying a pre-emergent herbicide. Pre-emergent herbicide needs water to activate the chemicals.

Weed Identification

• Weed identification will help you know when to apply pre-emergent herbicide and what type of post-emergent weed killer to use. Sedges, grasses and broad-leaved weeds make up the three general categories of weeds, according to the University of California. You can tell the difference by examining the weeds. Sedges have triangular tips, grasses have rounded tips and broad-leaved weeds are typically more colorful than the grassy weeds or sedges. Check the label of the pre-emergent and post-emergent herbicide to ensure that it treats the weed you are trying to kill.

Post-Emergent

• Post-emergent herbicide works best when weeds are actively growing. The more foliage on the weeds the better the absorption rate of the weed killer. Moreover, it is better to apply post-emergent herbicide on grassy weeds a few days before you mow the lawn. Pick a dry day to apply the post-emergent herbicide. This type of weed killer needs at least 24 hours to become absorbed into the root system of the plant. Avoid spraying weed killer on water-stressed lawns to prevent causing harm.

Establishing a Lawn

• Gardeners must make sure that their entire lawn is free from weeds before they can seed or lay sod. Weeds left in the lawn compete with seeds and sod for the soil's nutrients. Your newly planted lawn can suffer from the competition and keep from spreading. Use a broad-spectrum weed killer that contains glyphosate to kill off the weeds. Glyphosate is a popular weed killer because it leaves little chemical residue in the soil.