What are Pre-Emergent Herbicides?
Pre-emergent herbicides are chemicals that prevent undesired weeds in your lawn. They do not prevent the germination of the seed, but help control it so that it will not sprout. Due to the way these herbicides work, application timing is the most important aspect of weed prevention success. If the weed has already sprouted and is visible, pre-emergent herbicides will not solve the weed problem. Check out this guide to common Arizona weeds: https://www.preen.com/weeds/az
Using granules or liquid formulations are the two primary manners to apply herbicides. If you use granular pre-emergent herbicide it is very important to water the chemical into the ground because the active ingredient needs to penetrate the soil. The herbicide is trapped by the granule until it is activated by water. This is also true with liquid pre-emergent products because grass and other plants may trap the active ingredient keeping it from reaching the soil during the initial application.
When to Apply Pre-Emergent Herbicides?
Early Spring and Fall are the most effective times to apply pre-emergent herbicides. They can be applied throughout the year, and will still prevent new weeds that sprout, however most weeds sprout during spring and fall. Spring applications target different weed species than Fall applications, so it is very important to know the type of weed you are trying to prevent for best results. See below for help identifying your target weeds.
Spring pre-emergent herbicide applications are used to prevent summer annual weeds. Summer annual weeds are both grassy and broadleaf types. For best results apply herbicides when the soil temperature is around 55 degrees or above for at least 36 to 72 hours (ideally this will be two weeks before seed germination). The majority of the USA experiences these soil temperatures from March to April. Consult your local county extension service to get up to date soil temperatures in your area.