For most lawn weed killers, the best time to spray is after morning dew has dried or in early evening on a calm, dry day, with temperatures roughly 65°F to 85°F and no rain expected. Use post-emergent sprays when weeds are young and actively growing, use pre-emergents before weed seeds sprout, and never use regular Roundup or weed-and-grass killer inside a lawn unless you are willing to kill the grass too.
Quick answer: best timing by product type
The right timing depends less on the words “weed killer” and more on the product type. A selective lawn spray, a weed-and-feed granule, a crabgrass preventer, and a nonselective weed-and-grass killer all behave differently and require different approaches.
Product type
Best for
Best timing
Rain or water rule
Big caution
Selective liquid lawn weed killer
Existing broadleaf weeds, and some products also target crabgrass
Morning after dew dries or early evening, weeds actively growing, usually 65°F to 85°F
No rain for the label’s rainfast window, often 1 to 24 hours
Safe only on grass species listed on the label
Granular weed-and-feed
Existing broadleaf weeds plus fertilizer
Apply when weed leaves are wet from dew or light watering, temperatures commonly 60°F to 90°F depending on label
Keep rain and irrigation off for about 24 hours so granules stick
Not ideal during heat, drought, or if fertilizer timing is wrong
Pre-emergent crabgrass preventer
Crabgrass and other annual weeds before they sprout
Early spring before soil warms enough for germination, often before the 3rd or 4th mowing
Water in after application, commonly within 1 to 3 days
Does not kill most established weeds
Fall pre-emergent
Winter annual weeds like annual bluegrass, chickweed, henbit
Late summer to early fall, before fall germination
Water in after application
Can interfere with fall seeding unless label allows it
Roundup, glyphosate, and weed-and-grass killer
Existing weeds in beds, cracks, edging, gravel, or lawn renovation areas
Calm, dry weather when target weeds are actively growing
Rainfast timing varies, often 15 minutes to several hours
Kills desirable lawn grass unless it is a lawn-specific selective product
If you are treating a normal lawn and want the grass to survive, look for “for lawns” on the label and confirm your grass type is listed. If the product says “weed and grass killer,” “nonselective,” “kills to the root,” or lists glyphosate as the active ingredient, assume it can kill your lawn.
Best time of day and weather conditions
The best time of day to spray liquid weed killer is usually midmorning after dew has dried, or early evening if the label allows it and the grass will have time to dry before heavy dew settles. You want dry leaves for most liquid post-emergent sprays because the herbicide needs to sit on the weed long enough to be absorbed.
Aim for moderate temperatures. Most post-emergent broadleaf herbicides work best around 65°F to 85°F. Many labels allow
