After rain is the most efficient time to apply fertilizer and weed killer. The time to avoid putting down the product is just before rain is expected, as it may all be washed away.
Timing Is Everything
The best time for applying fertilizer and weed control is in early spring or autumn, when the weeds are having a growth spurt.
When to Apply
Spread the product when the lawn is wet--after a rain or after you have just watered completely. This helps the ingredients adhere to the foliage, both to feed grass and to kill broadleaf weeds.
If the weather cooperates, rain can direct the weed and feed product into the soil and root system. If not, water deeply after two days.
For Large Weeds
Large weeds will not be eradicated by this type of product. That is why it is applied during the vigorous growth periods in early spring and fall.
Environmentally Friendly Alternatives
Compost, manure, or mulch will prevent weeds by improving the soil, and adds organic matter, which helps soil utilize water and promotes the natural occurrence of micro-organisms. Compost also helps establish healthy plants that crowd out weeds.