Best Pre-emergent Time
Q: What is the best time to spread pre-emergent on garden areas containing flowers and shrubs?
A: The best time to apply is early March but it’s complicated because pre-emergent for a lawn might not be able to be put under shrubs.
Although the chemicals might be the same, legally you are supposed to only use in your situation pre-emergent products that are labeled for use on landscape plantings, not those labeled for lawn use. The application rate may vary between the two products; the rate for lawn application might hurt shrubs and perennials.
Although landscapers have access to several appropriate products, homeowners might only be able to find Preen Southern Weed Preventer, containing dithiopyr. Its label has a long list of ornamentals on which it can be used. As with any pre-emergent, best control comes when the chemical is applied directly to the soil surface and watered in.