Answer: I checked out the Barricade directions and product label at the product Web site (http://www.cp.us.novartis.com/info/splabel/3600_barricade.pd...) which shows a map for Crabgrass Emergence dates in the U.S. Beyond the map, here's some general information to fill out the "whys" and "hows" of preemergent herbicides:
Pre-emergent herbicides work by creating a chemical barrier through which seeds will not grow. To be most effective, the herbicide should be put down just before the germination time of the specific weed to be controlled. For example, pre-emergent herbicide is often used in lawns for control of crabgrass. Since it is an annual weed that germinates in early spring, recommendations are for applying it then. Specific dates vary by location but timing is critical since if applied too early the "barrier" can be broken and become ineffective. If applied too late, the seeds will germinate and control will be poor. Thus, it is critical to know what weeds are to be controlled, and when they usually germinate. Although there may be a few annual or perennial seeds with fall germination, by far most garden weeds germinate in spring, making spring applications most effective.
Besides herbicides, consider other non-chemical means of control such as cover and smother crops, mulches, and heavy seeding in gardens. In lawns, mowing high (2-3") is the first defense against weeds.
Q&A Library Searching Tips