Use the weed life cycle first
Spring applications target summer annual weeds like crabgrass and goosegrass. Fall applications target winter annual weeds like annual bluegrass, chickweed, henbit, and other cool-season germinators. A single spring application will not prevent every weed all year.
Soil temperature beats a fixed date
Purdue Extension describes crabgrass as a summer annual that germinates around 60 degrees Fahrenheit. Apply spring pre-emergent before the germination window, with enough time for rainfall or irrigation to move the product into the upper soil layer.
Check your current soil temperature before applying.
Do not block your own seed
Most pre-emergent herbicides can also prevent desirable grass seed from establishing. If you recently seeded or plan to seed, read the label carefully and choose a product that fits that situation. Purdue notes siduron is an exception for some cool-season seedings, but it is not a season-long solution.
Product choice matters
Common active ingredients include prodiamine, dithiopyr, pendimethalin, and dimension-style products. Use the Price Board to compare pre-emergent options after you confirm grass safety, target weeds, reseeding interval, and label rate.