I’ve read preemeegent you’re supposed to time around 55 and 70 degree soil temp. I’ve been poking around on
www.domyown.com and have found its pretty easy to buy professional strength products and dilute yourself. You can get Prodiamine (barricade pre emergent) at $75 for 5 pounds. Application rate is 4 oz per 1000 square feet in a tank sprayer. That brings my cost per application to about $15. Granted a bag of Scott’s Halts to cover the same are is about $18, but my Home Depot doesn’t stock it, and I feel the liquid is easier to apply.