Question
When is the best time to put crabgrass preventer on? Fall or spring or both? Also, I want to put buffalo grass on a steep slope in my backyard.  It was seeded (with buffalo grass) in June or July (construction contract) but I always thought the best time to seed for buffalot was fall.  Should I apply more? Thanks

Answer
I would expect about 6 weeks of control from a pre-emergent chemical; but (much) water and (high) temperatures will shorten this time. The chemical kills the young roots of germinating seedlings. So, the big question is when to apply the chemical to obtain the maximum effectiveness.
My best guideline is in the spring, when the temperature gets up to 50 degrees around the clock.
Some people say when the forsythia blooms; others, when the crocus blooms. Apparently when the soil temperature gets to 50 degrees, the crabgass seeds will germinate and start to grow roots. The pre-emergent must be in place for this event. Now, you have put down the chemical and you want the seeds to germinate. That calls for water; and if the weather is dry, you can tempt the seeds to germinate by irrigating. Remember that the seed requires a continued dampness for germination so it is up to you to judge if and when repeated irrigation is needed.
To sum up: apply the pre-emergent with the expectation that the crabgrass seeds will germinate soon; if they do not germinate, irrigate to create a soil dampness to stimulate germination.
And watch your program succeed.