How soon can you treat new grass that you might seed this fall or next spring to prevent crabgrass in the same season? The label is always right, (but it may be too conservative). As a preliminary test, I applied Dimension (Shaws 23-0-8 with 0.13% dithiopyr) on seed that was planted August 9th 2011. Grass seedlings were about 2 inches tall, and they were 9 days old, (Scotts "High Traffic" seed). I counted the pellets in a sample and used 14 pellets per flower pot as an estimate of 4 pounds per thousand sqft. The result was: the seedlings were not killed, but slightly stunted. I will have more information later.