Scotts Halts Crabgrass Preventer. When to Apply: Apply in early spring to prevent crabgrass all season long. For most areas this is going to be from early March until mid May. So it can vary from March 1st-May 20th.Crabgrass seeds can germinate when nighttime soil temperatures reach over 50 degrees for 3 days.uidelines to Apply: There are guidelines to applying Scotts Halts Crabgrass Preventer. Always apply in early spring from March to May. Either before dandelions reach the puffball stage or before Forsythia blooms. Forsythia is usually the first flowering shrub that blooms in the spring.