Guidelines 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.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.