When in 2017 to apply long term (dioptyr) pre-emergent preventer
Rule of thumb is to apply when soil temperature 4" down reaches 40 degrees (F). I'm not sure the temperature at that depth has reached 40 degrees; every time I measure it has shown one or two degrees above 40. So when should dioptyr be applied?d
The aim of using soil temperatures is to ensure that the soil has reached that minimum temperature of 40 degrees. You do not want the soil to be below 40 degrees.
The reasoning is that you want to put down the preemergent herbicide before the crabgrass seeds begin to germinate. However, if it is too cold, you'll be putting down the herbicide too early, and some of it will be wasted because the crabgrass is not germinating yet.
This is not an exact situation. Crabgrass will continue to germinate most of the summer. It will not even begin to germinate in all parts of your yard at the same time, because every yard has fluctuations in temperature. The general rule of thumb is to apply crabgrass preemergents when the forsythia sheds its spent blooms: http://extension.umd.edu/sites/extension.umd.edu/files/_images/programs/hgic/Publications/non_HGIC_F...