Crabgrass and The Pre-Emergent Paradox
Most homeowners are familiar with the concept of pre-emergent weed control. You put down this stuff in the spring and it kills the weeds before they have a chance to come up in your lawn. Easy peasy. Carrying this out to its logical conclusion means that you need to make sure you put this pre-emergent down well before the weeds get a chance to germinate or it won't work very well. While there are several weeds that can be controlled in the spring with pre-emergent, the number one target is crabgrass and putting crabgrass preventer down too early can sometimes lead to trouble.
Crabgrass Germination Conditions - No Easy Answers
So, when does crabgrass germinate? There is no straightforward answer to this question! If you are basing it on soil temperature, then you can count on crabgrass germinating when soil temps, to a depth of 1 inch, reach 57 to 64 degrees. That is, of course, except when it doesn't. You see a high percentage of crabgrass seeds do not germinate before reaching 73 degrees.
What about air temperature? Well...then we need to discuss the intricacies of 'degree days' if we want any quantifiable data. Trust me...neither of us want that.
Faced with the difficulty of when it might be too late to apply pre-emergent (especially with our weird weather), Virginia lawn people have turned to putting it down as early as the weather allows (February...early March).
This super early approach is problematic when the lawn care program you follow only has a single application of pre-emergent. You see, most pre-emergent weed controls have an effective life span of 80-110 days in the soil. This can be even less under conditions of excessive heat or moisture. If you make your application two weeks either side of March 1st that means the pre-emergent will run out of gas sometime in June or July. Since crabgrass seeds remain viable in the soil for years and years, this expected expiration of your preventive control halfway through summer can result in a bumper crop of crabgrass in August. Bummer.
The answer is to use a split application. Go ahead and get the first shot down way early. This ensures no early germinating crabgrass if the weather is unseasonably warm. Then make a follow up application about 6-8 weeks later for protection over the entire summer. Be careful when adding this extra pre-emergent treatment to your current program! Take into account any fertilizer that may be mixed with the pre-emergent. In other words, don't just do "Step 1" twice. This would likely lead to too much fertilizer being applied which has its own set of problems.