Is Summer Rain Making My Pre-Emergent Less Effective?

Q: My zoysia had quite a crop of crabgrass. I put down a pre-emergent in February but someone said summer rain made the pre-emergent less effective. 

A: Your friend might be right but there is another possibility as well. If you made your application in early February, the chemical might have “worn out” by June. But rains in summer made perfect conditions for more long-dormant crabgrass seed to germinate. If you have ever had crabgrass in your lawn, the seeds that were formed can last up to five years in the soil. That might explain why you have such a fine crop of weeds now. You probably would have gotten better crabgrass control if you had put down a half-strength application in February with another half-strength application in June. Do not exceed the rate on the bag’s label. But going back to your situation, you didn’t know at the time what the weather would be for the summer. Next year, apply a split application in March and May. That should give you better control.