Is there any issue to putting crabgrass preventer down today when the grass is still wet from rains in the night? The instructions say to apply when the grass is dry but I wasn't sure if that's a big deal or not.
You want the prills to fall below the leaf canopy, so you want to do it on dry grass and then later water it in. Pre-M works by getting into the soil and forming a vapor barrier below the soil surface that interferes with germination process of some plants. How long this vapor barrier lasts largely determines how long you get effective control. You can actually break the vapor barrier and create gaps in your protection if you later disturb your soil, so don't aerate in Spring after applying pre-emergent herbicide.
As far as harming grass by applying on wet turf, I honestly don't know how toxic the different pre-M chemistry is when it sticks to the grass blades and whether much would even get absorbed through the grass blades. Anyhow, the protection gained is through the soil route, so please do follow the label and apply to dry turf.
Conversely, weed 'n feed products must be applied to wet turf. The herbicide must actually stick to the weed leaf and be absorbed by the foilage. It's for that reason it's not my recommendation to kill weeds this way. You need to get lucky and have the weed killer actually land on the wet plant leaf, and so much is just wasted as it falls to the soil. Only if you had a lawn with like 50% weeds would this be a decent plan of attack and I'd still prefer a blanket liquid spray in that scenario.
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