re: Is it too late to put pre-emergent herbicide down?

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What’s the deal with 65°? Is that for protecting grass or making sure the 2-4,D is working?

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I’m guessing their recommendation is twofold.

One, the colder the temps, the longer the reaction time. 2-4,D usually reacts pretty quickly but cold temps could take a bit longer. During winter months, I’d just suggest that you spray with a 48 hr window of no rain or heavy fog.

Two, the surface inversion has usually lifted out of the way by the time the temps hit 65 degrees (not always but usually). What you doesn’t want to do is spray and have the chemical drift floating around in a suspension and land on off target areas.
If you spray with warmer than 65 degrees, there’s a much higher likelihood that the smaller drift particles will safely land in the target area.

Unless you grossly over apply, 2,4-D won’t impact your grasses at this point in the year.