what temperature does it need to be to use weed killer
3. There are some weed killers that will work when the temperature is 45 to 50 degrees F. and above and for 4 to 5 hours after applying. This includes Bonide âSuper Brush Killerâ â a weed killer that, when applied according to label directions, will not hurt bluegrass or turf type tall fescue grass.ome weed killers including Trimec, 2-4-D, Glysophate (Round-up, Kleen-up, and Hi-Yield Killzall II Weed and Grass Killer) do not work to maximum if the air temperature is below 55 to 60 degrees F. when applied and for 4 to 5 hours after applying.