Expert: Richard Clark - 8/8/2010

Question
I'm trying to decide by using the National Weather Service's hourly chart of dewpoint temperature versus air temperature when the dew formed last night.  I read that when they are the same, dew will form.  Last night I sprayed my weeds with herbacide between 7 PM and 8 PM.  By 11 PM, the dew point temp was about 74 but the air temp was about 85.  At 2 AM, they were 75 versus 82.  Right now it is approximately 5 AM, and they are 75 versus 80, and the grass is plenty wet already.  So that means that the dewpoint temp and the air temp were never the same, but dew formed anyway.  I'm trying to guess how long the weeds stayed dry after I sprayed them with herbicide.  I'd like to know for future reference also, because it's so hot right now that I need to wait until the sun goes down below the horizon to go outside to do gardening chores that I can't do in the early morning hours.  (I live in Memphis.)The reason I can't spray herbicides and insecticides in the early morning is because the chemical would be diluted by the dew on the plants, and would not be as effective.  However, if the dew forms shortly after the sun goes down, then the chemicals I have sprayed after sundown will be diluted that way too.  Am I right?  Thanks for your help.

Answer
Lana
You are correct, that with the dew the chemicals would be diluted. If you spray three or four hours before the dew forms the plant should absorb the chemicals that you had sprayed on them. You could wait until the dew goes off the plants in the morning and then spray, that way it could stay on the plants all day. Hope one of these work for you.
Richard