For the most part, drones can fly whenever a crop might need to be treated and in much more adverse conditions than a ground rig or crop duster would allow.
Pilots should avoid flight in heavy rain and extreme temperature conditions, generally below 40° or above 100°.
Common agricultural spraying drones can be flown reliably in winds up to 17mph. However, most pesticide labels restrict application at wind speeds above either 10mph or 15mph, so the drone itself is not usually the limiting factor.
Want to know if you are good to Fly & Apply? Check the UAV forecast here.