Howdy willythepic, It depends on what One uses in the foliar spray. I have used the GH Azamax as a foliar spray during the bloom phase (and I use the GH MaxiGrow and MaxiBloom line of nutrients). On the label it says that it can be used up to 24 hours prior to harvest, although I have not ever needed it that close to harvest. I assume that the active ingredients in the Azamax break down quickly as the instructions say to not mix up more than the amount which will be used within 24 hours.