Hi Amy,

Thank you for reaching out to OSU Ask Extension. Without more context from your question, I can give you a general recommendation on the timing of spraying. Also, I have attached a reference article that may offer some spraying guidelines. It is essential to read the label instructions carefully before applying any pesticide product to avoid environmental and human harm. When spraying you want to be mindful of the weather conditions. You can spray either very early in the morning or in the early evening. I would generally recommend the early evening during the warmer season in western Oregon because there will be more time for the active ingredient to affect the plants and at this time there is a cooling trend. In order for the droplets of the active ingredient-water mixture to adhere to the plant surface and adhere to it, the air humidity must be sufficiently high, otherwise the water that the active ingredient is dissolved in can evaporate, and the protection would be insufficient. Also, the active ingredient needs roughly 15 min to penetrate. Hopefully this helps.

Ohio State University spraying factsheet