Foliar sprays are a technique of feeding plants by applying liquid nutrients directly to the leaves where it can be directly absorbed. Foliar feeding is a method used in both vegetative and flowering stages and allows for nutrients to pass directly into the plant through the leaves and stems.

Don't use a foliar spray when your lights are on, instead the best time to apply any foliar applications would be right before or after the lights turn off. This gives the nutrients plenty of time to be absorbed without harming the leaves.