The first, and only hard and fast rule here is to avoid applying in direct light. Secondly, and especially in relationship to treating for PM taking into account relative humidity is important. When PM is an issue, applying the product at a time when the humidity is either low or about to drop is very beneficial to getting the desired outcomes. For outdoor farming, this could mean applying just before the dawn, or watching the humidity forecast and planning accordingly. For indoor applications, humidity control is very important, and Trifecta will combat PM most effectively is humidity is kept low.