In the Southern Appalachians, you can treat hemlocks almost any time of year, as long as the trees are actively transpiring. This occurs throughout the year unless the soil is too dry or, conversely, too saturated, the air too hot or humid, or the ground is frozen. That being said, we prefer to conduct hemlock treatments when the trees are most actively transpiring to facilitate efficient uptake of the chemical pesticides. Therefore, we do most of our treatments in the fall and spring when hemlocks are most active.
Returning NC's Hemlocks to Long-Term Health