I read in numerous places online that Aspens must be pruned when dormant. The local arborist insists he can safely do it now in August. I also have some Choke Cherry trees. What are the facts?
Teton County Wyoming
Dormant season pruning is a general recommendation for most trees and shrubs, but there are reasons to prune during the growing season. Pruning to remove root suckers, diseased branches, broken branches, and dying branches should be done as needed when they show up.
Otherwise, general pruning is preferably done while the trees are dormant. This allows the leaves to complete their purpose of photosynthesis and providing food and energy for the tree. Pruning while foliage is present on the tree can stunt the tree's growth, but late season pruning is less stunting than early season pruning. Pruning after the leaves have dropped does not impact the plant's photosynthesis.
In my opinion, if the tree were pruned in August it would probably be ok since we are so late into the growing season. But it is your tree and if you prefer that the tree is pruned while dormant I think it is fair to ask the arborist to wait. For general pruning, I recommend dormant season pruning.
Thank you very much, Chris.