I would think that the fall and winter would be the best time to trim lower branches off the trees around here. But how much can you safely trim without causing too much shock to the tree? Most are 15 -18 years old and it would be nice to be able to walk by a tree or mow under it, without ducking. Is there a set rule of thumb for how high you can trim? For instance if the tree is a 30â tall pin oak can you clear the bottom 10â?