Olives often exhibit alternate bearing patterns, so if you are growing the tree for edible rather than simply ornamental value, you should work with these patterns when pruning. Alternate bearing means that one year the tree fruits heavily, while the next it fruits very little to compensate and build its energy back up. During heavily fruiting years, the tree puts out more buds and fruits than it can manage, so prune then to lighten its load. Though you can prune in spring or summer, summer is better because blooms will indicate where fruit is going to set. During the off years, when it is recovering, avoid pruning. Doing so will help even out the manner in which the tree produces.
Olive trees need to be trained to their shape, but you shouldn’t do it until they are about 4 years old. After that, prune to let light into the center branches of the tree. Without enough sunlight penetration, olives will fruit only along the uppermost branches rather than all along them. Remove limbs that block other limbs, encouraging an open, breezy canopy.
If you have allowed an olive tree to become neglected and overgrown, you will need to prune it fairly seriously to get it to fruit again. Because olives tend to grow upright, when you need a broad, spreading canopy for best fruiting, you will need to top them considerably. Unfortunately, it can be mentally tough for some people to cut a tree down to the 4 or 5 feet you’ll need in order to encourage them to grow properly again. You can space it out over three years or so to make it easier on you and less shocking to the tree.
Two types of pruning cuts exist: thinning and heading. Thinning means that you are cutting branches off from their base, making more room inside the canopy, while heading means you are cutting them off midway along the branch. Unless you are topping an overgrown tree or using olives for hedges, you should avoid heading cuts. The other exception is if olive wood has died or been damaged by disease or a freeze. Usually, however, thinning cuts will do the trick, encouraging a wide, branching canopy into which sunlight can penetrate. Always disinfect your pruning tools before and after use to reduce the risk of spreading disease.