Q: Our new house came with a lovely old crab apple tree, but some branches are only 2 to 3 feet off the ground and shade a bed of sun-loving perennials. Is it okay to cut off the low branches? They’re 8 to 10 inches thick. — Andrea Schmitt, Canton, Mass.
A: Proceed with caution. Crab apple trunks are notoriously vulnerable to rot after large branches, like yours, have been pruned by inexperienced gardeners. Decay originating inside the cut may gradually, over the course of many years, penetrate to the center of the trunk. Even though crab apple trees with rotted, hollow cores have been known to live happily for decades, they can also split apart unexpectedly in a windstorm. It is wise to spread major pruning over several seasons. Never cut more than 20 percent of a crab apple’s canopy in one season: the tree will depend on nourishment from leaves growing on the remaining branches. And try to remove roughly equal amounts of branch wood from every side of the trunk to maintain aesthetic as well as physical balance. Rather than lopping off a major branch all at once, begin by working inward from the branch tips farthest from the trunk and closest to the ground. Look for the collar — the slightly raised area of bark where the smaller growth you want to prune joins the larger branch. Cut just in front of the collar; the tissue inside helps seal the wound. (There’s no need to apply tree paint or sealant.)
After several years of careful thinning, you will have elevated the base of the tree’s canopy without having cut any wood that’s more than a few inches thick. You will need to continue pruning a little every year: It is the nature of crab apple trees to keep reaching out close to the ground in their quest for sunlight. About those perennials: I would consider moving them to a sunnier part of the garden and replacing them with shade-loving varieties. Why risk overpruning a mature tree that is beautiful in every season?