Columnar apples trees, called "the fad tree of the decade" by one Seattle-area arborist, have become popular because of their ability to grow in much smaller spaces than required by conventional apple trees. Columnar apple trees thrive in containers or in small backyards. Trim columnar apple trees and conventional apple trees for the same reasons: to develop a strong structure in young trees; to bring young trees into production at an early age; to increase production in older trees; and to keep branches from having to bear too much fruit weight.
Determine that the tree is genuinely columnar. Some home growers will try to train conventional apple trees into columnar trees, but it's a constant battle to keep pruning back branches that want to grow up and out. Three popular varieties sold as columnar apple trees are Golden (yellow fruit) and Scarlet (greenish-yellow) Sentinal trees and North Pole (Macintosh-like) trees.
Prune in early spring, after the danger from frost has passed and before any new growth starts. Prune only broken or dead branches or suckers during the warmer growing season.
Prune young trees lightly. Older trees, especially those not producing much fruit, can be pruned more heavily. In either case, cut away dead branches, suckers, downward-growing branches, rubbing or criss-crossing branches and competing leaders. If the tree is like a whip, with few extending branches, cut some of the leader off to stimulate growth. If you have an abundance of branches or small spurs, trim to about five or six branches or spurs, depending on the height of the tree, several inches apart.
Avoid "wound dressing." There's no evidence that it helps. Trees protect themselves naturally by creating a barrier at a cleanly cut wound.
Sharpen the blades of the cutting tools before pruning. Sloppy cuts can cause damage to the tree and delay healing.