Cone-bearing, needle-leaved and evergreen, pine trees (Pinus spp) vary in height between species as short as dwarf mugo pines (Pinus mugo), which grow 1.2 metres tall (4 feet), or as tall as white pines (Pinus strobus), which reach 30 metres (100 feet). Pruning pine tree leaders, or central growing points, restrict their growth temporarily. To stop pines growing to their natural height, regular, careful cutting back is required.
Restricting pine tree height involves careful, annual pruning. Pines have central growing tips at their tops, called leaders. Cutting these back reduces tree height and limits growth for one year. Prune leaders down to a strong bud, leaving a 20 to 30 cm (12 to 18 inch) stub above the closest side branches. To balance the tree's shape, cut back side branches to 10 to 15 cm (4 to 6 inches) shorter than the leader. Removing up to two-thirds of leaders and side branches doesn't harm trees, but don't cut into brown areas, leaving no green needles on the leader or branches.
Cutting back pine leaders stops them from getting tall, but excessive pruning leaves unsightly brown patches. Pines put on a spurt of new growth yearly, growing outward and upward from their branch tips. This new, green growth then matures, and inner stems and branches lose their needles, becoming dry and brown. Pines don't sprout new growth from these brown areas. Cutting away all the green growth opens up areas that remain brown for the rest of the tree's life, although outer branches may grow or be trained to cover the gap.
Limiting pine height is most effective when trees are young, and before yearly growth begins in mature trees. Pruning the leader can take place when planting a young tree. Young pine stems are easy to cut, and trees quickly resume a natural shape. In established trees, pruning should take place in winter or early spring while trees are dormant, as cutting growing trees during spring or summer can introduce fungal infections.
Topping is the practice of severely pruning tree crowns to reduce their height or overall size, cutting off parts of large branches and leaving stubs. Topping stops pines from getting tall, but destroys their shape and exposes their permanently brown interiors. It also risks shocking and weakening trees, making them prone to infection. Sunlight on previously shaded areas can scald and damage bark, inviting insect infestation and decay. In the long term, topping can result in tree death.