Pleaching and pollarding are two special types of pruning used with street and shade trees. Pleaching is only rarely used today, but pollarding is common in street trees and parks.
Pollarding is a method of pruning back large trees to keep them small. The main branches of a pollarded tree are headed to the same spot on the primary branches near the main trunk once a year, usually in late winter. Vigorous upright stems arise from near the pruning cut. This forms a dense canopy of leaves and young branches during the growing season. Eventually the large branch ends form knobby, clublike stubs, which look strange when the trees are leafless.
This high-maintenance form of pruning can be used to give a small but formal appearance to large-growing trees. Many of the London plane trees along the narrow streets of San Francisco are pollarded. This keeps them within bounds on the city streets. Trees in Europe are also commonly pollarded. Actually, the practice was developed centuries ago in Europe as a way to grow and harvest firewood to make charcoal without killing a tree.
Branches can be pleached, which literally means woven, to create an allee or arbor. Almost any tree with strong, flexible branches that bend without breaking is suitable for this purpose. Beech and apple trees are ideal.
Build a framework of vertical and horizontal metal pipes or wood to support the branches over a walkway. Then plant a row of young trees next to the vertical posts on each side of the walk. Tie the main trunk to the horizontal support and bend it over the supports when it grows tall enough. Head the tip of the leader so that side branches develop; tie them to the overhead framework and interweave them with branches from the other side to form a leaf tunnel or allee.
Make an easy-to-build square or rectangular pleached arbor using four vertical supports of 3-inch galvanized pipe, cut to the desired height. Make the distance between the vertical posts appropriate for the size of the garden. Connect the four posts with an overhead frame of 2-inch pipe, spaced at equal intervals and crossed to make nine squares. As the trees at each post grow, remove any lateral shoots from the trunks. Bend and tie the vertical branches to the overhead pipe frame, interweaving them through and around to form a canopy of leaves.