Some fundamental knowledge about how plants grow explains a great deal about the way they respond to pruning cuts. By understanding the basics of plant growth a gardener is able to prune more intuitively.
The Role of the Bud
Almost all new growth on trees and shrubs develops from buds on the branches. There are three types of buds: dormant, latent, and adventitious.
Dormant buds form during one growing season and remain dormant until the next growth period when, depending upon the type of bud, they grow into stems, leaves, or flowers. New stems arise first from dormant buds at the branch tip (called terminal buds). Side branches develop from the dormant buds on the side of a branch (called lateral buds). Buds located at the base of a leaf are called axillary buds. Some axillary buds produce side branches and some produce leaves or flowers. Not every bud actually grows into a branch, leaf, or flower. Some buds on young twigs remain inactive for many seasons.
Latent buds are dormant buds that persist on older stems; they remain at or near the surface of the bark as the branch gets larger. Both dormant buds and latent buds are strongly linked to the stem’s pipeline of water and nutrients by a connection called a bud trace. These latent buds are the plant’s insurance. Should a branch be cut or broken above a dormant or latent bud, a new shoot can quickly grow from the bud.
Adventitious buds develop where no buds previously existed. These sometimes grow after a branch is wounded or cut back to mature tissue. These buds differ from latent buds because they develop close to the branch surface from deeper mature tissue and are not connected by a bud trace; consequently, the branches that develop from adventitious buds are not strongly connected to the trunk or main branches and can be easily broken during a storm.
Improper pruning techniques, such as cutting branches back to stubs, often activate dormant or latent buds or cause adventitious buds to form behind the stub. For a number of years, the young shoots that arise from these buds are weakly attached to the parent stems and are easily broken. Not until new layers of wood form annual growth rings around the branches are they strongly anchored.
By observing the current season’s growth of a woody plant, you can see that few, if any, lateral buds close to the tip of the branch have grown into side branches. Not until a lateral bud is a long enough distance from the terminal bud — usually as a result of two or three seasons of growth — does it begin to grow.
This phenomenon, called apical dominance, is controlled by a hormone in the terminal bud that is known as an auxin. This hormone suppresses the growth of the other buds, signaling them to remain dormant. Buds farther and farther away receive weaker and weaker signals, until they are released from dormancy and begin to grow.
This hormonal effect determines a plant’s branching pattern and its response to pruning. As long as the terminal bud remains alive, it will be the first to grow in the spring. This natural system results in an orderly, controlled growth rate and gives a characteristic shape to all species of plants — but prune off the terminal bud and growth patterns become drastically altered.
The science of pruning lies in understanding and manipulating bud growth. Removing a terminal bud releases dormant, latent, or adventitious buds from the growth inhibition caused by apical dominance. Removing the terminal bud alters the orderly, natural growth patterns. Many of the buds behind the cut sprout into branches; where one stem once grew, now a cluster of many stems emerges.
This growth pattern is nowhere more apparent than on a sheared hedge or on a tree cut flat below an electric utility line. Not only does the plant lose its natural shape, but exceptionally fast growth creates a branch structure that resembles a candelabra.
On the other hand, the growth pattern of a plant is preserved by pruning back a stem or side branch to where buds have already broken dormancy and formed a side shoot. The terminal bud on the lateral branch may then assume apical dominance.