Chapter 1: Anatomy of a Tree and a Bush
Certain characteristics make a tree, a tree and a bush, a bush. A tree is a perennial plant with a distinct woody main trunk and a series of branches growing from the main trunk. In addition, a tree’s branches form a crown, which is where the leaf and branch structures are located and what gives the tree its shape. It defines a tree and should be unaltered to allow the tree to form a natural shape. A tree also has an average height of 15 feet. A bush — or shrub as it is also known — does not have a main trunk and consists of a series of smaller branches that grow from the root crown. This is the transitional area at the surface of the soil where the roots turn into woody stems as they grow from the ground. Note: You should not confuse a tree with a bush pruned to resemble a tree.
The limbs and main trunk of a tree or bush serve two purposes: First, naturally structured limbs create an efficient circulatory system for the plant to transport water and nutrients from the roots of the plant to the leaves. Then, the limbs and main trunk hold the leaves at an optimal position to receive maximum exposure to light so that photosynthesis can occur. Photosynthesis is the process where the plant uses sunlight shining on the leaves to create a simple sugar to use as food and energy. This energy is then delivered down to the roots.
Removing too many leaves and limbs from a tree or shrub, so that it cannot produce enough energy to survive, can result in plant shock or death. That is why pruning is often done in stages, sometimes a year or more apart, so the plant has time to recover or heal between each pruning session.
The circulatory system of the plant depends on a process called transpiration because plants do not have a heart to pump water and nutrients from one area to another. Transpiration occurs as water evaporates from the pores in the undersides of leaves. As the water is pulled from the plant into the atmosphere, it moves from the roots of the plants to replace what is lost in the leaf structure. Some of the water is converted to sugar through photosynthesis and returned to the roots. The excess water is released through transpiration. A healthy and viable root system is just as important for the transpiration process to occur as a healthy crown, which is made up of all the limbs and leaves growing above ground. But, if there is too much top growth for the root system to support — because of low moisture levels in the soil, destruction of the roots by mechanical means, or root damage from disease or animals — the plant can suffer or die. This explains why pruning is often done to balance the amount of leaves or top growth with the available root system. Pruning done after someone moves a plant from one place to another, damaging roots in the process, provides on example of this.
Next is the internal part of a tree or branch. The rough brown or gray outer layer, known as the bark, covers the internal parts of the tree. The bark protects the tree from temperature extremes and also provides protection from insects and animals that can bore into the tree and disrupt the transpiration process. In young trees, you may only notice it as a smooth green or gray outer layer. Under the bark is the cork cambium, the layer of cells responsible for creating new bark. As the tree ages, the bark cells created by the cork cambium accumulate on the outside of the limbs and trunk and make the thick brown or gray corky layer recognized as the bark. A new layer of bark cells is created each year, creating a new ring — you can tell the age of a tree by simply counting the rings visible on a freshly cut stump. The next layer behind the cork cambium is the phloem, the layer of cells that conducts food from the leaves and moves it to areas of the plant that need food, often the roots. Behind the phloem is a thin layer of active tissue that produces new phloem cells on one side and new xylem cells on the other. It is known as the cambium layer.
The xylem is the entire inner part of the woody structure located behind the cambium layer; it is the supporting and water-conducting tissue of the plant. It can be separated into two sections: the sapwood and the heartwood. The sapwood is the place water and nutrients flow upward to the top of the tree or shrub during the transpiration process. The heartwood is the innermost layer and is dead tissue, but trees and shrubs depend on the strength of the heartwood for support as the tree ages. Most young trees or shrubs and their branches do not have an established layer of heartwood. Also, fast-growing trees, such as willows and some poplars, do not produce a thick or strong layer of heartwood, causing the limbs to break easily during ice and windstorms.
How a Plant Heals after Pruning
When a wound occurs on a large woody plant — and pruning is considered wounding the plant — the sapwood continues to send sap — a combination of water, nutrients, and sugar — toward the damaged area. The sap flows over the cut area and dries. The dried sap creates a barrier to protect the wound from common insects and disease. That is why the best place to cut a limb is near the main stem or trunk where there is a branch collar, an area that has robust cellular activity with strong sap flow and can, therefore, heal quickly and easily from mechanical damage. Also, the branch collar is located at an angle where water cannot collect and allow decay to establish.
For this reason, pruning must occur just outside the branch collar. Pruning flush against the trunk or main limb and cutting off the thick branch collar will damage the long-term health of the tree, allowing disease and insects to enter the main stem. If you prune by cutting somewhere along the branch instead of just outside the branch collar, the newly exposed end of the branch cannot heal properly and will begin to rot. The decay can spread down the limb past the collar and into the tree. Not until the limb completely decays and falls off can the root collar do its job of sealing off the wound. As you can tell, incorrect pruning is not conducive to the long-term health of trees and shrubs.
Left: Topping. Right: Tipping. Image courtesy of the USDA.
Left: Bark ripping. Right: Flesh cutting. Image courtesy of the USDA.
Stab cutting. Image courtesy of the USDA.
In situations where you cannot cut entire branches, such as fruit trees and rose bushes, you should make a cut a 1/4 inch above a leaf bud or leaf node growing in the desired direction, and you should make the cut at a slight angle with the top of the angle on the side of the cut that is away from center of the plant. This encourages new growth from the leaf bud in the desired direction, and the slight angle cut allows water to run off the damaged area. A branch cut in this manner heals at the leaf node where increased cellular activity helps seal the area for protection from insects and disease.