It is important to understand that these two types of cuts exist, and second, what response from the plant they provoke. When you read articles or any reference on pruning, directions will tell you to make one kind of cut or the other. Understanding these terms for pruning can be as necessary as knowing what a tablespoon is in cooking. It will help greatly if you first know what the nodes and internodes of a plant are.
Common to Both Types of Cuts
When you prune, you should virtually always be cutting back to a node. Nodes are not all created equal, though: some nodes have mature branches, some have just dormant, ungrown buds and other nodes have all kinds of growth in between, from the weak, flimsy, and misdirected to strong growth with potential.
A thinning cut goes back to a strong node that can take overgrowth without shock or interruption.A heading cut is the opposite, a cut to a node that is too undeveloped to grow strongly and smoothly replace the growth removed by pruning.
When to Use Thinning Cuts
Most of the cuts you make when pruning should be thinning cuts. An incomplete list of examples include:
Choosing certain branches, such as when training a climbing rose, choosing main branches of a hybrid tea rose, rejuvenating fast-growers like clematis and lilacs, and removing the branches between scaffold branches on fruit trees. Aesthetic pruning of a mature ornamental tree or shrub. For clearance, completely removing low branches.
Heading Cut
a) cutting a currently growing or one-year-old shoot back to a bud, or b) cutting a branch or a stem back to a stub or to a lateral branch too small to assume the terminal role. This is the opposite of a thinning cut. Unlike after thinning, the node you headed back is too small and weak to turn all the energy being poured into it into dominant growth. In broadleaf deciduous plants (excluding pines, spruces, cedars, and many other coniferous plants), a heading cut triggers a number of the top buds left behind to break dormancy and grow. The larger the branch, the larger the risk of a heading cut. They are not to be used as a matter of routine on large branches, but only for special purposes.
When to Use Heading Cuts
One bit of confusion is that thinning is also used to describe the practice of removing fruit from a plant to increase the quality of the remaining fruit. Take care when you see this term. Furthermore, when talking about the pruning cut, “reducing cut” is another term sometimes used to mean a thinning cut. Source: Harris, Richard W. “Clarifying Certain Pruning Terminology: Thinning, Heading, Pollarding.” Journal of Arboriculture 20(1): January 1994, 51–54.
