"Heading back" is one of those terms that means something quite different in a horticultural context than it does in everyday life, where it indicates movement, as in returning to a particular location.
When talking about gardening, "heading back" refers to pruning off the terminal, or "head," growth of a tree or shrub branch down to just above a lateral bud. "Lateral" means "side," and a lateral bud is one that grows from a node along the side of a branch. You will typically use pruners to make this cut.
Heading Back: A Primer
A heading cut preserves the lateral buds and encourages growth sideways, filling in the interior of the plant. When heading back a plant, you're pruning off its leader, thereby suppressing upward growth. The "leader" is the primary stem of a plant: the trunk of a tree (or tree-like shrub) or the main stems of a multi-stemmed shrub.
However, be careful—you can spoil the natural shape of a shrub by heading back. Sometimes, though, a gardener prunes not because he wants to, but because he has to. This is the case when, for example, a storm snaps the tip of a shrub's branch, leaving it hanging. In this instance, the branch is crying out for a heading cut.
Heading back can also be used to keep a shrub within bounds, such as when you have exercised poor plant selection and located a shrub in a spot way too small for it, or alter its shape. Regarding the latter, you can sometimes cut above a bud facing away from the center of the shrub to generate growth in that direction. This knowledge can come in handy if you're trying to correct the shape of a bush that has suffered damage or that has not been properly cared for in the past.
Sky Pencil holly is an example of a plant that can profit from a heading cut. The habit of this shrub is columnar, and heading back the plant won't spoil its natural shape: It will keep it shorter and bushier. Pruning just above a leaf node will stimulate new growth.
Because of Sky Pencil's rigidly upright growth habit, the new branch thus generated will stick straight up, rather than out to one side or the other. There is a node on each side of the branch, so you can cause two branches to grow with your cut where, before, there was just one branch (thus making the shrub bushier).
Types of Pruning Cuts
Heading back is only one type of pruning cut. Beside heading back, other kinds of pruning cuts include:
Pinching doesn't require using tools such as pruners. Instead, you literally "pinch" off the growth at the top of a plant's stem, using your fingers. For example, gardeners commonly pinch annuals, such as coleus, and perennials, such as chrysanthemums, up to a certain point to keep them bushier in the summer.
As with heading back, thinning requires the use of pruners or some other cutting tool, such as a bow saw. However, the similarity largely ends there; these two pruning operations are undertaken for very different reasons. As an example, consider a thinning cut to be made on a landscape tree. Possible reasons to prune off such a branch include:
- Opening up the canopy of a tree to create more space between the branches, just as when you "thin out" vegetable seedlings to increase the space between them.
- Removing a branch that's rubbing up against another one (in which case it's best to determine which branch you would rather keep and remove the other one).
Thinning cuts are also commonly made on shrubs, sometimes for the same reasons. In addition, sometimes you can rejuvenate a shrub, such as lilac, by entirely pruning out some of the older branches—one-third at a time over the course of three years—thereby giving higher priority to the newer branches. The energy that would've gone into the older branches is better put to use in the younger, more vigorous ones, which thereby grow even more vigorously.
Gardeners often make thinning cuts on forsythia shrubs for this reason. Homeowners who have a vague feeling that they should be pruning such a specimen shrub—but who don't know precisely why or how to go about it—will most often be better served by making a thinning cut on it rather than by heading it back.
Shearing is most commonly practiced on topiaries and hedges, such as those formed by boxwood shrubs. You may use a power hedge trimmer or manual hedge clippers for the job. The purpose behind shearing a hedge is to keep its growth within the desired bounds and make the hedge as dense as possible by stimulating new growth.