As a teacher, I see my job as putting into words what many landscape professionals know from experience. What is obvious to the experienced pruner can be a mystery to the novice. One of the concepts I use is the notion of the Pruning Budget.
Different plants can withstand different amounts and kinds of pruning without suffering dieback, watersprouting or looking bad. That is what I call the Pruning Budget. Too bad the Pruning Budget isn't posted on the trees and shrubs! If the pruner overdoes it, punishment will be in the form of watersprouts-those straight, skinny, rapid-growing shoots that appear at the site of the pruning cut. Not only are watersprouts ugly, they are impossible to get rid of with pruning. Every time one prunes them off, they come back, except each time they do, there are more! Watersprouts will increase exponentially and lock the property owner and landscape professional into a high-maintenance, and unwinnable, battle against their plants.
One of the problems with teaching pruning is that trees and shrubs are three-dimensional, making pruning difficult to illustrate or photograph successfully. Not only that, pruning actually has a fourth dimension, the dimension of time, because plants can respond to pruning cuts as much as four months after the cuts are made. People new to pruning tend to judge their pruning job by how it looks when they are done, not how it will look by the end of the next year. This delay in response is responsible for much of the mal-pruning seen everywhere, in Boston, the Northeast, the rest of the country and, indeed, the world.
The Pruning Budget is determined by three factors: the kind of cut, the size of cut and the total amount of foliage removed. My analogy is a person's body weight, which is determined by what one eats, how much one eats and how much exercise one gets. A heading cut spends more of the budget than a same-sized reduction cut, and a reduction cut spends more of the budget than the same-sized removal cut. A big cut spends more than a little cut, and removing more foliage spends more than less.
For example, witch hazel and double-file viburnum are members of a group I call the tree-likes with a small pruning budget. That means you can only prune a little bit, using relatively small removal cuts and forget size reduction. On the other hand, forsythias have a big budget. They are in the cane-grower category and you can confidently remove up to a third of the foliage, using a combination of reduction and removal cuts, some quite large, and even by using heading cuts that remove canes completely by cutting them to the ground. In between are the mounding-habit shrubs such as a boxwood or barberry that can also have up to a third of the foliage removed, using mostly reduction cuts to tidy them up and reduce the size somewhat.
Cass Turnbull is president of Plant Amnesty in Seattle, Wash. She will be speaking at New England Grows in Boston, Mass., on Thursday, Feb. 7, 2013, at 2 p.m. At that session, she'll provide a complete listing of shrubs and small trees with their categories and "Pruning Budgets". For more information, visit