I went to a pruning demonstration in town on Thursday night, in windy temps in the 20s. Brrr! Two women from the University of New Hampshire Cooperative Extension were there to tell us about equipment and to show us how to prune crabapples or any tree in the malus family, as well as lilacs, rhododendrons, junipers, forsythia, and others.
Equipment
Pruners: They recommended either the #6 or #2 Felco pruners. The #6 is for men or anyone with large hands, and the #2 for smaller hands. They are expensive but can be comfortably used for up to 6 hours of continuous pruning. (Mine are Fiskars, for $18, and I love them, but they do stick sometimes.) A pruner shouldn’t be used to cut branches larger than 3/8″.
Loppers: For branches larger than that, use a lopper, which can cut a maximum of 2″. If you get the Felco brand lopper, they’re about $90, but I have some very durable ones (Ace Compound Anvil) that I’ve been using for years that cost about a third of that.
Saws: For branches larger than 2″, and for tight spots, a pruning saw is called for. There were an assortment at the demo; I use the Felco 600 folding saw (about $30) and think it does a grand job. For anything more than 4″, you’d need a bigger saw, and for high branches you might want a telescoping saw, though it can be awkward to use.
For most trees, you will be using primarily the saw and lopper; for most shrubs, the lopper and pruners, but I find that on old rhododendrons, I need the saw pretty often.
We were also told to always wear leather gloves when pruning (or risk losing part of the thumb), though I don’t.
Pruning
Main rule of pruning is that pruning ALWAYS promotes growth. So it’s a bit tricky to prune a tree or shrub or make it smaller, but it can be done, usually over about 3 years, cutting down (quite far down, not just a little topping trim) some branches that are too tall each year.
Don’t prune more than 25-30% of any tree or shrub in one season.
Prune the 3Ds first: dead, diseased, and dying. After this, prune crossing branches that are rubbing against each other (prune one, so that there isn’t any rubbing.) After that, look for “watersprouts,” those annoying little upstarts that shoot directly upward from a limb, and cut them out unless you feel there is a reason to keep them (e.g., if the shrub is not very healthy and needs all the limbs it can grow). Watersprouts will not fruit well.
Prune to mimic the natural form of the plant — so know what the natural form is. Some are naturally free-flowing and leggy (forsythia, e.g.), some are shaped like a pyramid, some droop, some grow in horizontal tiers (like a Pagoda dogwood), some are tighter and more controlled by nature.
Remember that making big cuts — pruning large limbs and sections — gives you less bushiness and helps to keep the form correct.
Always make clean cuts at the outside edge of the branch collar (see what it looks like) so that it will heal over in time.
When to prune is partly determined by when buds set on the plant. Usually you would prune spring-flowering trees and shrubs in early to mid-summer, after they have flowered but before their buds for the next year have set. So crabapples should be pruned before June, when their buds set for the next year (malus trees in general are pruned from mid-March to the first of May here in NH). Lilacs can wait until July, as their buds set in early August. It’s best not to prune malus in the fall or early winter as it can make them prone to severe winter damage.
You can, and should, prune the 3Ds (dead, diseased, dying) any time. And you can prune-spring flowering shrubs and trees in early spring if you want, but remember that you will lose some flowers on the limbs you lop off. If in doubt, check to see when your tree flowers and prune after that, but remember that some shrubs (notably some hydrangeas) set buds on old wood.
You can cut a lilac, spirea, forsythia and most other healthy perennials down to about 6″ from the ground and it will come back, healthy, happy and probably more compact.
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Some resources you might find useful (all short PDF documents):
Training and Pruning Young Apple and Pear Trees