Why prune?
For plant health:
When woody plants branches cross, they rub up against each other and injure the bark. This becomes an entry point for diseases and pests, so pruning out crossing branches helps to keep the plant in good health
Prune to open up air circulation; this promotes health by making the plant less attractive to mildews
Prune to rejuvenate the plant
Prune to train the plant
Prune the way you want the plant to look
Prune so the plant will grow as you intended it
Prune to control size
Pruning cuts: Look at the illustration I found. You want to make the cut on an angle, not straight across. This has to do with not drying out the branch. If you are not taking out an entire branch, prune to a bud that is pointed in the direction you want the branch to grow. Cut just above the bud, about ¼”, on an angle. If you are taking out an entire branch, especially if you’re pruning a tree, cut close to the trunk but don’t damage the branch bark ridge. This is a ridge of cells that forms an upside down U on the top of the joint where the branch joins the trunk. You want to make a clean cut that leaves this intact, but that doesn’t leave a stub.
You’ll need a pair of good, sharp pruners for this work. Personally, if the pruning means I’d have to use loppers – extend my arms that way – I’d rather hire someone to do it than risk making sloppy cuts.
It’s worth cleaning and sharpening your pruners before you start. Clean them with some rubbing alcohol and sharpen with a whetstone. If you are pruning any diseased plant parts, clean your pruners before you move on to the next plant
The first rule of thumb for pruning flowering trees and shrubs is: if they flower in the spring, don’t prune till after they flower. These plants flower on last year’s growth, and they’ve already set their buds. If you cut now, you’ll lose the blooms. If they flower later in the summer, they probably flower on new growth, so you can prune early and not lose blooms.
If you don’t know whether your plant is early or late blooming, blooms on new growth or old, use your search engine to find out by typing in ‘prune lilac' or 'prune hydrangea' or whatever. You’ll get lots of information.
Wherever you live, now’s the time to prune away dead or crossing branches in shrubs and trees. You want to walk all around the plant – obviously this is easier if it doesn’t have leaves, but don’t let leaves get you off the hook – and identify the dead, damaged, diseased, and insect infested wood first. Remove those branches. Then determine if there are crossing branches that are or will soon be rubbing against each other. Choose which of these is easier to prune, and cut it right to the branch or trunk; see above for pruning cuts. You can also remove water sprouts – those funny looking branches that grow straight up. In addition, if the plant is too dense in the middle, it’s a good idea to thin it out a bit.
If you live where spring hasn’t sprung, and your trees and shrubs are still dormant, now is a good time to prune to rejuvenate older shrubs, to trim evergreens, to clean out storm damage, and to improve aesthetics. If spring is underway, be a little more careful, but if hte plant needs it, prune it!
Renewal and Rejuvenation pruning: Say your old red-twigged dogwood has lapsed into tan or brown twigs, say it’s gone a little shapeless, say the pruners are sharp. Renewal pruning involves cutting out about a third of the oldest stems. This gives the young, redder stems room to grow, better air circulation, and less competition. You can do this with lilacs too. In fact, there are plants that will be happy with a rejuvenation pruning, where you cut back the whole darned thing. The roots are still alive, and the plant will send out new shoots and reinvigorate itself. Plants in this category include Rose of Sharon, Lilac, Buddleia, Spirea, Forsythia, Honeysuckle, and Privet. This might be a strategy for an overgrown border or a distant shrub mass. Closer to the house, you’re probably better off doing renewal pruning in a 3 year cycle.
As for trimming evergreens, the biggest mistake people make is in pruning them so they’re broader at the top than at the base. This deprives the lower half of the plant of sun and results in those plants you see around the foundations of some homes where there’s abundant needles on top and bare legs beneath. You need to prune evergreen hedges so that the base of the plant is slightly wider than the top. If my keyboard would draw, I’d draw it for you. It’s best to trim evergreens before the new flush of growth in the spring, but if spring’s sprung for you, go ahead and do it anyway.
A note on evergreens: don’t ever cut past where there’s green. Many evergreens have a ‘dead zone’ close to the trunk or main stem, and if you cut away the live growth, they won’t regenerate.
Pruning for aesthetics is, of course, a matter of taste. The first rule is to have a goal. Decide what you want the plant to look like – do you want it not to sprawl over a walkway? Do you want it to grow more toward the shed? Do you want it to look like a pyramid or Mickey Mouse or would you rather have it follow its natural form? One thing to avoid in all cases is the buzzcut. If you still don't have leaves on plants in your area yet, walk around and look at those shrubs that have been buzz cut. You'll know them because they have a tangle of small sticks and twigs at the top of the plant, where last year's buzz cut spurred the plant to grow wild but only at the very top. If you prune more carefully, you can avoid this condition, which isn't good for the plant or the viewer.
Stand back and assess the plant, and then prune slowly, standing back after every couple of cuts to ensure you’re not overdoing one side or getting a little too enthusiastic. Is the tree or shrub retaining the form you want? Is it symmetrical? Is it beginning to look hollow? Don’t take too much – you can always go back tomorrow and prune some more, but only nature can produce the new growth.
If you live where spring is in progress, wait till your spring-flowering shrubs have finished before pruning them. Then be guided by your purpose – to ensure the plant’s health, to train it to a particular habit, to renew it. It’s also smart to deadhead spring-blooming shrubs so that next year’s blooms will be abundant. For many, just snip off the spent blooms. For shrubs that bloom on long stems, take the whole stem; forsythia, for instance, can be deadheaded to just below where the flowering began, and the same applies to spireas.
If you haven’t pruned your summer-flowering shrubs – rose of Sharon, potentilla, late hydrangeas – do it soon. Otherwise they’ll be setting buds on this season’s growth. Again, if you’re not sure when a plant blooms, do a little internet research.
Let me know if you’d like me to share what I know – which isn’t exhaustive – about pruning roses. Since I mostly only grow the very hardy ones that need almost no pruning, my contribution, such as it is, will be more book learning that field smarts. Cut away dead canes. Cut crossing canes. Cut to a bud. Cut for future growth. But I really don’t know much about hybrid teas – the roses that require the most care. Again, an internet search can’t hurt.
In the meantime, those of you in the joys of spring, happy, happy gardening. For the rest of us, it is, meteorologically, spring, and I’m sure we’ll get the real thing sooner or later. Patience is, after all , a gardener’s virtue.
