Your First Garden: What You Need to Know Before You Prune Shrubs and Trees

Proof that a little knowledge is a dangerous thing: My first year gardening, having read that pruning is important to hydrangea health, I vigorously cut back spent flowers and branches in early spring, crossing my fingers for another full bloom come summer. What I didn’t know: Timing is everything, and pruning doesn’t necessarily mean just cutting back old growth.

Bigleaf hydrangeas, which were what we had, need to be pruned right after they’re finished blooming, in the summer. I had waited far too long to cut them back. I had also made a huge mistake by pruning all the previous season’s branches (bigleaf hydrangeas flower on old wood). Oops. My mistake was costly. The next summer, we had healthy leaves on our hydrangea shrubs but no pom-pom blooms. Not one.

Pruning is clearly not for the faint of heart. Do it at the wrong time, on the wrong parts, and too much—and you’ll essentially be stymying growth as opposed to stimulating it. Which is why we’re going to make sure you know everything you need to know before you whip out the garden shears.

(Psst…now may be the best time to prune: See What to Do in the Garden in February.)

Is there a difference between pruning and trimming?

The easiest to way to understand the distinction between the two terms is to explain the motivation behind them. One prunes in order to stimulate growth, to get rid of dead or diseased parts, to thin out an overgrown plant, and, in the case of trees, to remove branches for safety reasons. One trims, in general, for aesthetic reasons. Another difference has to do with timing. You can trim whenever you want; when to prune, on the other hand, depends on the type of shrub or tree you’re dealing with. (Flowers, too, can be cut back; when to do it also depends on the specimen you have. See Your First Garden: What You Need to Know About Cutting Back Perennials in the Fall.)

When should you prune?

Prune deciduous trees in late winter/early spring (deciduous trees, unlike evergreens, lose their leaves in autumn and go dormant in winter). Same goes for deciduous shrubs. It’s easier to see what you’re doing when there’s less foliage. Furthermore, pruning is stressful to any plant; cutting a plant while it is dormant is less taxing. As for needle-bearing evergreens, they, too, should be pruned in late winter or early spring, when they’re dormant.

The caveat: Do not prune spring flowering trees and shrubs (e.g., dogwoods, magnolia trees, azaleas, lilacs, rhododendrons, forsythia, and yes, bigleaf hydrangeas) in the winter. These should be pruned immediately after their blooming cycle ends in late spring or early summer, before the next year’s buds set in.

What parts exactly should you prune?

Start by cutting off all diseased, dead, or damaged branches. Dead branches can be cut down to the crotch (where it meets another branch or the trunk); damaged or partially dead branches should be trimmed six inches into healthy wood. To figure out where the healthy wood starts, use your shears to gently chisel away the bark to reveal what’s underneath; if it’s green, it’s healthy.

You should also prune branches that cross or rub against each and remove those, too. If you’re pruning to thin out an overgrown plant, start with the older branches at the base. (Thinning it will allow more light to reach the center of the plant.)

Keep in mind that pruning works best for plant maintenance, shaping, and health; you shouldn’t prune to prevent height and size, as plants should be allowed to grow to their potential.

What are the different kinds of cuts?

There are two types of cuts to know when it comes to pruning shrubs and trees: thinning and heading. Thinning cuts remove branches and discourage regrowth; use thinning cuts, which entail cutting a branch down to the crotch, to tame overgrown plants. Heading cuts, meanwhile, are employed to encourage growth; to make a heading cut, prune at an angle to 1/4 inch above the bud.

Should beginner gardeners prune?

Yes! You’ll find on the Internet a plethora of how-tos for pruning specific plants—and you should seek them out as different plants have different pruning requirements. (I found this round-up of videos and instructional articles on the Fine Gardening site particularly helpful.) There are a few general rules of thumb when it comes to pruning, though. First, to avoid spreading diseases, you should disinfect your tools with isopropyl alcohol before you move on to the next plant. Second, pruning when the plant is dormant is preferable. Third, spring-blooming trees and shrubs should be pruned right after they’ve finished flowering; later-blooming specimens should be pruned late winter/early spring. Follow these rules, do research on the specific plants you’re looking to prune, and always err on the side of too little as opposed too much pruning, and you’ll have nothing to worry about.

For more on pruning, see:

• Landscaping 101: How to Tame Overgrown Shrubs
• A Master Class in Pruning Hydrangeas from White Flower Farm
• DIY: Pruning Pine Trees in Winter
• 10 Easy Pieces: Garden Pruners
• DIY: How to Clean and Care for Garden Pruners

For more in the Your First Garden series, see:

• Your First Garden: How to Start a Garden for Practically Free
• Your First Garden: 10 Compelling Reasons to Plant a Container Garden
• Your First Garden: The Easiest Vegetables to Grow