Don't Prune These Plants in Fall

Welcome to fall, where most of your gardening to-do list involves chopping back the flowers, shrubs, trees and other growing things in your yard. Pruning is such a big part of fall cleanup that it may come as a surprise to hear me ask you to put down the clippers down for a moment, because there are some plants that should never be pruned, at least not in the fall.

What is the purpose of fall pruning?

Understanding the philosophy behind pruning is helpful when you're out in your garden, wondering about the right course of action for a particular plant. There are multiple reasons to give your plants a chop: For trees, you can remove limbs threatening to encroach the street, your home, or the sidewalk. You can also shape shrubs and trees via pruning, but that is often cosmetic.

A better reason to prune can be to ensure that sunlight and airflow can penetrate through the middle of shrubs and trees, by taking off branches that cross others or are growing inward. Pruning encourages branching, meaning that a stem may often split into two stems where it is pruned, and that can have a positive effect on cutting flowers and fruiting vegetables like peppers. Lastly, pruning can help your plant survive extreme conditions like wild temperature swings by allowing the plant to focus on the roots, where energy is stored, instead of using that energy to support leaves and stems above ground.

When should you prune your plants?

Generally, you'll prune when a plant is dormant and dry, and plants go into dormancy during different seasons, although winter is normally consider a safe time for most. You can prune outside of that season if necessary, for example, to trim off a dangerous bough, but you want to be sure to offer more support to the plant if you do. Cut trees are more susceptible to virus and fungus in summer, for instance.

When pruning, the rule is to never take more than one third of a plant, which may seem like a lot, but most plants truly benefit from a hard chop. An excellent example are roses, which thrive after a winter where you've cut them all the way back.

Reasons not to prune a plant in fall

So when should you not prune a particular plant in fall? There are a few reasons to avoid it. The first is that it might not be the right time to do so-a good way to guess the dormancy period for a plant or tree is to look at when it flowers. Plants flower, fruit, and then go into their dormant period, so post-flowering or post-fruiting is usually a safe time to prune. Some plants are singular bloomers, like sunflowers; by pruning them, you remove the possibility for more blooms. Some plants simply do not need pruning at all, like annual mums. Lastly, a great reason not to prune is that plants, once they flower, produce fruit and seeds, and wildlife feed on those spoils through winter. Leaving a plant alone means you're supporting the local ecosystem.

Never prune these plants in the fall

• Azaleas, Rhododendrons, Camellia: People seem to adore flowering trees and shrubs like this trio. I have suffered through all three in my yard, and my ten year campaign to kill the azalea finally ended just this year. These trees tend to bloom in one flush, once a year, for a few weeks, and while those few weeks are quite lovely, the trees then proceed to unceremoniously drop all those blooms. This happens in spring, and so the time to prune them is summer. If you prune them in fall, you may well be affecting how many blooms you'll have next year. (Which may be fine if, like me, you'd prefer to not scrape camellia blossoms off your patio for a month.)

• Lilacs: Lilacs are delightfully tricky characters in your yard. Usually, they'll have one flush in spring and will fill the air nearby with the most intoxicating smell on earth, and then go back to being uninteresting shrubs. Most gardening reference sources will imply that if you give lilacs a summer trim, you may encourage a second flush in fall. I have never been successful there, but you can try. Either way, the appropriate time to thin and shape your lilac is in summer.

• Forsythia: One of the most electric plants in your fall garden, forsythia lights up like a firework in yellow and orange. But if you want that display, you'll need to leave the plant alone in fall.

• Flowering Pear, Cherry, and Crabapple: These non-fruiting trees create magical snowscapes of pink and white blossoms in late spring. The display is so beautiful, you'd be a true idiot to do anything to diminish the volume of blossoms trees like a flowering cherry will display in spring. These trees need their little flower buds to overwinter, so chopping them in fall is a no-no.

• Hydrangea: A divisive garden plant (people either adore or despise hydras"), this plant comes in a number of sub species that all require pruning at different times.The big blue and pink blooming hydras generally should be chopped back in May, which means that the buds that overwintered and survived are safe. Limelight hydras should be pruned after the new year, as we head into spring. Only oakleaf hydras should be pruned in fall, as the flowers begin to age and turn color.

• Echinacea and Black Eyed Susan: These perennial flowering shrubs can take a chop in fall, but you should consider sparing them. The seeds in the center of these plants can feed armies of birds all winter long, and won't do anything to harm the plant itself.

• Ferns: While your green leafy friends may look a little sad during the winter, losing color and vivaciousness, they do not enjoy a fall prune. If anything, wait to prune your ferns until late winter or early spring.

• Ornamental grasses: I love a plant that is self-sufficient, and ornamental grasses do a great job on their own. They require no chops or pruning for winter and will survive just fine. In fact, leaves that die will merely shrivel up and compost in place, feeding the plant.