Autumn is the time of year when many of us are tempted to do some serious pruning to reduce a plant’s size, remove dead wood or simply as part of the fall cleanup process. However, it’s important to know what should be pruned at this time of year, and what shouldn’t.
As you’ll see, it’s mostly “shouldn’t . . .
Here’s why: pruning stimulates new growth and doing that at a time when many plants are in the process of going dormant stresses and weakens them. If you prune while it’s still warm, the plant will send sap up the stems and the pruning “wounds” close more slowly than when they plant’s dormant. Woody stems have food-making ability and by removing them, you’re removing some of the plants ability to make food. Fall is also the time when disease spores and fungi spread, and having open wounds increases the risk of those diseases harming your plants.
If you’re growing roses, please don’t be tempted to cut them back too early because it can weaken the plant. Roses should be pruned while they’re dormant or just breaking dormancy and sending out new growth. This can range from January in the warmest zones to April in the coldest. And old rule of thumb on timing is to do it when forsythias start to bloom. Flower Carpet roses require no fancy pruning and can just be cut back with hedge clippers but hybrid tea roses and shrub roses require more specific pruning techniques.
Some woody shrubs that are grown primarily for their foliage can be pruned in the late fall once they’ve lost their leaves. If you don’t mind loosing the blooms which grow no old wood, Ninebark (Physocarpus opulifolius) can be pruned in the fall. Barberry bushes are easier to prune in the fall or winter once they’ve lost their foliage and you can see the thorns but in general, you’re better off waiting until late winter or early spring. Burning bush, dogwood, buffaloberry, sumac and smokebush can be pruned in late fall/early winter as well.
Spring and Summer Flowering Shrubs
Most spring or summer-flowering shrubs and trees set their new buds immediately after they bloom (generally by midsummer) so pruning them after mid to late June will result in dramatically fewer blooms (if any) the following year. This includes azaleas and rhododendrons, lilacs, forsythia, cinquefoil, spirea, magnolia, flowering plum and cherry and others. It’s fine to remove dead or diseased branches in the fall, but otherwise hold off on pruning these until mid-summer. Marie Iannotti, the Garden Expert at About.com provides a great tutorial on how and when to prune lilacs.
These can be a bit tricky because there’s no single rule for all. Because of that, it’s very important to know what type of hydrangeas you have. Some bloom on “new wood” while others bloom on “old wood”.
Pee Gee (‘Grandiflora’) hydrangeas (hydrangea paniculata) are upright deciduous shrubs and bloom on new wood, so you can prune these any time between late fall to very early spring and they’ll flower the same year. Examples of Pee Gee hydrangeas include those often old-fashioned varieties often seen in cemeteries along with Limelight, Pinky Winky, Quick Fire and Chantilly Lace.
Mophead and big-leaf hydrangea (Hydrangea macrophylla and Hydrangea quercifolia) bloom on old wood – meaning the previous season’s branches. These should only be cut back shortly after they flower – no later than mid-to late June in most parts of the country. These varieties include Endless Summer, Strawberries & Cream and many others.
The Hydrangeas! Hydrangeas! website offers a great tutorial on pruning all varieties of hydrangeas.
Deciduous trees are also best pruned once the trees are dormant. It’s important to wait until at least all their leaves have dropped, if not later. At that point you’ll also be better able to see the diseased and problem branches (ones that overlap, etc.). Evergreens can be pruned in late winter or early spring. By doing it at this time, the new spring growth will hide the harsh-looking pruned areas. Trees that produce a large amount of sap when cut should be pruned in the early summer, just after they’ve leafed out. Although the sap bleeding causes little or no harm, it can be messy. This group includes all varieties of maples, walnuts, butternut beech, and birch.
Other bits and pieces . . .
Do not prune lavender until new growth appears in the spring. After they’ve flowered you can cut them back to shape them and you’ll often get a second bloom. However, don’t cut into the old wood, as that’s the source of new growth. If your lavender plants get really straggly, you can cut them back by a third every few years.
Ornamental Grasses can be left standing to add interest to your winter garden, but if you’re in an area that gets heavy snow cover, you may find it easier to cut them back in the fall. We’ve found that small electric hedge clippers make this chore a breeze, especially if your grasses are more than a few feet in circumference. Spring is the time to divide overgrown ornamental grasses.
So, what SHOULD you prune in the fall?
Get rid of the dead wood . . . This is the time of year to remove dead branches, diseased areas or damaged wood on any trees or shrubs. Other than that, save your time and energy for that inevitable fall cleanup project and spend any extra time dreaming about next year’s garden!
There are a number of helpful step-by-step pruning books available. We recommend The Pruning Answer Book by Lewis Hill and Penelope O’Sullivan and Pruning Made Easy by Lewis Hill – both of which help take the mystery out of pruning!
Here are a few helpful pieces on springtime pruning tips and techniques:
Show Me How to Prune Roses (includes video)