Fall Pruning: Think twice

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Fall has always been considered the season of harvest. Images of corn, pumpkins and the bounty of Thanksgiving all create moving symbols of autumn. Fall brings to close another growing season, as plants prepare for winter by shedding their leaves or go into dormancy for a long winter’s rest.

Cooler temps and those feelings of closure signal to some that it is time to get out into the yard. Autumn projects include cleaning up the vegetable garden, the removal of faded annual flowers and the chore of raking leaves. Another chore that many people feel compelled to do in the cool, crisp days of fall is pruning.

Determine the right season for pruning the right plant
Is fall the right time to prune? We get asked that a lot. Before tackling any pruning project you might want to make sure that fall (September through November) is actually the correct time to prune. Even though plants are preparing for winter, that does not mean that it is a good time to prune.

Pruning, although a recommended practice, creates an injury. It is for that reason we must prune with care in the fall. One of the keys to proper pruning is to make the cut at the time of year when the plant can seal the wound as rapidly as possible. Unfortunately, fall is not that time, as the plant is going into dormancy and not focused on producing new tissue, which protects the injury created by the cut. Late winter and early spring are the periods of most rapid growth for plants and is the most desirable time to prune.

How trees recover from injury
It is also helpful to understand how trees recover from injury. Unlike people, trees really don’t heal. Instead, they seal, which is a completely different process. When we get a scratch or a cut we bleed a little, but then the wound develops a scab. Over time, the scab is slowly replaced with new skin and, with luck, there is no scaring. But the injury totally heals.

This process is not as simple when it comes to a tree. Once a tree develops an injury, whether made by a pruning cut or disease, the injury is sealed off by the tree. That means it is always there, simply covered over by new growth. Down the road, at a later date, you can cut into the tree and still see the sign of the injury, as it was sealed off by healthy plant tissue and wood. Arborists refer to this as compartmentalization. That is, the tree just surrounds the damaged wood with a layer of cells which function is to stop the injury from spreading.

Trees seal wounds through a process referred to as “CODIT” — compartmentalization of decay in trees. In simple terms, the tree develops a microscopic layer of cells around the damaged area. These cells prevent the movement of water and nutrients to the area through the xylem and phloem. The layer of cells is a maze of tissue which contains altered chemical compounds found in the tree. Some of these compounds can be toxic to microorganisms or other pests which like to attack trees through wounds.

This is a complex process which the tree uses to protect itself. Unfortunately many trees cannot stop the wound from spreading. This, then, is the source of where rot and decay occur. It is this decay that creates cavities, which at some point fail and lead to the demise of the tree. In summary, humans have the ability to grow new cells to replace the injured tissue. A tree does not have this ability. The tree simply seals off the injury and develops new wood to cover and provide protection.

That does not mean to never prune your trees but it does serve as a reminder that we must do it correctly. The goal of pruning is to improve the overall health and vigor of the tree, while inflicting as little injury as possible.

When you can prune trees and shrubs
Now that we understand about how a tree grows it should help us to understand that pruning should be best done heading into a season of rapid growth so that the tree can compartmentalize the wound quickly. For most trees that period is late winter or early spring.

With that being said, does that mean we should not prune in the fall? Fall pruning is best focused on the removal of dead, dying, broken or hazardous limbs. In fact, this maintenance pruning can be done anytime of the year, as safety of property and people is always the first concern.

What this does mean is that major corrective pruning to help maintain the overall health of the tree is best done during the dormant winter season or in early spring. During winter all potential damaging pests of the tree are dormant. Come spring, the tree rapidly grows to help seal off the wound and protect itself from harm.

In addition to trees, some people think the season of harvest is also good time to prune other plant materials. Good examples would be shrubs and roses. Like trees, pruning of these plant materials are best left till next season.

Timing for pruning shrubs
If the shrubs are flowering, then pruning is based on how the plant sets its flower buds. Some shrubs produce their flowers on the growth produced during the summer months. Then come spring the plants complete the development of the buds and flower. Good examples of shrubs that bloom on old growth are such favorites as lilac, forsythia and spirea. Pruning in the fall or even winter would remove the wood or growth that contains the flower buds. Thus the spring beauty would be lost.

These old wood blooming shrubs are best pruned within four to six weeks after flowering. This allows time for the plants to develop new growth and set flower buds for the following year.

Other shrubs bloom on what is referred to as new growth. This would be beauties such as butterfly bush, Rose of Sharon and crepe myrtle. Pruning on these plants is best done in late winter or early spring, as the flowers are set on the lush growth produced in the spring season.

Roses confuse a number of people. You have probably already guessed it but roses bloom on new wood. Rose canes can be subject to winter injury during a cold season. For this reason it is recommended to prune roses in late March to mid-April, just before the period of rapid growth.

Fall is the season of harvest and garden chores. As you can now see, pruning is maybe one of those tasks that might be best left until late winter next year. I am sure you can find plenty of things to do to keep you busy in the garden. But do think twice before deciding to prune, and do what is best for the plants.

Have questions?

The Garden Hotline is staffed by trained EMG volunteers and Extension staff who will assist you with questions.

Phone: (913) 715-7050

Email: garden.help@jocogov.org