Red Means Stop (Pruning)

Back when I was a baby arborist, I worked for an old-timer who told me that “The best time to prune trees is when the tools are sharp.” He also used to say “What doesn’t kill you makes you stronger,” even though he was beset with shoulder injuries, knee problems, and back pain. While he taught me a lot of useful things, I later learned that both of these old saws are dangerous lies.

Although tree-care companies have year-round expenses and need income in all seasons, a truly professional arborist knows there are certain times of the year when pruning should be avoided. Late spring between bud-break and full leaf-out is a key period of pruning abstinence.

The other no-pruning interval is from the time leaves start to change colour in the fall until trees are entirely bare. There are some good reasons to put away pruning tools at this time of year.

The Claw-Back Clause:

As days get shorter, deciduous trees and shrubs start to make a waxy layer between each leaf petiole base and the twig to which they’re attached. The wax is called suberin, which I mention because it’s also a popular make of vehicle that is the state car of Vermont. That’s what I read on the Internet, anyway.

Suberin will eventually plug the vessels that moved water and nutrients into, and sugars out of, leaves all summer. This blockage protects twigs from losing water over the winter. It also leads to the breakdown of green chlorophyll molecules, thus revealing the yellow and orange pigments already present in leaves.

Before the vascular tubes are entirely blocked though, trees “claw back” about half the nutrients from each leaf: nitrogen, potassium, iron, magnesium, manganese, and other essential elements. This recovery is quite important to the nutrient budget of woody plants.

Trees also move sugar, the product of a season’s worth of photosynthesis, out of the leaves. Much of the sugar clawed back from autumn leaves before they drop is transported down to the roots and lower trunk, as well as the branches, where it is stored as starch. In springtime, starch is turned back into sugar and distributed to developing buds and leaves. Pruning branches now will deprive trees of both nutrients and energy needed for the following year.

The Illness Angle:

All woody plants have internal defense systems that make special anti-fungal and antimicrobial compounds to fight infections at the site of injuries like pruning cuts. Readers may be familiar with the work of the renowned biologist and plant pathologist Dr. Alex Shigo, often called “the father of modern arboriculture.” Shigo described trees’ defensive processes as Compartmentalization of Decay in Trees, or CODIT. This “tree-mune response” becomes active in the early spring just before bud-break and is in full swing throughout the summer, after which it starts to shut down. By the time leaves are turning, the CODIT response is much attenuated.

Therefore, wounds made in the fall are at greater risk of being infected by fungal pathogens that cause persistent, or perennial, cankers. Nectria and Eutypella are the two primary genera of canker fungi, but there are more. And while spores from Nectria and other diseases are always present in the environment, they are most prevalent in the fall. In addition, the long rainy spells typical of autumn further raise the odds of pruning-wound infections, given that canker spores are spread by rain splash.
A few infected trees manage to live to old age with their ever-expanding perennial cankers, but for most, infection means a shorter lifespan. Perennial cankers disrupt vascular flow, and they create weak spots in trunks that are often the point of breakage. They also greatly reduce a tree’s commercial value.

The Best Times for Pruning:

Early spring prior to bud-break is the ideal time to prune. Not only is CODIT up and running, there is little chance that pruning sites will dry out too much, leading to bark cracks near the wound, as can happen with early winter pruning.
But the truly essential reason to stow the lopper and saw while leaves are expanding is that when trees are “busy” pushing out leaves, CODIT goes on a coffee break until full leaf-out. It’s not that it turns off completely, but due to a hormonal shift in a tree, which happens as leaves are forming, its defenses are weakened temporarily.

The Exceptions:

‘I’ before ‘e’ except after ‘c’ (except words like “height,” “seize,” etc.). Exceptions can complicate things.

Obviously, some pruning may need to be done right away to address safety concerns, regardless of the season. In such cases, my one-time mentor is right: the best time to prune is when the tools are sharp. But if the goal is to boost aesthetics, or get more light on the garden, or fewer leaves in the pool, or to lessen disease pressure in fruit trees, fall pruning should be off the table.

And then there’s oak wilt. To help prevent the spread of this devastating disease, the New York State Department of Environmental Conservation (NYSDEC) is asking us not to prune between April 15 and July 15, when the risk of spreading oak wilt is extreme. Depending on the year and where you live, mid-April might still be ahead of bud-break. The NYSDEC even suggests that to be on the safe side, to wait until October 1st to prune oaks, a time that overlaps the period of color change. Since oak wilt is far worse than perennial cankers or lost nutrients, oak-wilt prevention should always take priority over ideal pruning windows.

Do Conifers Count?

Although pines and spruces only lose a small portion of their needles in the fall, the same principles hold true for them. Cutting evergreen branches in autumn will rob trees of a share of the nutrients and sugars that they need. Another reason it’s best to cut conifer branches in late winter is that during July and August, a moth called the pitch-mass borer sniffs out fresh wounds in which to lay her eggs. Her babies become an issue the next spring, as they tunnel under the bark to feed on sap. The grubs enlarge the wound diameter, and trigger excess pitch accumulation. If you’ve ever noticed unsightly, oozing, pitch on the trunks of pines and spruces, it’s a result of summer pruning that invited pitch-mass borers to set up housekeeping. The pitch-blobs they leave behind can persist for years, diminishing a tree’s aesthetic appeal.

The Moral Imperative:

The last reason to stash the saw until trees are fully dormant is the danger of running into a self-righteous arborist who views fall pruning as a moral failure. They may give you the hairy eyeball, or worse yet, engage in “pruning shaming.” I’m not saying that I’ve ever done that kind of thing . . .

By Paul Hetzler, Copyright 2023

Paul Hetzler is now writing about nature for "The Saturday Evening Pos" and, of course, TI Life. He is a former Cornell Extension educator, and he also writes books!  Be sure to check out the description of his books and I promise you will not only smile but you will want to give them as gifts for the next birthday - Just the titles makes you smile! Head of the Class: Smart as a Slime Mold, Shady Characters: Leprechaun Trees, Plant Vampires and Caterpillar Soup.

And we certainly agree with Paul's email signature: Paul Hetzler has been an ISA-Certified Arborist since 1996. For shame-free consultations in the Ottawa-Gatineau region, call 613.255.4966.

Marie-Anne Erki

Marie-Anne Erki is TI Life's illustrator and accomplished artist. She is also Professor Emeritus of Civil Engineering at Canada's Royal Military College where she taught for twenty years. We are fortunate to have Marie-Anne offer to illustrate Paul Hetzler's articles - and I hope you enjoy them as much as we do.

If you wish to see all Paul Hetzler's amazing nature articles and Marie-Anne's imaginative drawings - all at the same time, just push here!

Posted in: Volume 19, Issue 10, October 2024, Nature, Essay