Paul Hetzler
The Advocate
Back when I was a baby arborist, I worked for an old-timer who told me: “The best time to prune trees is when the tools are sharp.”
He would also 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 things, I later learned that both of these old saws are dangerous lies.
There are, in fact, 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 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 the petiole base of each leaf and the twig to which they’re attached. The wax is called suberin.
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 out of the leaves. This sugar is the product of a season’s worth of photosynthesis. Much of the sugar clawed back from autumn leaves before they drop is transported down to the roots, lower trunk, as well as branches, where it is stored as starch. In spring, starch is turned back to sugar and distributed to developing buds and leaves. Pruning branches in autumn, therefore, deprives trees of both nutrients and energy needed for the following year.
The illness angle
All woody plants have internal defence systems that make anti-microbial compounds to fight infections at the site of injuries, like pruning cuts. The renowned biologist and plant pathologist Dr. Alex Shigo, often called “the father of modern arboriculture,” studied trees’ defensive process. He called it 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 types 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.
The best time to prune
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, which can lead to bark cracks near the wound, as can happen with early winter pruning.
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 trees that happen as leaves are forming, their defenses are weakened temporarily.
The exceptions
‘I’ before ‘e’ except after ‘c’ (well, except for 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, which can decimate oaks like Dutch elm disease did to elm trees. To help prevent the spread of this fearsome disease, the Canadian Food Inspection Agency recommends we not prune oaks 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, an optimal pruning time. Some authorities suggest that to be on the safe side, we wait until Oct. 1 to prune oaks, a time that overlaps the period of colour change. Since oak wilt is far worse than perennial cankers or lost nutrients, oak-wilt prevention should always take priority over ideal pruning windows.
Conifers count, too
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 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 to lay her eggs in. 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 might 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.
