Trees Need Leaves: How Biology Helps Us Understand How to Prune and Why We Should Not Top Trees
This is not what you want your tree to look like post-pruning. Credit: Vancouver Sun
As any quick internet search will tell you, pruning is both an art and a science. Whole courses and workshops are taught on pruning. There are a lot of fine details on proper pruning, with different techniques being taught for different kinds of trees. We’re not here to get that complicated. Most of us don’t need to be pruning pros in order to take care of our trees properly. But there is one critical thing everyone needs to know: topping trees is bad.
“Topping” refers to cutting off the uppermost parts of a tree’s canopy, leaving the remaining branches leveled out at the top. Topping triggers cascading stress reactions in trees--and high stress levels, much like in humans, causes health and performance issues.
One of the first reactions is starvation. Topping causes trees to lose a significant amount of leaves. Leaves are where trees transform sunlight into nutrients that feed the tree. When they lose a significant amount of leaves at once, survival mechanisms kick in to prevent starvation. Trees are forced to activate dormant buds and grow new leaves as quickly as possible. This process takes energy, often forcing trees to tap into their energy reserves. If they don’t have enough energy stored, trees are weakened and may die.
Credit: Vancouver Sun
The new growth that trees put out after being topped are weaker than the branches that were lost. Weak branches are at a much higher risk of falling from windstorms because they aren’t anchored as deeply in the tree. While some people may top trees for fear of their height making them more unstable, the risk is actually greater once trees have been topped.
When large branches are also part of the topping, trees become more susceptible to disease and decay as well. Trees can recover after minor pruning, but with multiple cuts to heal and new growth to rapidly put out, the overall rate of repair slows. This is also when trees are most vulnerable to insect infestations.
There are a lot of scientific reasons why topping is bad and shouldn’t be done. But one of the most persuasive reasons is actually aesthetic--topped trees are ugly trees. Trees have natural graceful forms that are unique to their species. Topping causes trees to lose that natural form, forever. Topping trees is the equivalent of creating a beautiful statue and then jaggedly cutting it off mid-shoulder. It doesn’t look right, it doesn’t look graceful, and no one’s yard is improved by it’s presence.
Removing branches that cross other ones is another general pruning rule.
Topping is often done by people looking to shorten the height of their tree or by utility companies in the right-of-way to make sure trees don’t grow into the wires. There are simple fixes for both of these. Other pruning practice, and the other can be avoided by planting the right tree in the right place. Sometimes, it’s easier to remove the tree that will need to be continually managed and replace it with one that is appropriate for the planting space. A list of utility appropriate trees in the Tacoma area can be found here (add link).
Most trees do not need to be pruned most of the time. Unless you top a tree, then you will have to continue to prune, though it is possible to do corrective pruning that helps the tree grow in a healthy way that is consistent with its form.
The priority with pruning should always be the removal of dead and dying branches, and to never remove more than a third of the canopy in a year.
If you must prune, here are a few basic tips:
Start your pruning with any branches that are dead, diseased, or dying. You can prune dead and dying branches any time of year.
Prune your tree based on when it flowers. Trees that flower in the spring should be pruned immediately after flowering. Trees that flower in the summer should be pruned during late winter or early spring, before new growth forms. For trees that aren’t grown for flowers, you can prune in the late winter or early spring as well.
As a general rule, avoid pruning from late summer to early winter. Plants can still create new growth during this time, that won’t have long enough to mature before winter.
Prune branches to the collar.
Remove branches that cross and rub against each other.
To see some good examples of what not to do while pruning, check out the @tacoma_wtf_trees on Instagram. We’re not associated with it, but we do appreciate their work finding such infamous Tacoma trees. We promise you won’t be disappointed.
And lastly, for any pruning questions you might have, feel free to reach out to us or to our local chapter of Master Gardeners--they’re an amazing resource and will be all too happy to help. We’ll also be adding a video about pruning to our YouTube page soon too, so stay tuned and check out our current offerings.
Grow strong, Tacoma!
With love,
Tacoma Tree Foundation