Photo: This ornamental Okame Cherry (Prunus 'Okame') flowers on the previous season's wood, so live branches cannot not be pruned in winter or early spring without sacrificing some flowers. Nassau County Museum of Art, Roslyn.
When people say that trees don’t need pruning, I bet they are thinking of trees they have seen that were “butchered,” with branches and limbs indiscriminately cut and lopped off, with complete disregard for the tree or the viewer’s eyes. And they are right – trees don’t need this type of “pruning.” I too have seen far too many examples of this type of hack job, also known as topping. Not only is butchering terrible for the tree’s health and longevity, but any Joe Schmo with a big enough chainsaw can do it. This practice has tarnished the reputation of other highly trained and certified tree care workers who don’t do this malpractice.
Photo: Topping trees like this Callery Pear (Pyrus calleryana) is a form of arboricultural malpractice, and is one of the worst things we can do to a tree. Mineola.
Although poor pruning can lead to hazardous, aesthetically unappealing, poorly functioning, and short-lived trees, there are many good reasons why trees need proper pruning. Let’s look at the big picture – in general, most trees evolved in a forest, where competition for light and space is relatively high. These conditions typically produce trees that have a single trunk, thin and short lateral branches, and branches concentrated towards the top of the tree. But in landscape settings, the conditions are often the opposite – competition for light and space is relatively low, producing trees with multiple trunks, large and thick lateral branches, and long, overextended limbs along the length of the trunk. These qualities can lead to a structurally unsound growth form over time, and increased risk of storm damage as the tree ages. Therefore, one of the main goals of pruning is to develop a more “forest-like” tree form – a tree that has a single dominant trunk and smaller, shorter branches that can withstand high winds.
Photo: Trees that develop multiple trunks have a higher risk of splitting and breaking under high winds, like this 'Princeton' Elm (Ulmus americana 'Princeton') that has developed a hazardous crack. Oyster Bay
Other important reasons to prune a tree include providing clearance, improving a view, reducing shade or wind resistance, reducing risk of tree failure, maintaining health, and improving aesthetics. Whatever your reason for pruning is, it’s very important to establish a clear objective, because you don’t want to remove any more live branches than you have to. Live branches mean photosynthesizing leaves. If you are removing live branches, it means you are removing the food source of the tree. A healthy tree will be able to endure a short diet, but will not be able to endure starvation. So prune as few live branches as needed in order to get the job done. Some reasons that don’t justify removing any live branches include, you haven’t pruned in a while, you’ve gotta do something in the garden, you wish the damn tree would just grow faster!, the tree is declining and you don’t know what else to do, or it’s easier than removing or replacing the tree.
Once you have determined a good reason for pruning a tree, the next logical question is, how much should you prune? Which brings us to the next myth – it is NOT better to remove a whole bunch of branches all at once, than the same amount of branches spread over time. The fact is, removing live branches is stressful for any tree, especially very old trees. Being patient, and removing a small amount of branches during multiple pruning sessions over a few years, is less stressful for the tree than removing all the branches to be pruned all at once. As a rule of thumb, you should never remove more than ¼ of the live foliage in a single year.
As for timing, dead or broken branches can be pruned anytime – really as soon as possible is best. This is because pruning out dead branches will potentially prevent further spread of decay and disease, and will minimize the risk of branches falling on targets and people below. In general, it is best to prune live branches when they are dormant in winter or early spring, before the buds begin to swell. At this time, it’s easy to see the branch structure of deciduous trees and any defects that may be present. The impact on tree growth is minimized because the tree is dormant. Pests are also dormant, so it can minimize any potential diseases or insects. Furthermore, pruning wounds have the entire season to begin closing over.
The worst time to prune live branches is during or soon after the initial growth flush – a great amount of stored energy is expended to produce new foliage, shoots, and root growth every year. You want to allow at least some time for that new foliage to photosynthesize and produce new food and energy for the tree. Also, you want to make sure to not prune ornamentals in a way that minimizes flowering. If a tree or shrub blooms on the new, current season’s growth, then pruning live branches in late winter or early spring will not reduce flowering. But if a tree or shrub blooms on last season’s old wood, like the forsythia and flowering cherries, pears, and plums that are blooming now, pruning live branches in late winter or early spring WILL reduce flowering. It is best to prune trees and shrubs that flower on old wood after flowering and after the leaves have been out for a few weeks and photosynthesizing.
Photo: Properly pruned trees and shrubs will be both healthy and beautiful. Baiting Hollow
Another common pruning myth is that certain species of trees, including maples and birches, should not be pruned in early spring when sap flow is heavy. Sap flow, or “bleeding,” is not like human bleeding – although unattractive, it has little negative impact on tree growth.
The last pruning myth is that wound dressings should always be used after pruning to accelerate wound closure and reduce decay. This is another instance where trees are being anthropomorphized – covering a wound will not help a tree like a band-aid helps a person. In fact, pruning wound dressings and paint can actually negatively impact a tree by interfering with the pruning cut closing over properly. This practice was common fifty or sixty years ago, but better science continues to guide the practice of arboriculture.
When you prune your trees this season, avoid these pruning myths and pitfalls, so that you can create healthy, long-lived, and beautiful trees!
