I get asked a lot of questions about trees. But that’s okay because answering questions, about trees is one of my favorite parts of my job. One of the most common questions I am asked is, “When should I prune my tree?” My most common answer is simpler than you might think. The answer, “When the saw is sharp.”
“Wait just a minute,” you might say, “I’ve heard it’s best to prune in the winter when the sap isn’t flowing.”
Or another common one, “You have to wait until after it sets seed/fruit to prune.”
Or one you probably haven’t heard, “ You can only prune a tree after the waning of the moon in months whose second letter is shared by the genus name of the tree…”
Okay, so I just totally made up that last one. But it articulates an interesting point, which is namely, why prune a tree at a particular time? Or, more specifically, what are you trying to accomplish in doing so? Are you pruning for:
- the health of the tree?
- the showiest foliage/flowering?
- the most productive crop of fruits?
- The structural needs of the surroundings such as keeping branches off of your house, driveway, or other trees?
- non-specific reasons such as, “I just feel like it needs to be pruned?”
These are the kinds of questions I like to ask back when I’m asked, “When is the best time to prune my tree.” Because here is the interesting thing, each of the questions in the list above might have a different pruning prescription, and thus a different timing, associated with it. Let’s investigate this further.
When should I prune my tree to keep it healthy?
Before we talk about pruning for health, we need at least a basic understanding of how trees are different from people. The most relevant difference in this situation is that, unlike humans, trees do not heal themselves. If a tree limb becomes damaged or diseased, the tree goes to nearest proximal branch union (toward the trunk) and builds a wall at a predetermined structure called the branch collar.
Once this wall is in place, the tree no longer sends nutrients to that branch, nor can any pathogen travel the other direction into the tree. At this point, the branch begins to rot. We see this all the time with deadwood still attached to the tree. This is not bad for the tree, it is a natural part of its life cycle. (Though it may be hazardous to people recreating in your yard, more on this later.)
A way that I like to think of this is as follows. Say you cut your finger, your body begins producing a suite of hormones as well as physical clotting agents and sends them via your bloodstream to begin the process of healing the wound. Your cut begins to scab over, and if it wasn’t too deep, you’ll have some new, pink skin in a week or so. Healed, simple.
A tree, on the other hand, breaks a small branch in a storm. The branch is still intact, but its structure is damaged such that it's losing more energy than it’s making through its leaves. The tree builds a wall, its branch begins to rot, and eventually falls off. Walled off, also simple.
This strategy works great for a tree since, if a branch is equivalent to a finger, a tree has hundreds if not thousands, whereas a human only has ten (most of the time.) No need to expend significant energy healing when you have ready replacements.
The exact amount of damage that leads to the walling off and shedding of a branch varies from species to species. For example eastern red cedars hang on to broken branches and persistently continue to send nutrients until they’re all but fully severed, whereas tulip poplars readily shed branches, sometimes before damage is readily detectable.
Now that we know a bit about tree physiology, I’m going to share a pretty important secret with you. More often than not, trees don’t need our help to be healthy. The occasional dead branch in a tree canopy is not a sign of distress and leaving a decaying branch attached to a tree is not a vector for disease. Since they’ve built a wall to stop travel between the dead branch and the living tree, any decay can’t get into the vasculature of the tree (with the exception of a few unusually aggressive fungi that we’ll talk about some other time).
As mentioned above, trees and humans just aren’t the same. A gangrenous arm on a human is a real threat to that person’s health, because the rot that’s happening in the arm could get in the bloodstream and cause systemic infection and death, but the wall at the branch collar stops this from happening in a tree.
So, in conclusion, if you were to ask me, “Is now a good time to prune my tree,” and if I then asked you, “Why do you want to prune your tree,” and you answer, “It’s got some dead branches here and there in it, I think it’s unhealthy.” Then I’d likely tell you to leave it be, it doesn’t need our help to be healthy and anything we do to intervene with normal patterns of decay and shedding of unproductive branches is unnecessary and in some cases can even cause harm.
When should I prune the deadwood I see up in my tree?
So here’s the catch. If the tree discussed above were in a forest, or out in a field with no structures or other trees around it, I’d say leave it alone. In fact I’d insist, leave it alone. Take a walk through any forest or field to which you have ready access and you’ll find some big, old, majestic trees that have been growing very successfully without human help, and I guarantee they’ve got dead in them. They might even have large branch stubs where branches died years ago and have since broken out and rotted away.
I’d even argue that in some cases, the reason these trees have grown so large, and so mature is precisely because they’ve had little to no human intervention. It’s rare and marvelous to find a tree over 100 years old in a neighborhood these days. And it’s almost impossible to find one in a neighborhood built in the past thirty years. It’s this observation that is at the root of my insistence that we be very conservative in our pruning prescriptions and not intervene without good reason.
So, when then is a good time to prune? In the case of the deadwood we’ve been discussing, I’d say it’s when that deadwood presents a hazard. Note that hazard is a uniquely human framework through which to see the tree. Hazard has less to do with the tree itself and more to do with what is around the tree, or what might come and go underneath the tree. A great example of a situation where hazard wood is extremely relevant is a public park.
In a public park, people come and go underneath the trees and don’t think much of them unless something is obviously out of place. The average park goer simply isn’t conditioned to think of a green, living tree as a threat and thinks of it more in terms of shade, or beauty, or wildlife viewing. So if a group of people were to place their picnic blanket under a large dead branch that they can’t see or didn’t notice, they’re unwittingly exposing themselves to a foreseeable hazard.
At Valley and Ridge Tree Care, we generally define hazard as greater than 2 inches in diameter. At this size, we expect a branch falling from the upper canopy to have the potential to hospitalize an adult, not necessarily critically, but as far as I’m concerned hospitals are to be avoided whenever possible.
When should I prune my tree to reduce its hazard potential?
Now that we have a frame of reference for hazard wood, let’s put it into perspective. Hazard wood is a dead or damaged branch greater than 2 inches in diameter that has the potential to do bodily harm or property damage due to where the tree is growing. This means, the big dead branch growing out over your roof is hazard wood. And the old dead branch hanging over the section of the yard where your grandchild plays is definitely hazard wood. Or, one of my favorites (that I have legitimately encountered) is the dead branch on your tree that reaches out over your neighbor’s driveway where they park their sixty thousand dollar car out of doors. In any of these situations, the risk of the dead branch causing harm is legitimate, and even though I’ve already established that it’s not necessarily better for the tree, it’s surely the kind of thing I’d advocate we remove.
And here’s the cool thing. All this fuss about timing doesn’t even matter when it comes to dead hazard wood. The tree has already written it off, walled it off, and forgotten about it. So, if we perform our pruning cuts properly and don’t cut the branch collar or do any unnecessary damage to the tree (like climbing with our spikes on as though we are removing the tree), this kind of pruning can be done year-round, regardless of other goals for pruning.
There’s one thing I’d like to emphasize though, given all these things we’re considering. Just because a tree has aspects that are hazardous, does not necessarily mean the tree as a whole is hazardous. It is very much within the capacity of Valley and Ridge Tree Care to remove the hazardous portions of the tree and leave the tree standing healthy and now contributing positively to its surroundings rather than negatively.
In fact, this is a key component to our approach to tree care. It is my goal on all of our free at-home consultations to provide wherever possible, a pruning based solution to a tree problem. Rather than coming in, seeing some hazardous branches in a tree, and condemning the entire tree, I want to find a way to reduce or remove the hazard and allow the tree to continue contributing to the character of the property and the quality of life of those that live under its shade.
When should I prune my tree?
Essentially, we’ve switched the question of, “Should I prune the deadwood from my tree in order to keep it healthy,” to the statement, “I should only prune the deadwood in my tree when it becomes a hazard to myself (and other humans who hang out under it) or my property.”
The next question on our list, “When should I prune my tree for better foliage and showier flowers?” requires a different approach, which I’ll discuss, along with more details about tree physiology, in my next post. Whatever your reason for wanting to prune your tree, we at Valley and Ridge Tree Care are ready to help. After all, we care about your trees, it’s in our name.