Proper pruning techniques to get young trees off to a great start

(This article was first published in our Spring 2020 Newsletter)

We’re wrapping up another annual pruning cycle at the Arboretum. The majority of our pruning is completed in the dormant season when it is easiest to see the structure of the trees and when there is less likelihood of introducing disease.

Poor pruning practices as well as waiting too long to prune can have life-long impacts on tree health, so it’s important to know the basics before you embark with tools in hand!

Choosing your tools:

For pruning young trees, a bypass pruner and a quality handsaw are likely sufficient. Choosing the right tool for the job will facilitate clean cuts that seal quickly. For most cuts over half an inch, the handsaw will usually be the best tool, while anything smaller can be managed with a pruner. Starting with tools that are sharp will make for smoother cuts, not to mention less work for you.

Sanitize:

When moving from one tree to the next, it is a good idea to sanitize tools to prevent the spread of plant disease. We don’t want to take any chances in the oak collection when the likes of oak wilt, bacterial leaf scorch, and oth-er harmful diseases could lurk undetected. Pruning in the dormant season does lesson the likelihood of spreading some plant diseases, but we take extra precautions. We utilize a spray bottle of concentrated isopropyl alcohol to mist blades between trees for this purpose. Bleach and other cleaning products work well to sanitize, however they can be corrosive.

Removing small branches is best:

Trees don’t have the ability to “heal” when cuts are made. Instead, they seal up wounds like pruning cuts and wall off damaged or decaying tissue on the interior. This is known as compartmentalization. Small cuts can be compartmentalized quickly, limiting exposure to decay and disease. Large wounds, no matter how carefully made, will take years to compartmentalize internally and seal over externally, leaving the tree with a large section of decay.

What are you removing?

Before the first cut is made, a brief visual assessment of the tree’s overall structure and health should be complet-ed. Our motto when pruning is to look for branches that have no future. Ultimately, we want to leave behind a strong scaffold of well-spaced permanent branches. The first and easiest decision is removing any dead, diseased or damaged branches. Next, look for vertical sprouts and suckers. These branches are weakly attached, detracting from the tree’s structure and appearance. Next, look for crossing or rubbing branches, as these will only get worse as they grow. Where needed, make cuts that will improve structure over time, such as removing double leaders and branches with included bark. Crowded branches should be thinned to leave behind a well-spaced scaffold. This means reducing the number of branches that form a whorl at one height on the trunk as well as parallel branches that are closely spaced.

You might also consider raising the crown if desired for clearance. Be sure not to remove too many branches at the same height on the trunk in a single year, as this risks girdling the tree. Low branches that will eventual-ly be removed can be shortened to slow down their growth while they still help to build trunk caliper. Leaving low branches also offers some deterrent to deer looking for a place to rub their antlers.

Be sure not to over do it! It is not recommended to remove more than 25% of the canopy in one year when pruning a young tree, even less on a mature tree, as it can lead to stress and possibly decline in the tree.

Cut outside the branch collar:

The branch collar is a (usually) visible swelling where the branch meets the trunk. Cutting just outside the col-lar will allow for good formation of callus wood to seal over the cut surface. See the Year 2 photo, a good ex-ample of a complete ring of callus wood forming around a pruning cut. Flush cuts that remove the branch col-lar leave the tree without the plant tissue responsible for callus wood, making it difficult if not impossible for the wound to be sealed. Leaving a stub also prevents the formation of callus tissue that will seal out insects, disease, and on-going decay.

Avoid tear outs:

When removing larger branches (anything over 1”), be sure to either support the branch or use the three cut method to prevent the tearing of bark and damage to future callus tissue. To do this, make an undercut part way through the branch six inches or more from the final cut. The second cut is made at least a couple inches past the first cut, removing most of the branch. The undercut creates a break in the bark, preventing the weight of the falling limb from tearing the bark down the trunk. Finally, the stub can be carefully pruned away just outside the branch collar.

Photos below: These are three good examples of oaks with proper pruning cuts. In the Year 1 example, a clean cut was made outside the branch collar. The Year 2 example shows good formation of callus wood around the entire edge of the pruning wound. Note the small fresh cut on the bottom, where a sucker was re-moved. It is not uncommon to see the development of vigorous but ultimately weakly attached sprouts or suckers at the site of recent pruning cuts. It is more common in some species than others and can also be de-pendent on the tree’s health or the amount of pruning that was done. The Year 3 photo shows a wound that has been completely sealed. While there is an amount of dead wood at the site, the vigorous growth of wound wood has sealed over the cut surface, protecting it from insects, disease, and further decay.

Making the Cut