A Guide to the Pruning Decision Process

As arborists, pruning is a very important part of our work. Most arborists and horticulturalists are passionate about pruning. Most enjoy pruning and believe in their methods. The question is, how do you know what’s right? Pruning should be more than just removing branches. Before you prune trees and shrubs, you need to ask a few important questions that lead to a specific plan.

So let’s consider the pruning decision process.

Intent

What are you trying to accomplish with this particular plant? It can be as simple as removing dead material or as complicated as espalier (shaping the growth of branches). Do you want the plant to look attractive when the pruning is complete, or will it take time to respond to the pruning? What are your aesthetic expectations?

Intent is the easiest question to answer, but you need an answer before you begin. You need to realize that pruning live growth will cause a reaction from the plant. Your intent also means being responsible for how the plant reacts.

Timing

What is the best time of the year to prune? The general rule is, if you are going to prune something aggressively, the best time of year to do so is late winter or early spring. The idea is that the plant is capturing all the energy it is generating to push out new growth. This energy is channeled to react to the wounding. This pruning leaves the plant with a more natural shape from late spring through the end of the year.

The classic example of aggressive pruning is rejuvenation or basal pruning, cutting down stems of shrubs to get new sprouts from the base of the plant. This is very species specific, but if you do this to the right species, it can rejuvenate old plants. This is best done in March or April, depending on the length of winter in your area. January and February are a great time to prune most deciduous plants, especially large trees. But it is not a good time to prune most evergreens. some evergreens could desiccate (dry out) if they have experienced a dry summer/fall followed by a cold winter.

If your intent is to have the least reaction or sprouting, the best time of year to prune is late summer and fall. If you prefer plants to have a manicured appearance, then schedule pruning in the summer and early fall. The plants will have a neater look until late spring the next year, when the new growth emerges. Be careful not to prune too aggressively in the later summer and fall, especially if it’s been dry.

People always ask about losing flower buds and fruit. This should be a lower priority, because you want to manage the plant with a longer view than one season. I’ve seen too many plants become leggy and unkempt by lack of pruning, because people were overly concerned about one season’s blossom.

Dose or amount

How much can you remove at once? Some plants lend themselves to aggressive pruning and some plants do not like to be pruned. How you prune a sugar maple should be very different than how you prune a silver maple.

How often will you need to prune this plant to achieve your intent? The more formal your objective, the more pruning is required. Take espalier, for example. Bonsai and topiaries will require frequent pruning throughout the year. Once you have the desired result, what is the pruning schedule going forward? These decisions require a basic understanding of the particular plant.

This prompts another question: Do you – or does your client – have the time and resources to commit to your intent? Do not start something that will require multiple prunings if you or your client cannot stay the course.

In some cases, over pruning is worse than not pruning at all. Timing and species are critical to the amount of pruning. A general rule is not to remove more than 25% of the foliage at one time. Most of the time that is a good rule, but there are instances when less or more is appropriate.

Condition

What is the condition of the tree or shrub? If the plant’s health is compromised, pruning may further stress the plant and possibly kill it. Old age, insects, disease, shade and drought stress can all cause a plant to be vulnerable. Reducing the plant’s ability to photosynthesize when it’s compromised is usually a bad idea.

Species?

Will this particular plant respond well to your intent? This is the most difficult question. It requires strong knowledge of the plants you work with and experience with how each species reacts to pruning.

Reaction

What will the reaction be? As already mentioned, when you remove live growth from a plant, there will be a reaction. Knowing how the plant will react requires a great deal of experience. It also will determine the next steps. If you prune something aggressively or try to create a formal look, you need to commit to follow up.

This process is really difficult to explain, because it is so specific to the plant, your intent and the time of year. At a minimum, realize that plants respond to injury. Whether it is Mother Nature or us, the plant will react.

Working the plan

You need to have a specific plan for each plant, because there are so many factors to consider, but using this pruning decision process provides a guide. These rules are pragmatic, but not set in stone. Pruning is a mix of science and art. As you know, plants can be forgiving, and a big part of learning is trial and error.
Like most things, people have their biases about their pruning styles. This is wonderful, but we should always be open to learning and debate. However, there are a few important things that should not be compromised.

Proper cuts

Making the proper pruning cuts separates the professionals from the posers. As noted earlier, pruning is wounding the plants. It is imperative that these wounds are made in the best possible manner. Our industry does a very good job with this. However, it needs to be mentioned, because making proper cuts and committing to follow up to monitor the plant’s reaction is essential. You are leaving your fingerprints.

Pruning Safety

Another obvious factor is safety. Heights, wires, structural defects, pruning ladders, tools, weather, terrain, animals, bees’ nests and many other factors make pruning potentially dangerous.

A plan in action

In March 2023. I aggressively pruned a scintillation rhododendron that was overgrown (15×15 feet). Even though I am very confident in my pruning ability, I was concerned.

Even though rhododendrons generally respond well to pruning, I was nervous, because this particular species does not usually respond well. To hedge my bets, I fertilized it the fall before. Fortunately, this plant reacted with several large, healthy leaves sprouting beneath the pruning wounds – cause and effect. (Frankly, better than I expected.)

The accompanying rhododendron photos illustrate the pruning decision process. My intent was to reduce the size of this plant over time. I pruned it in March 2023 to get the most sprouting. Because I was concerned about how this species would react, I fertilized and was less aggressive than I would have been with some other species of rhododendron.

I will wait for this lower growth to mature, and will reduce it more in March 2025. My goal is that by 2027, this overgrown shrub will be a third the original size and healthy and full.

Conclusion

There is so much more that can be discussed and written. This is a simple guide to help you start the process. Many arborists are biased by the “way they learned.” I get that; obviously, I am. I want to give credit to Hartney Greymont in Needham, Massachusetts, now a Davey Company, where I learned these concepts. However, I want to challenge everyone to be open minded and to experiment. The best way to learn is by doing.

Pruning is a very important maintenance practice, and it can be rewarding. When you set a goal using the pruning decision process and it comes to fruition, you know you have accomplished something special. It gives you a sense of pride seeing a beautiful, healthy plant because of your work. Isn’t that, ultimately, why we work with plants?

David M. Anderson, CTSP and Massachusetts certified arborist, is a manager with Mayer Tree Service Inc., a 31-year TCIA member company based in Essex, Mass., and is a member of the TCI Magazine Editorial Advisory Committee.