I love to prune. I find it relaxing.

One of the first chores we attack in our gardens each spring is pruning, but I find that most people are do not know how to prune properly or understand the best time to prune. 

I cannot stress enough that proper pruning techniques can make all the difference.  Pruning promotes flowering, reduces pests and diseases, and contributes to a plant’s overall health, all while helping it maintain an attractive shape.

The longevity of a plant is also determined by how well it has been pruned over the years. 

It wasn’t until I really understood what was going on inside a plant that pruning started to make sense.

Here are three scientific facts to help answer some questions you may have about pruning deciduous trees and shrubs.

FACT #1: Four types of buds

Four types of buds are found on any branch: terminal, lateral, latent and adventitious buds.

The terminal bud (a.k.a. the apical bud) is found on the tip of any branch.

Lateral buds form during one growing season and remain dormant until the next growth period when they develop into stems, leaves or flowers. New leaves emerge first from the terminal buds then the lateral buds.

Not every bud grows into a branch, leaf or flower, though. Some buds on young twigs remain inactive for many seasons.

These are the latent buds, which are the plant’s insurance. Should a branch be cut off or broken above a dormant latent bud a new shoot can quickly develop.

Adventitious buds develop where no buds previously existed. These sometimes grow after a branch is wounded or cut back to mature tissue. These are different in that they develop close to the branch surface from deeper mature tissue.

FACT #2: Apical dominance

All plants grow from the tips of branches, or to put it more scientifically, all plants go through a phenomenon called apical dominance.

There are many different shapes of plants available to choose from. Some are more upright, some are round and some grow flat to the ground.

But what causes plants to grow into different forms?

All plants produce a hormone in the terminal buds known as auxin. This hormone supresses the growth of the other buds, signally them to remain dormant. 

The hormone is strongest at the tips so the buds closest to the terminal bud are stunted the most. As the auxins move farther from the terminal buds, they are less stunted, which causes more growth.

This hormonal effect determines a plant’s branching pattern and its response to pruning. A pyramidal plant produces more auxin than a low-growing plant. 

When you prune off the terminal bud, the growth patterns become drastically altered.  The science of pruning lies in understanding the growth pattern of plants.  If you remove a terminal bud, it releases the lateral, latent and adventitious buds from the growth inhibition caused by apical dominance. 

Many of the buds behind the pruning cut sprout into branches; where one stem once grew, now a cluster of stems might emerge. 

If you are trying to maintain a certain shape of plant, special attention must be taken as to how you prune.

FACT #3: The food cycle

When we head into autumn and start seeing plants go dormant and the leaves turn beautiful fall colours, what we don’t see is what is going on inside of the plant. 

All summer, the leaves have been storing up food from the sun in a process called photosynthesis. As the fall approaches, all this stored energy starts to move from the leaves, down the stem and back into the roots by late winter. 

In the spring, that same energy (food) then moves from the roots, through the stem and forces out new leaves.

If this process did not occur, when a plant drops its leaves it would lose all its stored energy, weakening the plant. 

As temperatures start to warm up in the spring and we start to tap maple trees to collect sap to make syrup, we often say “the sap is rising” and that is exactly what is happening. 

The plants food is rising from the base into the branches and is what forces out the new leaves.

Knowing this information, the best time to prune most deciduous shrubs is late dormant period to early spring (March-early April) before the new leaves emerge.

Next week, we will look at some general rules for pruning deciduous shrubs.

Joanne Young is a Niagara-on-the-Lake garden expert and coach. See her website at joanneyoung.ca.