To prune or not to prune? That is one of the most often asked questions. One of the first chores that we attack in our gardens each spring is pruning, but I find that most people are not quite sure what it is that they are supposed to do. Knowing the proper pruning times and proper pruning techniques seem to be the most misunderstood areas of gardening. Pruning promotes flowering, reduces pests and contributes to the plant’s overall health, all while helping it maintain an attractive shape. The longevity of a plant, its productivity and the overall health of a plant is mainly determined by how well it has been pruned over the years, so it is very important to know and understand the proper way to prune your shrubs.
UNDERSTANDING HOW A PLANT GROWS
To understand pruning better it is important to first understand how a plant grows. There is a lot more happening inside a plant than what is apparent on the outside. Here is a little science to help us out.
There are four types of buds found on any branch: terminal buds, lateral buds, latent buds and adventitious buds.
- The terminal bud (also known as the apical bud) is found on the tip of any branch.
- Lateral buds form during one growing season and remains dormant until the next growth period when they grow into stems, leaves or flowers. New leaves emerge first from the terminal buds then the lateral buds.
- Latent buds – not every bud actually grows into a branch, leaf or flower. Some buds on young twigs remain inactive for many seasons. They remain at or near the surface of the bark, as the branch gets larger. Latent buds are the plants 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. Branches formed from these buds are usually weaker and can be easily broken in a storm.
All plants grow from the tips, or to put it more scientifically, all plants go through a phenomenon called APICAL DOMINANCE.
What is apical dominance?
There are many different shapes of plants available to choose from. Some plants are pyramidal, some rounded and some grow flat to the ground. But what causes plants to grow into different forms? By observing the current season’s growth of a woody plant, you can see that few, if any, lateral buds that are close to the tip of a branch has grown into side branches. Not until a lateral bud is a long enough distance from the terminal bud – usually the result of two to three seasons of growth – does it begin to grow. This is what is called apical dominance.
All plants produce a hormone in the terminal bud that is known as auxin. This hormone supresses the growth of the other buds, signally them to remain dormant. The farther the lateral buds are away from the terminal bud, the weaker the signals are. In other words, the buds closest to the terminal bud are stunted the most. As the auxins move farther from the terminal those buds are less stunted causing more growth to occur.
This hormonal effect determines a plants branching pattern and it’s response to pruning. As long as the terminal buds remains, it will be the first to grow in the spring. This natural system results in an orderly, controlled growth rate and gives characteristic shape to all species of plants – but to prune off the terminal bud and 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 cut sprout into branches; where one stem once grew, now a cluster of stems may emerge. This growth pattern is very evident on a sheared hedge. Not only does the plant lose its natural form but also exceptionally fast growth creates a branch structure that resembles a candelabra. On the other hand, the growth pattern of a plant is preserved by pruning back a stem or side branch to where buds have already broken dormancy and formed a side shoot. The terminal bud on the lateral branch then assumes apical dominance. Pruning invigorates new growth.
As we go into autumn and start seeing plants go into dormancy and the leaves start to 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 and air in a process called photosynthesis. As the fall approaches, all of this stored energy starts to move from the leaves, down the stem and back into the roots. In the spring, that same energy then moves from the roots, up the stem and forces out new leaves. If this whole process did not occur, a plant, when it dropped its leaves, would lose all of its stored energy, resulting in a weakened plant.
PRUNING DECIDUOUS TREES AND SHRUBS
General Rule Of Thumb
In general, the best time to prune any deciduous woody plant is just before the new growth starts (late dormant period – March – mid April). Pruning at other times can rob the plant of stored food and energy. It may also mean a loss of flowers or fruit. With that said all rules has some exceptions. Here are those exceptions:
- Any early spring flowering plants (plants that bloom off old wood) should be pruned immediately after flowering and before leaves unfold. E.g. Magnolia, Forsythia, Wisteria, Serviceberry, Lilac, Quince, Bridal Wreath Spirea, most Viburnums.
- Trees such as Maples (including Japanese Maples), Flowering Dogwoods, Birch and Elm will bleed if pruned in late winter or early spring. A better time to prune these trees would be mid-summer.
When To Prune
In general, the best time to prune most deciduous woody plants is just before the new growth starts in the spring or, in other words, late dormant period (early March to mid -April). Pruning at other times can rob the plant of its stored food and energy. It may also mean a loss of flowers or fruit.
With that said, all rules have some exceptions. Here are those exceptions:
- Any early spring flowering plants (plants that bloom off old wood) should be pruned immediately after flowering and before leaves unfold. These are some plants that this exception applies to: Magnolia, Forsythia, Wisteria, Serviceberry, Lilac, Quince, Bridal Wreath Spirea, Viburnums, Serviceberry and Wisteria
- Trees such as Maples (including Japanese Maples), Flowering Dogwoods, Birch and Elm, Poplar, Laburnum and Lindens will bleed if pruned in late winter or early spring. A better time to prune these trees would be mid-summer to early fall.
Where To Start – THE 3 D’s OF PRUNING
Many people, when pruning deciduous trees and shrubs, will just shear back the top of the plant to reduce the overall height of the plant, but this is only a part of proper pruning techniques. So what else is involved in pruning? What steps should be taken when starting to prune?
Step 1. When pruning any shrub or tree, you always start out with the 3 D’s of pruning. This means that you prune out any DEAD, DISEASED or DAMAGED branches first.
Step 2. Remove any branches that closely cross with other branches or that cross through the middles of the plant. You want to have branches that come from the center of the plant and heads outward.
Step 3. The next step is to consider your pruning goal.