Pruning Spring Flowering Trees and Shrubs

As a mild mannered horticulturist and garden writer, I usually do not get too upset about how people garden. I do admit to a certain passionate response when I see people abuse their plants. Since the plants have no other advocate, I must speak up and try to help people understand how and when to prune their plants.

When I ask gardeners to define pruning the response is, "cutting off part of the plant." While that covers the first part of the definition, they almost never add the second part which is "to produce a desired growth response." You always have to determine what a plant does when you prune it.

Pruning skills require learning about how the plant grows. The theory is important but practice is equally important. Since different plants have different pruning requirements, this weeks column is focusing on spring flowering trees and shrubs.

Perhaps the biggest abuse I see to the shrubs and trees that bloom in the spring come from people who have already pruned those plants. Spring flowering woody plants are always pruned after they flower. This allows for vigorous summertime growth and results in plenty of flower buds the following year.

The horticultural reason for this is these plants are grown primarily for their spring color. If you prune them prior to the time they flower, you are cutting off the blossoms before they bloom. While that is not exactly like lighting dollar bills on fire, you are not getting full value from your plants.

Plant that grow in temperate zone meaning those areas where they go dormant in the winter have an built in safeguard so the buds only bloom in the spring when they are going to have a long enough growth period that they can form fruits and seeds.

Because of this, all spring flowering woody plants including fruit trees form their buds the summer before they bloom. If you prune any time between when the buds form the previous summer and when the plants finish blooming, you reduce the flower quantity.

So why do fruit trees get pruned before they flower? You grow those trees to produce fruit and with some kinds of fruit trees you want to eliminate as many as ninety percent of the flowers to divert the energy into the remaining fruit so it will be much larger.

Prune evergreens anytime the wood is not frozen, but spring is best because if you make a mistake, the new growth covers it.

Now that we have established the time to prune these plants, the next question is how to prune them. For convenience, we can divide them into flowering shrubs with multiple stems or trunks and flowering trees that have single trunks.

Pruning recommendations for most deciduous shrubs consist of thinning out, renewal and rejuvenation pruning. With thinning out cuts, remove a branch or twig at its point of origin at either the parent stem or ground level.

Thinning out pruning gives a more open plant because it does not stimulate excessive new growth, but allow room for the side branches to grow. You can remove considerable growth without changing the plant's natural appearance or growth habit. Thinning out lets you maintain your plants at a given height and width for many years.

Prune your spring flowering deciduous shrubs with a renewal pruning system. The plants will need little or no pruning the first three years because the plants are still getting up to size. Limit your pruning to removing dead, broken or diseased branches or other obvious problems but do not reduce the height or spread of the plants.

After plants finish blooming the fourth year, providing they are large enough, start renewal pruning. Your plant is in full leaf but that does not hurt it. Remember your goal is to maximize the flowers for next season not the green growth during this summer.

In gradual renewal pruning, select about one-third of the oldest and tallest branches to remove clear down to ground level annually. Thin them as needed to shorten long branches or maintain a symmetrical shape.

For each old stem you remove, you will let one new shoot come back up. That keeps the shrub from getting to thick and overgrown. Each subsequent year you follow the same procedure so you maintain the size and quality of the plant.

If your shrubs are badly overgrown and need considerable attention, you are going to follow the same general procedure but you may need to spend additional years to get the neglected plants back into shape

Rejuvenation pruning is for plants that are so overgrown and neglected that they cannot be pruned any other way. The entire plant is cut to the ground and then new shoots grow from the base. If you do this radical pruning, you will likely not get flowers the next year and in some cases plant may not grow back.

Prune your spring flowering trees after they flower to keep them strong and healthy. Remove the mandatory dead and broken branches but also take out crossing branches, water sprouts and branches that for narrow V shaped crotch angles.

Some examples of shrubs that bloom on last season's growth are Japanese quince, forsythia, honeysuckle, cotoneaster, dogwood, rambling rose species, lilac, spiraea (early white species), mock orange and viburnum species. Some examples of trees that bloom on last season's growth are redbud, flowering crab apples, fringe tree, flowering cherry, plum and peach, and ornamental pears. In addition, hawthorns, horse chestnut and Japanese lilac are pruned after flowering.

One of the most serious problems is using hedge shears to pruning these plants. Rounding over these flowering shrubs destroys their form and their blossoms. Limit shears to pruning hedges. Do your part to make our area more beautiful by stamping out woody plant abuse and learning to prune the right way at the right time. I will thank you and so will your trees and shrubs.

Larry Sagers
Horticultural Specialist
Utah State University Extension Service
Thanksgiving Point Office
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