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Knowing when to prune is key to healthy plants

October 30, 2015
Juliana Jensen

When we first moved into our house with a garden, my mother-in-law gave me a big pair of lopping shears. What a fantastic housewarming gift! I went at it with gusto, toning my upper arms and getting out any frustration.

The only thing is, it was the wrong time of year. I pruned off the next year’s flowers; I pruned the fruit trees when they weren’t dormant; I pruned anything that grew. I have since learned that pruning must be done judiciously, and in season.

There are four major reasons you might prune. You prune for the health of the plant and to remove deadwood. You prune to control the plant size. You prune to make the plant more beautiful or to reveal the underlying character of the plant, such as a Japanese maple. And you prune to increase the bounty of the fruit or flowers the plant produces.

Although there is a particular season for each type of pruning, taking out deadwood can occur anytime. There is a lot of good advice at homeorchard.ucanr.edu. Click on the link to “Pruning and Training”.

Young fruit trees need quite a bit of pruning for shape and strength. Once the tree is mature, you want to keep the fruit within reach, allow sunlight to reach all the leaves, remove the dead leaves and renew the fruitwood. The optimal time to do this is when the tree is dormant: the leaves are off the tree and you can see the structure. Steven Swain, environmental horticulture advisor for Marin and Sonoma, recommends using caution on timing.

“Don’t prune fruit trees just before they bloom, or during bloom. Only prune them when they’re totally dormant (December through January) or in late summer (August through September), except apricots and cherries, which should only be pruned in the summer, due to the risk of Eutypa infection,” he says.

For most other deciduous trees, prune in the late winter or early spring (February, March, early April). Don’t prune at the end of the growing season because the pruning would stimulate growth at a time when the tree is preparing to go dormant.

For deciduous shrubs, you prune to thin out the plant, renew it gradually, or rejuvenate it. Thinning out opens the plant while maintaining its natural shape without over-stimulating top growth. For gradual renewal, some of the larger, older branches are removed each year, while longer remaining branches may be shortened. Rejuvenation pruning removes one-third of the older branches just above ground level before the beginning of the growth season.

Evergreens do best if they are pruned in early spring, just before the growing season starts. But Swain points out exceptions. “Don’t prune oaks in the spring (SOD transmission risk is highest in the spring); don’t prune pines in the summer (the resins in the cuts attract beetles).”

When it comes to flowering shrubs, you need to be aware of when the flowers bloom. Spring-flowering plants like forsythia, rhododendron, azalea, and weigela bloom on growth from the previous season. Lightly prune these plants right after they bloom to maximize the time they have to set flowers for the next spring. An older, neglected plant may need a rejuvenating pruning in late winter to early spring. This will reduce the flowering that spring, but create a healthier plant in the long term.

Summer-blooming plants (those that bloom after June) like butterfly bush and crepe myrtle bloom on the current year’s growth. They should be pruned in late winter to promote vigorous spring growth.

In some flowering plant types, such as hydrangea, it is important to know if you have a variety that produces flowers on new growth or old growth. If the flowers bloom on the previous year’s growth, then prune them back after they bloom. If they produce on new growth, prune them during the dormant season.

Ah, the rose, the queen of the garden! The best time to prune most roses is during the dormant period (late December to end February). However, once-blooming old roses like Chevy Chase bloom on old wood. If you prune an old rose in winter, you prune off that year’s blossoms. Now, that is a heartbreak! Wait for the bloom, and then prune lightly.

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