A Primer on Pruning
We prune plants to control their size, to produce a desirable shape, to increase vigor, and to increase flowering and fruiting. Experienced gardeners will tell you the best time to prune is whenever it's necessary - anytime of the year! But there are some general guidelines we can follow. In this presentation we'll discuss when to prune, provide some useful tips and cautions, and describe the five main kinds of pruning cuts.
Buds at the branch tips are called terminal buds; those on the sides are lateral buds. Terminal buds produce a hormone (called auxin) that suppresses the growth of other buds close to it. This phenomenon is called apical dominance, and is why lateral buds close to a terminal bud do not grow into side branches. Buds farther away from the terminal bud get a weaker dose of auxin and at some distance from the terminal bud they are released from dormancy and begin growth into side branches. When the terminal leader is cut, the hormone no longer flows to the dormant lateral buds below and they develop into branches.
Dormant buds are formed in one growing season and in the next growing season they grow into stems, leaves or flowers, depending on the type of dormant bud. Some dormant buds don't develop into stems, leaves or flowers right away and instead become latent buds, persisting for many seasons as a plant's "insurance", ready to turn into dormant buds and grow into new shoots in case a branch is cut or broken. Adventitious buds develop beneath the bark and are usually not visible. Adventitious buds and latent buds can grow into unsightly water sprouts - fast growing, tender and fragile upright shoots that develop in response to injury or sudden release from shade.
When to Prune Trees and Shrubs
Don't prune trees and shrubs during active growth. The worst time to prune is when the new leaves are first forming. The best time to prune most trees and shrubs is during their dormant period. Plants that bloom on the new growth of the current season should be pruned in late winter. That way, we won't be cutting off spring flowers. We like to prune near the end of the dormant period, before new growth begins, so that there is less loss of stored energy (in the form of sugars). If we do major pruning during the growing season, we eliminate a lot of newly produced sugars and we cause a significant dwarfing effect. (This is okay if dwarfing is the goal.) Pruning in autumn can induce tender new growth to sprout that will surely be damaged by the first frosts. If we prune in early winter, we may still be removing sugars stored in the branches. (Those sugars will be needed to initiate next spring's growth.) By the end of winter, most sugars have moved farther down the branches and into the main trunk and roots, and we can safely prune without depleting as much of the stored energy. Some trees bleed sap profusely when pruned in late winter. To avoid excessive loss of sap, maples, birches, cherries, and walnuts should be pruned after the new growth has matured in late summer.
Blooms on New Growth-Prune in Winter
Most conifers and many of the fruit trees are pruned in winter. Here's a list of some familiar species that are pruned in winter:
- butterfly bush
- citrus trees
- crape myrtle
Bloom on Last Season's Growth - Prune After Blooming
Trees and shrubs that bloom on the previous season's growth can be pruned in late spring, soon after they bloom, and before they set the flower buds for next year's blooming. If we prune these plants after next season's flower buds have formed (in autumn or winter, for example) we will reduce the number of flowers produced next year. Of course, these plants can be pruned in late winter too (when you can see their structure better) - you'll just lose some flowers.
- African tulip-tree
- banana shrub
- flowering quince
- fringe tree
General Rules for Pruning Trees and Shrubs
- Use bypass clippers or loppers; anvil clippers tend to crush the branches.
- To encourage the most rapid shoot development and greatest plant growth, prune trees and shrubs just prior to bud swell in late winter.
- To retard growth for maximum dwarfing effect, prune right after each flush of new growth throughout spring and summer.
- You don't need tree-wound dressing; if you make pruning cuts properly, the plants are quite capable of healing themselves.
- Prune very little at the time of transplanting new shrubs and trees - they need all the leaves they have to grow roots and get established. Instead water them every day or two for the first six months.
- Leave the lower lateral branches on young trees until they are an inch in diameter. The little guys need those leaves.
- Prevent co-dominant leaders by removing the weaker one. You usually don't want forked trunks.
- Branches that come off the trunk at narrow angles have bark-to-bark squeezed between the branch and trunk, and this makes for a weak branch attachment that will surely break off some day in the future. Remove branches with narrow angle attachments and embedded bark.
- Water sprouts should be removed whenever they are noticed. You can usually just break them off with a sharp sideways yank.
- Shearing to maintain geometric shapes for formal hedges and topiary may be required 4-5 times a year.
Feel free to perform minor touch-up and maintenance pruning all year long. No unruly branch, divided leader or crossed stem is safe from my trusty, ever-ready pruning shears when I walk around the yard!
There are five main kinds of pruning cuts (click each to learn more):
Steve Christman 3/6/01; updated 12/26/03, 9/21/15