When and How to to Prune Tress and Shrubs

Phyllis Heuerman
Frederick County Master Gardener Program

People are often confused about when to prune their trees and shrubs, particularly if they are of the flowering type. There are a few general rules to follow. Non-flowering deciduous trees generally should be pruned when they are dormant, in the winter or very early spring. The exception is trees that bleed heavy sap like Maple (Acer), Dogwood (Cornus), and Birch (Betula). These should be pruned in late autumn.

Conifers may be pruned lightly each spring, removing up to 2/3 of the new growth. Never prune Conifers heavily because new growth may not develop from dormant buds in old wood. Evergreens and Hollies (Ilex) may also be pruned by thinning in the dormant season -- i.e., November through February -- for holiday decorations.

Spring flowering (before June) ornamental trees and shrubs flower on old wood from the previous summer. Prune them right after blooming, before new growth has begun. If you prune then now, at the end of the summer, you will not have any flowers in the spring. Examples of plants that fall in this category include Lilac, Forsythia, Flowering Cherry (Prunus), and Azalea. Delay pruning shrubs grown for ornamental fruits until after the fruit drops, e.g., Pyracantha, Viburnum, Holly (Ilex) and roses grown for hips.

Summer flowering trees and shrubs bloom on new growth produced in the spring. They should pruned right after blooming if it is not later than August, or in the dormant season, around November or later. Avoid pruning anything in September or October because new growth will occur and it may not have time to harden off before the cold season starts.

The reasons for pruning are many and varied. Newly planted trees may have the lower limbs removed to raise the canopy. Young shrubs are pruned to encourage branching and to shape growth. Formal hedges require regular pruning to maintain shape. We prune to improve air circulation, exposure to sunlight and overall plant health. Roses and lilacs that are prone to mildew are prime candidates for this type of pruning. We prune to remove dead, broken and diseased branches, crossed branches, and suckers at the base of trees or shrubs. Some shrubs, like Buddleia, or Butterfly Bush should be cut back severely each spring, for rejuvenation. Some old and declining shrubs may be completely renewed by cutting out old growth. We may also prune to maintain a special form, as in topiary or espalier.

Why have I not mentioned pruning to restrict growth? There are times when you will have to use your shears or pruners to restrict the size of a plant. However, as a general rule you should know the mature size of a plant before you select and place it. Do not depend upon pruning to restrict a plant that it too large for its location. It is better to move the plant, or replace it. When you are planting new trees and shrubs, do not plant them too close to the foundation of your house, or too close together. You will only be forced into excessive pruning in the future.

A word about basic pruning techniques. -- the simplest is pinching. Using your thumb and forefinger, or your hand pruners, you nip off the tips of new growth. This can be used to encourage branching in new shrubs, such as Rhododendrons. This technique may be used with Conifers, like pine, fir and spruce, to restrict size. You may remove up to 2/3 of the new growth each year.

The most common pruning cut you should be making on woody plants is called thinning. With this technique you remove an entire stem or branch, either back to its point of origin or to its junction with another branch. This technique is used to remove old and unproductive branches, to eliminate crossed or competing stems, and to open up a plant so that more sunlight and air can reach its interior. It may also be used to reduce overall size.

Thinning is generally a much more desirable pruning technique than heading, in which you remove just part of a stem or branch, at a bud or leaf node. The immediate result of heading is a smaller, more compact plant - but it does not last for long. Once headed, the plant will put out vigorous new growth, generally in candelabra at the point of the cut. Over time it will lose its natural shape. Too often Crepe Myrtle's (Lagerstroemia) are headed back instead of being properly pruned by thinning.

Shearing is a more extreme form of heading, in which plant material is removed without regard to location of bud or leaf nodes, It is not generally recommended but is useful in some circumstances, such as maintaining formal hedges. Shearing can also be used with some small woody plants that have small leaves, such as Lavender (Lavandula), Spirea (Blue Mist Shrub), and Perovskia or Russian Sage.

Rejuvenation is another exception to the rule that you should not use heading cuts. In rejuvenation you cut all of the stems to within 1 to 8 inches of the ground, generally in spring. You should try to cut near a bud or leaf node. This technique is used annually for Buddleia, or Butterfly Bushes. It can also be used for Chaenomeles (Flowering Quince) and Forsythia.

The final technique I should mention is renewal pruning. Renewal is removal of the oldest branches of a shrub at ground level, generally over a period of 2 to 3 years. It is used on declining shrubs like old Lilacs. It may also be used on Pussy Willow (Salix), Crepe Myrtle (Lagerstroemia), and Red Twig Dogwood (Cornus stolonifera or Cornus sericea).

Read other articles about tree care

Read other articles by Phyllis Heuerman