Let’s consider evergreen shrubs, which differ from deciduous shrubs by continuing in leaf year-round. This is not the same as retaining leaves all year: evergreen shrubs sometimes are described as dropping their leaves year-round, for on-going renewal.
A separate group of evergreens is comprised of coniferous trees and shrubs, which are pruned primarily in late winter or early spring, before the appearance of new growth. It is now generally too late in the year for pruning coniferous evergreens. Exceptions include removing unwanted whole branches of spruces and junipers, which may be done at any time, and trimming yews and arborvitae can be done when they have a second flush of growth in mid-summer.
We will address the pruning of conifers next winter. These plants require minimal pruning, except as needed to control their size and shape. If you have such plants in your landscape, mark your calendar with a reminder to consider pruning needs around next February.
The larger group of evergreen shrubs should be pruned as needed in April or May, i.e., in mid-spring, after any risk of frost has passed and ideally before new growth starts. In the Monterey Bay area’s moderate climate, there is little chance of frost except in inland regions, and new growth might appear early in the spring. If the gardener initiates light pruning after new growth can be seen, the downside is that some of the plant’s energy will have been wasted, but the plant will simply replace the shoots that have been trimmed.
For pruning purposes, evergreen shrubs can be regarded in one of three groups.
- Early flowering. Examples include Berberis, Camellia, Ceanothus, Daphne, Mahonia, Pieris, Azalea, Rhododendron, and Viburnum. They bloom in winter, spring or early summer. Many shrubs in this group should be pruned only lightly and deadheaded.
- Late flowering. Examples include Eucryphia (Leatherwood), and laurels (e.g., English Laurel). These plants bloom in summer and late autumn on either old or new growth. They need little pruning.
- Mid-season flowering. Examples include Calluna (Heather), Erica, Lavandula (Lavender), Santolina, and Thymus (Thyme). These bloom on old growth in spring or early summer, or on new growth in late summer and autumn. Generally, pruning involves removing shoots after flowering to about one inch of the previous year’s growth.
The pruning strategy for these plants follows these following basic steps;
First, remove any diseased, damaged, or dead branches. They will not heal themselves and could spread disease, so prompt removal benefits the healthy parts of the plant and helps the gardener to evaluate other needs for pruning.
The second step, then, is to remove branches that are crowding other branches, or compromising the desired appearance of the shrub. The pruning objectives might include reducing the overall size of the plant, either to work better within the landscape plan or to clear a walkway.
A common problem arises when an established, healthy plant grows larger than expected or wanted. Such outcomes should not be surprising: information on the mature size of a plant is readily available at the garden center or in a reference book or website. A bit of research during plant selection can save future effort.
Shrubs that have become badly overgrown might need rejuvenation pruning. In such cases, remove one-third to one-half of the branches to ground level, and reduce all other branches by one-third. In the following two years, remove half of the older branches to ground level.
Pruning time also should be used as an opportunity to evaluate the overall health of the plant. If it has sparse or leggy growth, consider the need for greater exposure to sunlight. For example, a nearby tree might have grown to shade a plant that grows best in full sun, or the plant might have been installed originally in partial shade. In such cases, prune the tree that blocks the sun, or move the shrub to a sunnier spot.
Another factor limiting the plant’s growth might be poor soil, which can be treated with fertilization during the growth period, and regular applications of compost. The gardener should avoid planting in soil with minimal nutrient value, e.g., sandy soil or sub-soil (lacking loamy top soil). If this is unavoidable, consider planting in better soil in mounds, raised beds, or containers.
A third factor might be insufficient drainage. Some shrubs thrive in soggy soil, but the large majority need oxygen at their roots, so the surrounding soil must be allowed to dry out between irrigations. This can be a problem that results when plants are placed in low-lying areas, or in moisture-retaining clay soil.
The third step in pruning includes mulching and feeding. These actions minimize weeds around the plant and help the plant to grow.
The final step is to stand back to appreciate a job well done.