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Description

Shrubs are woody perennials that produce multiple stems, shoots and branches from the base of the plant. Generally, they survive winter without dying back to the ground as herbaceous perennials do, although there are a few exceptions. There is no clear distinction between a shrub and a tree; a shrub is usually low growing and multi stemmed, but it can also be single stemmed, like a tree.

Shrubs have no defined maximum height.

In this factsheet, the recommended shrubs range in height from 2 ft to 10 ft, like a sunny site and are deciduous, meaning they shed their leaves in the fall.

Considerations Related to Choice

When selecting a shrub, it is important to check that it is hardy in our climate; the hardiness zone, usually located on the plant's tag, indicates the minimum winter temperature the shrub can tolerate. Choosing plants suited to the growing conditions of your site, is always preferable to changing the conditions of the site to suit the plant. Conditions to take into consideration are the amount of sun/shade, acidity or alkalinity of the soil, whether the soil is primarily clay, loam or sand and proximity of other plants and associated root competition.

Another important, but often missed consideration, is the shrub's size at maturity. Checking mature height and width will tell you the amount of space the shrub will need for optimum growth.

Location

Since our focus is on ornamental shrubs for shade, it is important to look at how light conditions are classified. Most shrubs adapt well to either sun or shade; however, some may produce fewer flowers in a shady location.

• Full sun - plants require at least 6 hours of direct, late-morning/afternoon sun
• Partial shade - plants need a minimum of 4 hours of morning or afternoon sun, but should be shaded from the hot, midday sun.
• Full shade - plants can thrive with less than 4 hours of sun. A bright location that receives no direct sun would be classified as full shade.

Uses

Shrubs may be used in foundation plantings, hedges, shrub borders, rock gardens, as specimen plants, groundcovers and in mixed borders with perennials.

In addition to their growth habit (height, width, form), shrubs are chosen for characteristics, such as:

• Foliage (colour, form, texture)
• Flowers, fragrance
• Attract wildlife, birds and insects for food source, habitat, nesting material.
• Fall interest with fruit or berries and colourful foliage.
• Winter interest through bark, structure or form.

Maintenance

Once established, shrubs require very low maintenance. They need to be watered regularly when first planted and in times of drought. If the soil was properly amended, at the time they were planted they will only require a feeding in early spring, either with a top-dressing of well-rotted manure or a water-soluble fertilizer, to meet their basic needs.

Individual moisture and nutrient requirements will vary from one shrub to another.

Pruning

Shrubs are pruned for many reasons, including shaping, reducing size, removal of broken and diseased branches, thinning, rejuvenation and increasing flower and fruit production.

The most common mistake made when pruning flowering shrubs, is pruning at the wrong time. It is always advisable to find out the exact pruning requirements because there are variations within the same family of shrubs.

Flowering shrubs fall into two groups. The first group blooms on new wood and produces flowers on this year's growth. These are the late flowering shrubs, like hydrangeas, and should be pruned in late fall or early spring, before growth begins.

The second group blooms on old wood produced the previous season. These are the early bloomers, like forsythia, and should be pruned immediately after flowering. These shrubs will produce new growth during the rest of the growing season.

Deadheading (the removal of spent flowers) is a form of pruning. Shrubs, such as lilacs, benefit from deadheading, because this process will help the plant put more of its energy into growth, rather than seed formation.

Pruning should be avoided in late fall, to allow cuts to heal over and prevent winter dieback. Spring pruning can be performed as soon as the temperatures rise around the freezing mark and are guaranteed not to plunge dramatically.

Pests and Diseases

Shrubs, in general, are easy to grow and have fewer insect and disease problems than many other plants. Common pests, such as aphids, leafminers, caterpillars, beetles, borers or scale may attack, but the viburnum family, is often the most seriously damaged by insects each year.

The Viburnum Leaf Beetle quickly skeletonizes the foliage of any viburnum it attacks; two of the most popular varieties, the Fragrant Snowball Viburnum and the Highbush Cranberry are its usual targets. This beetle was introduced from Europe in 1996, and it has no natural predators in Canada.

Fungal diseases, such as verticillium wilt, anthracnose or cankers can infect dogwoods, and honeysuckles and lilacs are prone to powdery mildew.

Organic Management/Control Strategies

The City of Toronto adopted a new municipal by-law (spring 2004) that restricts the use of pesticides. For details about the by-law visit the Pesticide by-law page on Toronto Public Health's Web site at www.city.toronto.on.ca/pesticides or www.toronto.ca/health. Questions may be e-mailed to pesticide@toronto.ca.

Integrated Pest Management (IPM) is an approach to gardening, where different methods of control are selected and used in combination, in order to treat plant problems effectively and in an environmentally responsible manner. This approach can be used in the care of ornamental shrubs.

IPM uses four general control methods:

Sanitation

Good sanitation practices are important. Esthetics aside, a tidy garden is generally not a desirable environment for pests and disease. Weeds and plant debris provide shelter for insect eggs, as well as food for larvae and adult insects that may host a variety of pathogens. For these reasons it is important to clean up decaying plant material and remove any weeds as they appear and before they set seed.

Diseased plant material (including infected deciduous leaves shed in the fall) should be removed and burnt, or disposed of in the garbage, but never composted.

Pruning out diseased branches promptly and proper sterilization of your work tools will also help to eliminate or minimize the spread of disease.

Cultural Methods

The principle behind cultural controls is that a healthy plant is far more likely to be able to withstand disease and pest problems than a plant that is under stress.

Begin by selecting plants that are well suited to the existing growing conditions of your site; native plants are always a good choice. Look for disease-resistant varieties. When planting, prepare the soil well and give your plants plenty of space to grow. Over-crowded plants become stressed trying to compete with neighbouring plants for light, nutrients and moisture and are more susceptible to disease, due to poor air circulation. Make sure that your plants are watered deeply when needed. Mulching will help reduce the need for watering and weeding. Do not over-fertilize. Soil amendments such as compost, bonemeal and manure applied in the growing season help to reduce the need for chemical fertilizers. Closely monitor all plants for any signs of disease or pest problems.

Insect Control

Due to the restrictions on pesticide use, this fact sheet only references organic controls.

There are many effective non-chemical methods of controlling insects. Some examples are:

• Spray infested plants with water, to wash off aphids and spider mites.
• Handpick and destroy insects, such as potato and lily beetles
• Use insecticidal soaps to control insects such as aphids, spider mites, thrips and scale.
• Traps may be used for slugs, snails and earwigs
• Physical barriers, such as plant collars, act as deterrents to cutworms

Biological Controls

Insects have predators, parasites and pathogens as natural enemies. Biological control takes advantage of this fact. Some beneficial insects, that are predators to other insects, include ladybugs and lacewings, which feed on aphids, scales, mealybugs and mites, and parasitic wasps, which feed on aphids, whiteflies and caterpillars. Many of these insects can be purchased at garden centers for use as a natural insect control.

Creating a natural habitat in your garden will encourage wildlife such as birds, toads, and snakes that feed on insects. Creating such habitats is another form of biological control.

Recommended Species/varieties/cultivars

Ornamental Shrubs for Full Sun or Part Shade

Ornamental Shrubs for Part Shade

Ornamental Shrubs for Part Shade

References

The Toronto Botanical Garden Weston Family Library is an excellent source for horticultural information.

Books

• Damrosch, Barbara. The Garden Primer. New York, NY: Workman Publishing Company, Inc., 1988.
• Taylor's Guide to Shrubs. Boston, Massachusetts: Houghton Mifflin Company, 1987.

Garden Guide

Many nurseries publish Garden Guides that provide descriptions and information related to preferred growing conditions for the plants available.

Factsheets are produced by the Toronto Master Gardeners in association with the Toronto Botanical Garden. They provide introductory information about a broad range of horticultural topics and are intended for personal use and study purposes. Should your gardening group or organization wish to use multiple copies we ask that you inform the Toronto Botanical Garden at info@torontobotanicalgarden.ca.

Date revised: November 22, 2007