Toronto
Master Gardener Factsheet 
Toronto Master Gardeners are trained volunteers dedicated to providing horticultural information to the public.
For answers to horticultural questions contact the Toronto Master Gardeners' Info Line at the Toronto Botanical Garden (416) 397-1345 (Mon. to Fri. 10 a.m. to 1 p.m and Sat., Sun. and Holidays noon to 3 p.m.) or our web site.
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Toronto Botanical Garden is a volunteer-based, charitable organization whose purpose is to inspire passion, respect and understanding of gardening, horticulture, the natural landscape and a healthy environment.
777 Lawrence Avenue East
Toronto, ON M3C 1P2
Tel. 416-397-1340
Fax. 416-397-1354
www.torontobotanicalgarden.ca
Drought Tolerant Perennials
Perennials are plants that renew themselves each year growing from their hardy roots. Although there are woody perennials, such as tree and shrubs, the plants most people refer to as perennials, are herbaceous plants that die back to the ground at the end of each growing season.
The life span of a perennial will vary with the individual species/variety; some live a few years, while others, like peonies, may live for decades.
People require drought tolerant plants for many reasons: climatic conditions, municipal watering restrictions, a limited water supply from a well or cistern, water conservation preferences and for lifestyle considerations, such as a cottage garden, that must be left unattended for extended periods of time.
When selecting perennials, it is important to check that they are hardy in our climate.
The hardiness zone, usually located on the plant's tag, indicates the minimum winter temperature the plant can tolerate. Choosing plants suited to the growing conditions of your site, is always preferable to changing the site to suit the plant.
When choosing a plant consider the following: the amount of available sun/shade, the acidity or alkalinity of the soil, soil texture (e.g. primarily clay, loam or sand), the proximity of other plants and associated root competition.
Another important, but often missed consideration, is the perennial's size at maturity. Checking its mature height and spread will tell you the amount of space the perennial will require in your garden for optimum growth.
Drought tolerant perennials share many of the following characteristics that, when established, enable them to withstand periods of prolonged drought.
Many have silver or grey foliage resulting from a coating of fine hairs on the surface of their leaves. These hairs reflect light and heat, and provide shade, thus reducing the amount of moisture lost to the atmosphere.
Waxed or furry leaves, designed to keep plants cool and retain moisture, are also characteristics of drought tolerant plants.
Leaf size also affects moisture loss. Many plants, that have adapted to dry conditions, have small or narrow leaves, in order to minimize the amount of exposed leaf surface. Thick leaves are another common adaptation. Fleshy, succulent plants, such as Sedum, are popular choices for dry locations.
Root systems also affect drought tolerance. Perennials with large fleshy, fibrous roots or deep tap roots are able to reach down into the soil to collect water and also store moisture for future use.
Most drought tolerant perennials grow in sunny locations and it is important to understand how light conditions are classified.
Most perennials develop deep root systems and require beds prepared to a depth of at least 12 inches. Soil should be amended with generous amounts of organic matter, such as peat moss and compost, in order to improve drainage and nutrient content. Perennials can rot in wet soil, especially in the winter.
Perennials that grow in drought conditions do not need a lot of fertilizer and do best when they receive their nutrients from natural sources, such as an annual application of manure or compost. Remember, however, to check for specific nutrient requirements.
Perennials should be planted away large trees and shrubs, so that their roots do not have to compete for nutrients and water.
A sheltered location is best, as it helps protect perennials, and soil, from excessive moisture loss to warm, summer winds.
Perennials may be used alone, in mixed borders or for container gardening.
It is a common misconception that perennials bloom for the entire growing season, like annuals. While there are some perennials that will bloom for many weeks, most are in flower for an average of three to four weeks.
The challenge of planning a perennial garden is to choose a combination of plants that will collectively give colour throughout the growing season. Gardeners are naturally drawn to plants that are in bloom. Because many perennial plant purchases are made in the spring, this attraction can result in a garden filled with blooms only until the end of June, unless care is taken to also select plants that come into bloom later in the growing season.
It is important to take note of the mature height of your perennials when planning the layout of your perennial beds. In a traditional border, the tallest plants are placed at the back of the bed, moving down in height to the lowest perennials at the front. If planting an island bed, which is viewed from all sides, the tallest perennials should be in the centre, with height decreasing outwards to the perimeter of the bed.
Given that most perennials only bloom for a short time, attractive foliage that lasts throughout the growing season is a very desirable feature. Combining perennials that provide contrast in form and texture is important. 'Texture' mainly refers to the foliage of plants (e.g. thick heart-shaped leaves compared to highly serrated leaves) and 'form' refers to the overall shape of a plant, such as conical, upright, spreading or vase-shaped. Many gardeners focus more on selecting plant material that will provide these contrasts rather that selecting plants based solely on their flowers.
Once perennials are well established, after two of three years, they can usually survive with the water they receive, naturally, through rainfall. However, until that point, they must be watered regularly.
Regular weeding is necessary, because weeds can quickly take over a perennial bed.
A layer of mulch, 2 inches thick, will suppress weeds.
In dry locations, mulch is essential. Mulch forms a barrier, which stops moisture loss from the soil below. The rough surface traps water preventing runoff, and allows the moisture to drain slowly down into the ground.
Perennials should be dead-headed regularly to keep their appearance tidy, and to encourage plants to put their energy into flower production, rather than seeds.
A mulch of straw can be also helpful when over-wintering tender, or newly-planted perennials.
Perennials, in general, are easy to grow and have fewer insect and disease problems than many other plants. Common perennial pests, such as aphids, slugs and caterpillars are easily controlled organically. At present, the Scarlet Lily Beetle, Lilioceris lilii, is causing the most serious damage. It lays its eggs on lilies and fritillaries, (Lilium and Fritillaria spp.). Both the larvae and adult beetles feed on the host plant, resulting in defoliation, and in severe infestations, the flowers and stems are also eaten. Neem Oil is the most effective organic control at present.
Fungal diseases, such as powdery mildew and rust, are often a problem in our hot, humid summer weather. A constant supply of moisture deters these diseases, but in dry-land gardens, that is not always possible, so it is important to mulch well and select drought and disease resistant plants.
The cultural controls, described below, will also help to minimize the spread of fungal diseases.
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. IPM adapts well to the care of perennials.
IPM uses four general control methods:
Good sanitation practices are important. Aesthetics aside, a tidy garden is not a desirable environment for pests and disease, therefore it is important to clean up decaying plant material and remove any weeds as they appear.
Both weeds and plant debris, offer shelter for eggs and food for larvae and adult insects, that may also be hosts for a variety of pathogens.
Diseased plant material should be removed and burnt or disposed of in the garbage, but never composted; this includes infected leaves, shed by deciduous trees in the fall.
Pruning out diseased branches promptly and proper sterilization of your work tools helps stop the spread of disease.
The principle behind using 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.
You 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 they are well watered and do not over-fertilize. Closely monitor all plants for any signs of disease or pest problems.
Due to the restrictions on pesticide use, this factsheet only references organic controls. There are many effective non-chemical methods of controlling insects. Some examples are:
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, that feed on aphids, scales, mealybugs and mites, and parasitic wasps, that feed on aphids, whiteflies and caterpillars. Many of these insects can be purchased at garden centers for use as a natural insect control.
Create a natural habitat in your garden that will attract and support wildlife such as birds, toads, and snakes. These all feed on insects and offer another form of biological control.
The Toronto Botanical Garden Weston Family Library is an excellent source for horticultural information.
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: March 4, 2007
