Time in the garden
Shade Gardening
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Mr. Wright enjoys gardening in Calgary, 51o 02’ N, at 1060 m in zone 3, and at 49 m in zone 8b, in North Yorkshire, England, 54o 26’ N.



Calgary’s Gardening Climate


“Climate” describes the atmospheric characteristics of a region: a summation of our day to day weather over several decades. To describe climate norms, the World Meteorological Organization uses data collected over a thirty year period: ideally, measurements of temperature, precipitation, wind, evaporation, and humidity are reported each day from standardized weather stations. In Calgary we have had over 120 years of observations, but numbers alone don’t tell the whole story because the past weather patterns will not be predictive of the future. We live in a semi-arid continental climate with long, cold winters and short, cool summers. May is the windiest month, June the wettest, and July, the hottest and sunniest. Our notoriously rapid weather changes can be caused by the juxtaposition of Arctic, and southern, “American” air masses, or moist Pacific systems, and daily extremes in our weather are not unusual, such as the 48cm of snow in May, 1981 and the 95mm of rain in July, 1927. Variations from the average are frequent and important from the gardening perspective, although it appears as if the very dry years are getting wetter and the very wet years are getting drier. As gardeners, we Calgarians are well aware that the weather in our specific community and gardens has huge impact on plant health and appearance.

One very important aspect of our northern climate is the number of frost free days. The expression “frost free days” has to be refined to indicate consecutive frost free days during our growing season. A week above zero in January does not help our plants - quite the opposite in fact. The last frost typically occurs on May 23 and the first fall frost on September 15, giving an average of 115 consecutive frost free days; however, the minimum number is about 50 days and the maximum about 150 days: clearly a great range. The number of consecutive frost-free days is a guide for plant growth or time to maturity and does not take into account the desired optimum growing temperatures. 5°Celsius is often the minimum temperature needed for plant growth. The minimum threshold for lettuce is 4°C, potatoes 7°C, and for corn (and grasshoppers!), 10°C. Farmers may use Corn Heat units as a guide to growing conditions for some crops, but we can more readily appreciate the nature of the growing season by the number of “growing degree-days” (the difference between the mean temperature and 5°Celsius, multiplied by the number of those warm days). If the climate in Calgary becomes warmer, we may need gardening practices more like those of Lethbridge where the number of growing degree-days from April to October totals 1723, compared to Calgary’s current figure of around 1400.

A killing frost is defined as -2°Celsius or below. This temperature is measured 1.2 metres above the ground surface. The median date for the last killing frost at the airport is May 13, but it can be as late as June 13. Many perennials begin to grow after April 17 and some seeds begin the germination process about then too: a few robust seedlings can survive to -6°Celsius. Generally speaking, eastern Calgary has a longer growing season than to the west. If fall flowering plants such as chrysanthemums can withstand an early frost, they will benefit - even flourish - from the warm days that typically persist into October and the later the first frost, the redder the leaves of Virginia Creeper will become.

Total precipitation (i.e. rain plus snow equivalent) is another important measurement for any garden because we need compatibility between plants and available moisture. The average precipitation in Calgary, measured principally at the airport weather station between 1885 and 2003, is 420 mm and the majority of this falls in the growing season. One significant aspect of Calgary’s annual precipitation is its variation: the range from 201mm and 878mm is huge; greater than at most other Alberta weather stations. The timing of rain or snowfall is also important. In spring, snow acts as a great insulator and modifier of temperature, but if it evaporates rather than being absorbed into the soil, plants don’t benefit. The annual number of days of snow cover (greater than 2 mm) in Edmonton is 144 days compared to 118 in Calgary and 114 in Lethbridge.

Evaporation is another aspect of growing conditions. More specifically, evapotranspiration is the combination of evaporation from the soil and transpiration from trees and plants, a phenomenon accentuated by wind. Winter wind direction is typically from the west; the term “Chinook” may have originated because during Chinook conditions, the wind comes from west of the Rockies, where the Chinookan language was spoken by some aboriginal nations. Dry air is a phenomenon of Calgary and area, and our particularly dry fall and winter can give plants (including indoor varieties) problems unless we provide sufficient moisture. In March of 1968 Calgary had probably the lowest relative humidity (6%) of all Canadian sites recorded during the 20th Century. Summer winds are most often from the W or NNW and evaporation rates increase to the south and south east of the city of Calgary and, although evaporation often exceeds rainfall, a well designed and managed soaker or irrigation system will offset this (check out the Dorothy Harvie Gardens at the zoo). Evaporation rates vary during the day and it is best to water in the very early morning, before 6 am, when humidity is high and less water is lost through evaporation: we need to direct water to the roots.

Probably drought is the biggest threat to plants. We may need to look to Medicine Hat for our gardening methods. Historically, 1967 was one of our drier summers with only 72mm of rain. Occasionally, July and September have virtually no rain and August 1886 had none at all. Less rain means we have clearer skies, and therefore more evaporation, and of course, more frost danger. In Alberta, one of the most rapid reductions in precipitation occurs as we move geographically from the foothills south-eastwards to the prairies. If we wish to garden well, knowing a plant’s water requirements is essential, and the birch tree provides an example of this requirement. We do not find river birch - nor do we find poplar, and certainly not spruce - on top of the bald prairie. Where there is inadequate soil moisture, we must add water if we want trees to thrive.

Weather conditions are cyclic, ranging from daily to thousands of years. There have been many fluctuations since the disappearance of the most recent ice sheets: the soil types we have today are the long term consequence of the climate since that time, and our flora and fauna are a result of northward-marching warmer climates. From the early 20th century until the wet 1940’s, dry and wet periods came in cycles of about seven years. After that, the cycles are not so obvious. The variations in the weather from day to day, month to month, and year to year are large and statistics must be handled cautiously. For example, the average precipitation over the last 30 years or so is 413mm; 7mm less than the 420mm average over the last century. Has cloud seeding affected this? Has this reduction been more acute over the last 10 to 15 years? What changes will come in the next decades? Will we have to deal with a changing diseases and a complement of insects both beneficial and harmful? Our cool nights allow shade-loving plants to survive in sunnier locations than expected: again, this will change if we have hotter and drier summers.


The Government of Canada plant hardiness zone map published in 2000 shows Calgary to be generally in zone 3a. Higher, colder and windy elevations are zone 2b and the warmer areas are zone 3b. These zones are not defined by minimum temperatures alone, but in combination with six other parameters; note that the USDA plant hardiness map is based solely on temperature - and commercial growers can use different definitions of “zone”. The City of Calgary and environs exhibit several microclimates dependant on topography (elevation, slope gradient and orientation, exposure to sun and wind), exposure to Chinooks, proximity to the rivers, urban heat sources, regional temperature gradients and on the varying paths taken by hail and thunderstorms. Chinooks cause a rapid rise in temperature and consequently the desiccation of vegetation and soils. Higher ground in the city and wherever the base of the Chinook “touches down” as it travels eastwards are more likely to be damaged than protected areas of the Bow and Elbow Valleys. Within Calgary, the number of growing-days can vary. At the airport the total is almost 1370, whereas near the Glenmore Reservoir and the University the totals are 1352 and 1349 respectively, but each property has microclimates. The natural vegetation around the city reflects climatic variations well, as shown by the poplars, aspen and deciduous shrubs in the valleys, by the evergreens on north-facing slopes as on the Douglas Fir Trail at Edworthy Park, and the short grasses of south-facing slopes.


Elevation, as mentioned earlier, is a major factor in defining Calgary’s microclimates. The International Airport weather information is collected at an elevation of 1084m above sea level. The highest point within the city is at 1295m and the lowest is 975m; a difference of 320m or 1050 feet. Depending on its moisture content, air can cool at rates between 0.5° and 0.8°Celsius per 100m rise in elevation, so the higher lands to the north and west are cooler. Frost pockets can also occur in ravines and valleys as cold dense air funnels down from higher ground. Both of these features are important when temperatures hover around freezing.


As the city has grown, the urban heat source (read “pollution”) has increased, and again, helps in the creation of microclimates within the city. One positive consequence is that we have a warmer growing season in and downwind of the high traffic zones and densely populated areas. Indeed, the city centre’s annual temperature has risen several degrees above the suburbs in the last few decades. In general though, gardening is easier - or at least there’s a more moderate climate to work with - in the southeast, particularly on the flood plain of the Bow River Valley. However, as trees mature and gardeners improve their soil, each suburban property becomes easier to cultivate.


A superb example of a unique microclimate is the Calgary Zoo and Botanical Gardens’ site on the flood plain of the Bow River which has become a resource to many Southern Alberta gardeners. The gardens benefit from river moisture and a high water table in well-drained sandy soil. There is a good canopy of mature trees and bushes designed in part to protect much of the area from the extremes of wind, hail, rain and sun; the soil has been cultivated and well mulched for decades by expert gardeners. Extra heat is provided because of the proximity to the downtown core and the nearby major traffic routes. A consequence of all these factors is an array of trees, shrubs, annuals and perennials that thrive in a few communities in Calgary, but can be replicated in some areas of many gardens, especially near downtown Calgary and within the Bow and Elbow flood plains. The Perennial Trial Gardens sites at the zoo, Olds College and at the Muttart Conservatory in Edmonton test and publicize plant varieties that should grow well over much of south-central Alberta. At the zoo, some of the more unusual trees include the Manchurian walnut, and linden, but some “exotics” like the Hick’s yew (Taxus x media ‘Hicksii’) can be susceptible to winter kill.


Certainly, microclimates exist throughout the city, but vital to each gardener are the specific microclimates within his or her garden. For instance, we might need zone 2b plants in one area, and use zone 3 in more sheltered spots. Zone 4 grasses often do well here. The best ways to garden are by observation of our neighbouring communities and by experimentation, and note June Flanagan’s advice … “Native plants make great garden candidates because they have evolved with the local climate.” Of particular importance is the consideration of different plantings north and south of buildings, taking into account shady areas and those areas that have the shortest and the longest growing season. Know where the spring sun causes an early thaw for bulbs, where heat is provided by your house and how the angle of the sun and exposure to it changes with time of year. In the summer in areas without shade, hours of sunshine typically vary between 8.5 and 10.5 hours per day and this is a consideration when looking at a plant’s shade and sun requirements. Identify the best locations for alpines that like our cool, dry evenings. Guard hostas from hail with barriers. Use grasses like local fescues and taller imports, and drought-resistant plants or deep-rooted plants in dry, windy spots. Recognize where early flowering vines will flourish before nearby trees are in full leaf. Utilize mulch to insulate and preserve the precious soil and moisture: if we have less rainfall, our soils will become more alkaline; expect less black spot but more powdery mildew. Where needed, add fast-growing shade trees to protect your garden from too much sun and add wind breaks for shelter too. The creation of a variety of “rooms” allows flexibility in plant varieties that can survive climate changes, especially annuals.


Past Society Newsletters and governments are great sources of information on our variable and changing climate. Assuming that Calgary will have longer, warmer summers, we should be able to make good plant choices for eco-compatible gardens. Take some risks with planting early and extending the gardening season and try the occasional tender group of plants: nothing ventured, nothing gained!


Glynn Wright

Click on this link to see the Plant Hardiness Zone Guide


Selected references & advisers:


Andrey, J.C, Spatial study of precipitation in Calgary, U of C, MSc thesis, 1980.

Chetner, S. et al, Agroclimatic Atlas of Alberta, 1971 – 2000, Agdex 071-1, 2003

Hodgson, I. (editor), RHS magazine “The Garden”, January 2008

Klivokiotis, P, and Thomson, R.B., The Climate of Calgary, 1986, ISBN 0660121514


Judith Doyle, Corinne Hannah, Jane Reksten, Ann Van de Reep.


May 5, 2008