Our Growing Conditions
Lethbridge is sunny, dry, windy and high, with a short season and winters that swing between deep cold and chinook thaws. None of that stops a good garden, but each one changes how you plant, water and protect. Here’s what you’re working with, and what to do about it.
On this page
Lethbridge at a glance
Climatologists call our climate semi-arid: warm summers, mild winters for the prairies, and not much rain. Lethbridge is one of the driest and windiest cities in Canada, and one of the sunniest. Summer days average about 26°C in late July, and January nights average about −12°C, with chinook winds breaking up the cold several times most winters.
Our hardiness zone
A hardiness zone tells you which perennials, shrubs and trees can survive the winter here. Lethbridge sits in Canadian zone 4. Most maps put the city in 4a or 4b, and sheltered yards in town often do a little better than open country.
Canadian and American zones are not the same. Canada’s zones are worked out from several things (winter lows, summer heat, rainfall, snow cover, wind and frost-free days), while the US system uses only the coldest winter temperature. A plant tag from a US supplier that says “zone 4” may mean something different. Look for Canadian zone ratings, and when in doubt, choose plants rated zone 3 or lower for exposed spots.
What it means for you: perennials, shrubs and trees rated for zones 2, 3 and 4 are your safest bets. Zone 5 plants can sometimes survive against a south-facing wall with good snow cover, but treat them as an experiment, not a sure thing. Annuals and vegetables don’t care about zones; for them, the frost-free season is what matters.
A short season
Our frost-free season runs roughly from late May to mid-September. At the Lethbridge research station, Environment Canada’s records give about a one-in-three chance of frost after May 25 and about a one-in-three chance of frost before September 11. That leaves around 108 to 120 frost-free days in most years, and some years less.
That’s enough for most vegetables and annual flowers, but not for crops that need a long, hot summer. Tomatoes, peppers and squash need a head start indoors, and long-season varieties of melons and corn can run out of time.
What it means for you: pick varieties that list a short “days to maturity” (under about 75 days for tomatoes and corn), start heat-lovers indoors, and use covers in spring and fall to stretch the season a few weeks at each end. The frost page covers dates and protection in detail.
Dry air, little rain
We get only about 375 mm of precipitation a year, and roughly a quarter of that falls as snow. The wettest stretch is usually late May and June, which helps gardens get going, but July and August are often hot, dry and windy. Low humidity and wind pull moisture out of soil and leaves fast.
What it means for you: water deeply and less often, so roots grow down rather than staying at the surface. Mulch everything: a layer of straw, shredded leaves or wood chips keeps soil moist and cool, and cuts weeds. Water in the morning, when less is lost to evaporation and wind. A rain barrel on a downspout catches the June rain for July.
Wind
Lethbridge averages 116 days a year with wind gusts of 40 km/h or more, the second-most of any city in Canada. Wind dries plants out, snaps tall stems, strips blossoms, tips over pots and makes young transplants struggle for days.
What it means for you:
- Plant tall things (sunflowers, corn, gladiolus, staked tomatoes) where a fence, house or hedge breaks the prevailing west and southwest wind.
- A windbreak works best when it slows the wind rather than blocking it solid: a slatted fence or a hedge shelters a strip several times its own height behind it.
- Stake or cage tall plants early, before they need it.
- Choose low, bushy varieties for exposed spots, and give new transplants a temporary wind shield for their first week.
Chinooks and winter swings
A chinook can lift the temperature 20 degrees or more in a few hours and keep it above zero for days, in the middle of winter. Then the cold comes back. Those swings are hard on plants in three ways:
- Winter burn: evergreens lose water through their needles on warm, windy, sunny winter days, but can’t replace it from frozen ground. The result is brown, dried-out needles in spring, usually on the south and west sides.
- Waking up too early: a long mild spell can coax buds to swell, and the next cold snap kills them. Fruit trees and early-flowering shrubs are most at risk.
- Heaving: repeated freezing and thawing can push shallow-rooted perennials up out of the ground.
What it means for you: give evergreens a deep soaking in October before freeze-up, and again during a long winter chinook if the ground is thawed. Mulch perennial beds after the ground freezes to keep it frozen, rather than to keep it warm. Choose fruit varieties bred for the prairies, which tend to stay dormant through mild spells.
Hail
Southern Alberta has some of the most frequent hail in Canada, mostly from June to August, and usually on hot afternoons when thunderstorms build over the foothills and drift east. A few minutes of hail can strip a vegetable garden.
What it means for you: watch the forecast on stormy afternoons and keep an old sheet, a tarp on stakes or a piece of shade cloth handy to throw over the most precious beds. Hardware cloth or hail netting over a frame is the long-term answer for serious vegetable gardens. Many plants recover from hail better than they first look: give them a week before you pull anything out.
Strong sun at 900 m
At about 910 m above sea level, with clear, dry air, our sunlight is strong. That’s good news for tomatoes and sun-loving flowers, but it can scorch shade plants and stress new transplants.
- Sunscald on young trees: on bright winter days, the sun warms the southwest side of thin-barked young trees, which then freezes hard at night and splits. Wrap young trunks with a light-coloured tree guard from fall to spring for the first few years.
- Shade plants need real shade: a plant labelled “part shade” usually wants afternoon shade here, not morning shade.
Our soil
The natural soil around Lethbridge is a fertile, dark brown prairie soil. Many city yards, though, sit on clay subsoil or fill left over from construction, with little organic matter and a hard, compacted layer under the topsoil. Our soils are also usually alkaline (a pH above 7), which is fine for most vegetables and prairie plants but makes life hard for acid-lovers.
What it means for you: add compost every year, because it’s the single best thing you can do for clay. Don’t work clay when it’s wet; wait until a handful crumbles. Skip plants that need acidic soil, like blueberries, rhododendrons and azaleas, or grow them in containers. See soil, compost and watering for the full story.
Your yard’s microclimate
Within the city, conditions vary more than you’d think:
- River valley and coulee bottoms are frost pockets. On clear, calm nights, cold air drains downhill and pools in low spots, so the valley can be several degrees colder than the top of the coulees.
- West Lethbridge and other open, high ground catch more wind but drain cold air away, so they often escape light frosts that hit the valley.
- South- and west-facing walls soak up heat and give it back at night: the warmest spot in most yards, and the place for tomatoes, peppers and slightly tender perennials.
- Older neighbourhoods with big trees are more sheltered, but shadier and drier under the trees.
Try this: Spend one season watching your own yard. Notice where the snow melts first in spring and stays longest, where frost lies on a cold morning, and which corner the wind hits hardest. That map of your yard is worth more than any book.
Our West Lethbridge station records temperature, wind, humidity and rainfall around the clock. See today’s readings and forecast on the forecast page, or learn more about our weather in Weather School.
Sources
- Environment and Climate Change Canada, Canadian Climate Normals 1991–2020, Lethbridge Airport (precipitation, temperature, sunshine, wind days).
- Environment and Climate Change Canada, Canadian Climate Normals 1981–2010, Lethbridge CDA (frost probabilities).
- Natural Resources Canada, Plant Hardiness of Canada.
- Alberta Agriculture, Agroclimatic Atlas of Alberta.