Frost Dates, Frost Warnings and Protecting Plants
Frost decides when the gardening season starts and ends in Lethbridge. Here’s when to expect it, what Environment Canada’s frost advisories mean, which plants can take a nip and which can’t, and how to get your garden through a cold night.
On this page
Frost watch for tonight
Checking tonight's forecast…
Current readings from our West Lethbridge station. Forecast low and alerts from Environment and Climate Change Canada for Lethbridge. Low spots, the river valley and coulee bottoms can be several degrees colder.
Our frost dates
Frost dates are averages and odds, not promises. For the Lethbridge research station, Environment Canada’s climate records give:
In plain words: in about two springs out of three, the last frost has come and gone by May 25, but in the third year it hasn’t. In a cold year, frost can come in early June, and the long weekend in May is not a safe planting date for tender plants, however warm it feels.
The rule of thumb for Lethbridge: hardy crops go in as soon as the soil can be worked in late April or early May. Frost-tender plants go out around the last week of May, with something handy to cover them. Heat-lovers like peppers, melons and basil are happier waiting until early June. Even then our nights often dip below 10°C, which they dislike, so keep a cover handy.
How frost forms on a clear night
Most spring and fall frosts here are radiation frosts. On a clear, calm night, the ground and plants radiate their heat straight up into the sky. With no cloud to hold it in and no wind to mix the air, the coldest air settles right at the ground. That’s why:
- Frost can form when the forecast low is 2 or 3°C. Forecast temperatures are measured about 1.5 m above the ground. At plant level it can be several degrees colder.
- Clear and calm is the danger sign. A cloudy or breezy night usually stays warmer, even when the forecast low looks similar.
- Low spots freeze first. Cold air flows downhill like water and pools in the river valley, coulee bottoms and low corners of a yard.
The other kind, an advective freeze, comes when a mass of arctic air pushes in on a north wind. It freezes everything, high ground and low, and covers don’t help much. Luckily these are rare once spring is well under way.
For more on why clear, calm nights get so cold, see Sun, Seasons and Temperature in Weather School.
Frost advisories and warnings
During the growing season, Environment and Climate Change Canada issues a frost advisory when temperatures are expected to fall to near the freezing mark overnight, cold enough to damage frost-sensitive plants. They show up on the forecast for Lethbridge and on the WeatherCAN app.
- Advisory issued: cover tender plants, bring containers and hanging baskets in, and harvest anything ripe that can’t take frost.
- No advisory, but clear, calm and a low of 3°C or less: cover the tender plants anyway, especially in low spots. The advisory covers a wide region, and your yard may be colder.
- Cloudy or windy with a low of 3°C: usually safe, but check the forecast again before bed.
Check tonight’s forecast low and sky for West Lethbridge on the forecast page. Or use the frost watch at the top of this page, which does the checking for you.
Which plants can take a frost?
| Group | What they can take | Examples |
|---|---|---|
| Tender | Damaged or killed at about 0°C. Cover on any frosty night. | Tomatoes, peppers, eggplant, cucumbers, squash, pumpkins, melons, beans, corn, basil, potatoes (the tops), most bedding annuals such as petunias, begonias, impatiens and marigolds, and tropical houseplants summering outdoors. |
| Semi-hardy | Take a light frost, down to about −1 or −2°C, especially if hardened off. | Lettuce, Swiss chard, beets, carrots (tops), cauliflower, celery, potatoes (tubers in the ground), snapdragons, dusty miller. |
| Hardy | Shrug off frosts of −3 to −4°C or colder. Many taste sweeter after a frost. | Peas, spinach, kale, cabbage, broccoli, Brussels sprouts, onions, garlic, radishes, turnips, parsnips, pansies, violas, sweet alyssum. |
Temperatures are approximate. Young seedlings, plants straight from a greenhouse that haven’t been hardened off, and plants in pots are all more easily damaged.
Protecting plants on a frosty night
- Water during the day. Moist soil stores more heat than dry soil and releases it through the night. Water in the morning or early afternoon, not at dusk on the plants themselves.
- Cover before sunset. The goal is to trap the heat the ground stored during the day, so get covers on while the ground is still warm. Old bed sheets, blankets, burlap, cardboard boxes, buckets and garden frost cloth all work.
- Cover to the ground. A cover that reaches the soil on all sides holds in the ground’s warmth like a tent. Weigh the edges down with rocks or bricks; Lethbridge wind will find any loose corner.
- Keep plastic off the leaves. Plastic touching leaves conducts the cold straight through. If plastic is all you have, hold it up on stakes or hoops so it doesn’t touch the plants, or put a sheet underneath.
- Bring pots inside. Containers and hanging baskets freeze much faster than plants in the ground. A garage or porch is plenty.
- Take covers off in the morning once the temperature is above freezing, so the plants don’t cook under them in the sun.
Season extenders: water-filled plant protectors (“walls of water”), floating row cover on wire hoops, cold frames and cloches let you plant tomatoes and peppers a week or two early and protect them through cold nights. They also shield young plants from the wind.
The morning after
- Don’t panic, and don’t prune yet. Frost-damaged leaves look dark and water-soaked at first. Wait a few days to see how far the damage goes before cutting anything off.
- Shade frosted plants from early morning sun if you can. A slow thaw is gentler than a fast one.
- Tender crops that are blackened right to the ground (tomatoes, beans, squash) usually won’t come back. There’s still time to replant in early June, and many garden centres stock replacements for exactly this reason.
- Perennials and shrubs that lose new leaves to a late frost almost always push out a second set.
Fall frost and stretching the season
The first fall frost often comes in early or mid-September, and it’s frequently followed by two or three weeks of warm, sunny weather. If you can get tender crops through that first cold night, you often gain weeks of harvest.
- Cover tomatoes, peppers and squash on the first frosty nights rather than giving up on them.
- When a hard frost is forecast, pick every tomato showing any colour, and the full-sized green ones too. They ripen indoors on a counter, out of direct sun.
- Harvest squash and pumpkins before a hard frost and leave a few centimetres of stem on.
- Hardy crops like kale, carrots, parsnips, Brussels sprouts and leeks can stay in the garden well into October and taste sweeter after frost.
- Dig gladiolus corms and dahlia tubers after the first frost blackens the leaves, dry them off, and store them indoors for the winter.
Sources
- Environment and Climate Change Canada, Canadian Climate Normals 1981–2010, Lethbridge CDA, frost probabilities.
- Environment and Climate Change Canada, public weather alert criteria (frost advisory).
- Alberta Agriculture, Agroclimatic Atlas of Alberta.