Accurate Frost Forecast = Happy Gardener


By John Sauder

As a broadcast meteorologist, I’m always very careful when it comes to issuing a forecast because I know that it affects how Manitobans will plan their day. When it comes to a forecast for frost, I’m extra careful because it affects the incredible effort that gardeners undertake in spring and all season. And I have extra pressure; my wife is a gardener!

A forecast that includes a risk of frost, especially in late spring, really gets the attention of gardeners all over the forecast region. It can be a tricky forecast. I don’t want to tell people to cover their plants when it’s not required, because that can be a lot of work. On the other hand, I don’t want to see newly planted flowers and gardens destroyed by frost either, because that makes for a very unpopular local weatherman. When that happens (not often), I spend a lot of time hiding in the basement, and you won’t see me walking through the local garden centre for a while.

All official watches, warnings, and advisories are issued by Environment and Climate Change Canada. A Frost Advisory is issued during the growing season when widespread frost formation is expected over an extensive area and surface temperatures are expected to fall near freezing in the overnight period. Sometimes there is no frost advisory but I will mention a “risk of frost” in my forecast. That’s different from a frost advisory. When I talk about a “risk of frost”, I usually say something like “If your area usually gets frost when the low is going to be +3 Celsius (C), cover your garden”! That’s because frost will usually occur in quieter wind conditions where the coldest air sinks into low lying areas. Even a difference in elevation of one meter can make a +3 C low for your general area turn into a zero or -1 C in your particular location, and produce some early morning frost on your garden and flowers.

We know that frost happens when the temperature dips below zero and something called deposition happens. The water vapour in the air goes from a gas state directly to a solid state without passing through the liquid stage first. Ice crystals deposit on surfaces like tree limbs, your car window, and on your plants. If that phase change goes in the other direction from a solid to a vapour, that’s called sublimation.

When plants get cold enough to have frost form, the plant cells freeze, which disrupts the movement of fluids and damages the plant tissues. It’s nasty stuff for a plant when that happens. A light frost between -2 and 0 C won’t hurt your plants too much, but a hard frost that happens at temperatures colder than -2 will wreak havoc on your garden, and unless you receive that warning from your meteorologist, my inbox fills up with emails from upset gardeners.

The lowest temperature will typically happen in the early morning just before sunrise after a clear and calm overnight period. That’s when we have had the longest period of radiational heat loss. Unless you don’t mind getting up really early, it’s best to cover the plants with a sheet or blanket the night before.

Here are the top three weather questions your local meteorologist (that’s me) gets in the spring:
3. Is this summer going to be warm?
2. What’s the weather going to be like at 3:00pm on my wedding day?
(still 10 weeks away)
And the #1 question… when can I safely plant my flowers/garden?

For the Winnipeg area, the average last frost of the spring season is around May 24th. The average growing season is around 121 days. (Winnipeg airport data)

The latest spring frost was June 21, in 1940, 1969 and 1972. I bet there were a few upset gardeners during those late frost events. The earliest spring frost was April 26, 1922, but I don’t think Manitobans generally plant outdoors that early in the season.

At the other end of the season, the earliest first fall frost (temp of 0.0 C or lower) in Winnipeg was August 20, 2004. I can’t imagine having put in all that effort to grow your garden and flowers to have them destroyed by frost during the third week of August. The latest first fall frost in Winnipeg was October 27, 1963.

The longest growing season in Winnipeg was in 1963 (157 days). The shortest growing season was in 1875 (69 days).

The growing season varies across southern Manitoba and so does the average date of the last spring frost. Below is a map that shows the average dates of the last spring frost. Keep in mind the term average in this case means a 50% risk as explained on the right hand side of the map.

Here is a map that shows the average date of the first fall frost at 0 C.

As temperatures continue to drop through September and October, know that I will be watching for the risk of frost and doing my best to warn gardeners when that frost may happen.

Happy harvesting, and I hope you embrace and enjoy the change of season.

CBC Meteorologist John Sauder provides you with daily comprehensive weather forecasting and reports on CBC News Winnipeg.
He earned a certificate in broadcast meteorology from Mississippi State University in 2007 and is endorsed by the Canadian Meteorological and Oceanographic Society and holds the American Meteorological Society’s Seal of Approval for Weathercasting.
John started his broadcasting career in 1984 as a commercial pilot, flying a Cessna 152 over Winnipeg broadcasting traffic reports on two local radio stations. During ten years on that job, he earned 7,800 hours of flying time.
John transitioned into television in 1995 and went back to university in 2004 to study meteorology. He and his wife Kim are an active couple who love all kinds of sports and the great outdoors in all seasons.