Gardeners, rejoice: Toronto has (probably) seen the spring’s last frost

We don’t want to jinx it.

But a spring frost after May 10 has become increasingly rare in Toronto in the past 70-odd years, weather records kept at the Pearson International Airport site since 1938 indicate.

Over a 75-year period, only 22 per cent of Toronto’s last frosts of the spring have happened after May 10. After May 12, that drops below 20 per cent.

We crunched 75 years of weather data from the Pearson International Airport site, looking for the last date in the spring where the low was below zero.

The median final frost date has been edging earlier since 1980, says Environment Canada’s senior climatologist David Phillips – from May 8 during 1951-80, to April 30 now.

“It’s the urban heat island, but it’s also climate change,” Phillips said.

The central city tends to be warmer than the 905 because of heat stored in buildings and concrete.

“The more urbanized you are, you can grow more exotic tomatoes, because your backyard has got a different kind of climate. It really does show up at this time of the year, in terms of planting.”

For the cautious, the traditional frost-free planting date of May 24 works fine – 95 per cent of Toronto’s final frosts have fallen before Queen Victoria’s birthday.

But the truly risk-averse can wait until May 29 – the latest spring frost in the data was on May 28, 1949.

If the temperature hovers around freezing, local factors can influence whether tender plants get nipped by frost, Phillips explains. Cooler air can pool in low areas:

“You can have valley locations that would be touched by frost, while a hilly location would not be touched by frost.”

Soil makes a difference, as well.

“With a sandy soil, there’s going to be more heat radiated at night, so there’s a chance that you’re going to see frost.”

But, as always, Mother Nature has the last word, Phillips warns:

“There are always wild cards in this thing. There is no guarantee with weather in this country.”