Toronto’s Last Frost Date

Frost dates.
They dictate many of our actions in the garden. What are frost dates? How do we use frost dates and most important, when are Toronto’s frost dates?

Last frost occurs first in a calendar year. This often confuses new gardeners when reading plant labels. As winter wraps up and transitions to spring, it will give us one last freeze, our “Last Frost”.

First frost occurs towards the end of our calendar year, as Autumn transitions into Winter. The first cold snap we receive marks the end of our growing season for certain crops, our “First Frost”.

How are Frost Dates determined?

Frost dates are educated guesses based on historical climate data. In Canada, our hardiness zones are based on the same collection of data, calculated using 1981-2010 Climate Normals. Since they are guesses, frost may occur before or after these dates. The farmers almanac states a 30% probability of this occurring.

When is Toronto’s last frost date?

May 4.

When is Toronto’s first frost date?

October 13.

What is the length of Toronto’s growing season?

161 Days.

When we count the days between these two frost dates we find the length of our growing season. Why would we do this?

Annuals, for example, your vegetable garden, each have a “days to maturity” or “days to harvest” associated with them. If this number is larger then the length of our growing season, we give them a head start by starting seeds indoors. How much of a head start depends on the difference between “days to harvest” and “length of growing season”.

Now you might be wondering about the common “plant on the May 24 weekend” or “sow seed on St. Patrick’s Day” and we dive into that here.