If you plan to make a living off of growing vegetables, it’s in your best interest to know your number of frost free days in your area.
As most vegetable crop development is referred to as the ‘days to maturity (DTM)‘, it’s vital you know how many frost free days you have so you can pick the best crops for your growing environment.
It’s important to note that variability in daily temperatures and light intensity affect the measure of DTM, causing it to not always reflect the rate of crop growth under actual growing conditions.
We will be focusing on frost-free days in Ontario for the sake of this article since that is where we are starting our market farm.
Ontario Ministry of Agriculture, Food, and Rural Affairs (OMAFRA) is a great resource for growers in Ontario and is where we will be getting the information in this article.
Climate Zones In Ontario
Ontario has developed a Climate Zone Map (1976-2005), which shows the Ontario climatic zones based on the average frost-free period.
Zone A has the longest growing season.
The growing seasons of the other zones are progressively shorter.
Note that the zone lines on the map are approximate and do not represent sharp changes in climate.
The length of the growing season and temperatures change gradually from one zone to another.
I found it a little tricky to find my exact climate zone according to this map.
Luckily, they have a text version where the names of each county are written out and categorized according to its climate zone.
You can find the written version here.
Each climate zone has its own Average Frost Free Period, presented in a convenient table which gives the average dates of first and last frosts for each zone.
Interpreting The Frost Free Table
The dates listed in the Table indicate that 50% of the time (one year in two) there will be no frost in these locations after that date.
There is a 25% risk of a frost occurring seven days after the listed last spring frost date.
Fourteen days after the listed date, the risk decreases to 10%.
On the fall side, there is a similar risk associated with frost occurring before the first fall frost date.
The frost dates refer to the occurrence of 0°C at a standard height of 1.5m, which tender crops will often be damaged.
Frost-hardier crops are not usually damaged until temps at this height drop below -2°C.
The date of occurrence of -2°C is usually about 2 weeks earlier in the spring or later in the fall than the dates mentioned on the table.
Growing conditions are also influenced by local variations in topography, altitude, natural air drainage, and proximity to water.
Cold air tends to flow in low-lying areas.
Plants at the bottom of a slope are most often the first to suffer frost injury.
Take local variations into consideration when deciding the location and time of planting your crops.
Here is a list of vegetable crops and their comparative hardiness, according to their climate zones.
First Field Seeding or Planting Dates for Commercial Production Table.
There are a few frost-free calculators out there on the internet to try as well, I like this one here.
Importance Of Knowing Number Of Frost Free Days
Now that you know about frost free days and climate zones, how does this apply market farmers?
While this information is not absolute, as there are many variables affecting the amount of frost free days, it is a good indication of how long your growing season will be.
Having this information on hand can help you plan what veggies to grow, as well as when to start each crop.
Whether you’re starting your seeds indoors or planting directly in your garden, you will want to coordinate your schedule with the last frost date.
This does not necessarily mean that you will have to wait until the last spring frost to plant.
If you want to get an early start to your growing season or extend your season into the fall and winter, there are things you can do to provide protection for your plants.
You can check out some season extension articles here for more information.
How many frost free days do you have each season? Let us know in the comments below, along with your location.
I’ll go first, we are growing in Grafton, Ontario, and have approximately 146 frost free days.
Kathy & Jon
your friendly neighbourhood growers