Climate zones and planting dates for vegetables in Ontario

Introduction

Most vegetable production is concentrated in Southern Ontario. There are differences in climate even in this small area. The length of the growing season and average temperatures can vary greatly from one area to the next. Vegetable crop development is usually referred to by the number of "days to maturity." This measure indicates the average number of calendar days from planting to harvest maturity. However, due to variability in daily temperatures and light intensity, this measure of "days to maturity" does not always reflect the rate of crop growth under actual growing conditions.

Climate Zones

Figure 1, Climate Zone Map of Ontario (1976-2005), shows the Ontario climatic zones based on the average frost-free period. This updated climate zone map of Ontario was created using a larger dataset than the previously published 1961-90 map. As a result, the amended zones reflect a higher degree of accuracy when compared to the previous version.

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Figure 1. Climate Zone Map of Ontario (1976-2005). Source: Weather Innovations Incorporated

Zone A, in Essex and parts of Kent County, has the longest growing season. The growing seasons of the other zones are progressively shorter. 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.
Table 1, Average Frost-Free Period in Climatic Zones of Ontario (1976-2005) gives the average dates of first and last frost (0°C) for each zone.

Risk of Frost

The dates listed in Table 1 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 7 days after the listed last spring frost date. Fourteen days after the listed date, the risk decreases to 10%. Conversely, there is a similar risk factor associated with frost occurring before the first fall frost dates.

Note that the frost dates in Table 1 refer to the occurrences of 0°C at a standard height of 1.5 m, at which time tender crops will often be damaged. Frost-hardier crops are not usually damaged until temperatures 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 in the table. For a list of vegetable crops and their comparative hardiness, see Tables 2a-2d, First Field Seeding or Planting Dates for Commercial Production.

Growing conditions are also influenced by local variations in topography, altitude, natural air drainage and proximity to water. Cold air tends to flow into low-lying areas. Plants at the bottom of a slope are often the first to suffer frost injury. Local variations must be considered in deciding the location and time of planting.

For more information on recommended planting conditions, see OMAFRA’s vegetable publications.