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Hardiness Zones & Frost Dates

Written by Guillaume Corriveau

2 February 2021

The Plant Hardiness Zones (PHZ) map is often used/referred by gardeners to find out what can grow in their areas. However, you need to be careful while using this tool..

The PHZ map was first developed in the United States. Attempts date back as far as 1927, but the modern PHZ map promoted by USDA was first published in 1960. In the United States, the PHZ map is based on the annual average minimum temperature. It goes from zone 0 to 13 in 10°F (5.6°C) increments. In 1990, subdivisions “a” and “b” were introduced in each zone resulting in an increment of 5°F (2.8°C). The latest version of the PHZ map was released by USDA in 2012 with updated data from 1976 to 2005. Many areas are now considered to be half zone warmer compared to the 1990 PHZ map.

The PHZ map can be a useful indicator, but since only one parameter is being monitored, it doesn’t tell the whole story gardeners would like to hear. The Canadian PHZ map, originally developed in 1967, is a prime example of a more refined approach. Indeed, 7 factors are included in an index describing the zones ranging from 0 to 9 with a/b subdivisions. These factors include the minimum temperature of the coldest months of the year, the maximum temperature of the hottest months of the year, the amount of precipitation 

during the growing season, the frost-free period, snow cover, maximum wind gusts and a winter factor. While the Canadian PHZ Index is quite extensive, even its maintainers point out that zones are only a general guide.

Now that you know more about PHZ maps, note that the benefit of these maps is mainly for perennials. Indeed, for annual plants, constituting most of the plants in the vegetable garden, the winter conditions described by the PHZ map will never be met. Therefore, as a gardener you need to follow other indicators.

hardiness zones are only a general guide

Besides extremely specific parameters related to your site, such as soil quality and sun exposure, what really matters is the last frost date in the spring and the first frost date in the fall. With these settings you can determine when to start your seedlings and when to transplant those that have been started indoors. With frost dates, you can also make sure that the growing season in your area is long enough to get your frost-sensitive plants to maturity!

To get your hardiness zone and associated frost dates for your location for free and easily, simply sign up for the HireNature ecosystem and go to your Dashboard.

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