Garden Hardiness Zones and Frost Dates Explained

Plant hardiness zones are a way for gardeners to determine what can successfully be grown in their particular region. Every plant has a threshold where temperatures become too cold for it to survive. Individual mechanisms within a plant respond to cold temperatures in different ways while tender plants have no mechanisms to survive freezing temperatures.

For example, a basil plant is considered tender and will not withstand even a single degree of frost, while a rhubarb plant can withstand extremely cold temperatures by going dormant and concentrating sugars in the roots.

Plant hardiness zones aid gardeners in making informed decisions about what to plant in their garden. They are a reflection of many years of minimum temperature data for a region and that is all they can provide. To find out your hardiness zone in Canada or USA follow the maps and find your location then match the colour to the zone legend. Use your hardiness zone as a guide for growing perennials, shrubs and trees in your garden and you will lose less plants to frost.

There are however a few more considerations that your zone map will not help you with. A problem with hardiness zone maps is that they don’t take into consideration the conditions for any region. They are a measure of minimum cold temperatures. If you live in a region that has frequent freeze/thaw cycles in the winter you may still be unsuccessful with plants that are hardy for your region.

For example, the Pacific Northwest gets a great deal of rain in the winter which may kill off plants that are fine in a dry region of the same zone such as Northern Texas. Both regions are zone 8 on a hardiness zone map but they have vastly different climates.

Another great illustration of regional variation is snow cover. Because of the insulating qualities of snow, a region that gets a good cover of snow that remains all winter will perhaps be able to grow perennial plants that are not considered hardy in that zone.

A microclimate is an area where the temperatures of a certain place are below or above the average temperature of the surrounding region. These microclimates can occur naturally because of geographic features such as a body of water or a mountain. It is possible to create your own microclimate with some planning. South facing gardens (Northern Hemisphere) that are protected from chilling winds by buildings, hedges or fencing will be substantially warmer than surrounding areas.

Also the placement of rock or concrete can act as heat sinks that gather heat from the sun and release it slowly at night to subtly affect the climate in small pockets and make conditions more favourable for certain plants.

Hardiness zones will not tell you about frost dates for your area. Frost dates are probably a more important guide for success when growing annual vegetable crops than hardiness zones. They will give average dates for the last frost in the spring and for the first frosts in the fall. Obviously this is very important for vegetable crop planning and determining best times for starting and planting out seedlings.

Andrew Couzens is a software engineer turned soil scientist.

A life long sufferer with inflammatory bowel disease, he was motivated to enter the agricultural space in an effort to “be the change” he believed was necessary to heal his own body.

Healthy plants come from healthy soil, and healthy soil comes from working with nature, not against it.

Leveraging his knowledge and experience in software engineering, he started Terra Flora Organics with a goal of helping conventional growers move from unsustainable practices that destroy soil and negatively affect the health of people and the planet, to regenerative practices that allow intensive farming whilst building soil and healing our minds and our bodies.

Andrew Couzens