By Robert Pavlis
Most common garden plants have been assigned a hardiness zone range that indicates if a particular plant will grow in your climate. It is a range of numbers with both a high and a low limit because most plants have trouble growing in very cold or very hot regions. Some take hardiness zone numbers as gospel, but they are only an approximation.
Each country or region has developed their own zone numbers. A zone 6 plant in one country may mean something completely different in another country. Reference material, especially on the internet, rarely mentions which country is used, although most sources use the US zone numbers.
Canada and the US use a very similar numbering system, but you may live in zone 5 on the Canadian system and zone 6 on the American system. The UK does have a numbering system, but since they don’t have very cold or very hot regions, they tend to simply state that a plant is hardy or not.
The hardiness zone numbers in the US, as with most systems, only look at temperatures, but other factors may be just as significant. For example, many alpine plants can grow in very cold conditions if they remain very dry. These plants quickly die in the warmer, wet winters found in the Northeastern US. Soil drainage can also play a big role in the hardiness of a plant. Winter in clay soils can kill some plants that would survive well in the same area if they were provided with sandy soil.
How are the zone numbers developed for plants? When a new cultivar is developed, it has no zone rating. To establish a zone value, the plants are grown in a variety of locations. This provides some information about the hardiness, but it is still only an approximation. After introduction, the plant is grown by more people, which provide even more data. Over time the zone value for the plant is fine-tuned.
No official organisation regulates these numbers. Each grower or seller will provide the number they think is best. If you Google a plant, you will find a wide range of values. Nurseries in warmer areas don’t really care too much about the cold end of the range and underestimate it, and northern nurseries are always trying to stretch the lower end of the range to get more sales.
Snow cover plays a big role. Plants will survive in much colder regions if they’re covered by snow all winter. The snow protects the plant from extreme cold by holding the heat from the soil around the plant. The same plant will parish in warmer areas that don’t get reliable snow.
Summer heat and humidity also plays a factor. The famous blue poppy does not grow well in the humid, warm summer found in Ontario, Canada, and subsequently dies in winter, but it grows well a few hundred miles away in Quebec, Canada.
Local microclimates can also play a role. Cold air is heavy and settles in low areas. The bottom of a hill is usually colder than higher up on the hill. Inside the city is warmer than the outskirts. On a hardiness zone map, all of these areas are given the same number.
Zone numbers also get updated from time to time. Canada had a major revision in 2000, and the US updated theirs in 2012. That means that older books and web pages may be out of date.
Plant hardiness zones are a very useful tool for determining the likelihood that a plant will survive in your garden, but it is not a guarantee. Use them as a guide only.
Written by Robert Pavlis, author of three books, , Garden Myths , Soil Science for Gardeners and Building Natural Ponds.
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