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Plant Hardiness Zones, (US)

Plant Hardiness Zones

State by state detail maps for United States


Plant hardiness maps allow producers to label their plants as being suitable for particular areas, and, in theory at least, this results in happy customers who can confidently buy plants that will survive in their locality. Early in 1990 the United States Department of Agriculture (USDA) published an updated version of their map of plant hardiness zones. This divides the United States into 11 zones (1-11), characterized by their average minimum temperature. Zones 2 to 10 are also subdivided into a or b, giving a total of 20 zones or sub-zones. These zones of course only apply to plants growing out of doors with no protection, but which are provided with adequate water.

Plant hardiness refers to their ability to survive the conditions of a particular location, including tolerance of heat, soil moisture, humidity and so on. This map is based only on how well they survive low temperatures in winter. Even that is a gross oversimplification. For example, are plants affected more by a single extremely low temperature night, or is the number of days of frost (the duration of winter) more important? In fact both are important, but the statistic for the map only relates directly to the former. Another limitation is that often plants will survive in an area for some time, but every now and then there will be a catastrophic cold snap that will kill them. Some risk evaluation - the probability of getting a particularly severe low temperature - often would be more useful for each locality rather than the average conditions.

Low temperature is not the only determinant of plant survival. Other environmental factors such as high summer temperature, humidity, soil temperature, etc. may be equally important. Also, many plants will survive in a locality but won't flower if the day length is inappropriate or if they require vernalisation (a particular duration of low temperature). The low temperature statistic is only appropriate for woody perennial species, and even then its use is limited. With annuals the time of planting can often be adjusted to allow growth beyond their normal geographical range.

The map is thus only useful as a very broad guide. It needs interpretation that takes into account factors other than low temperature that limit plant growth as well as local knowledge.

For an Australian Plant Hardiness Map (which has an excellent overview of Zone Maps in general), click here.

This article was originally published in 'Australian Horticulture' 90 (8) 37-39, 1991, when the author was employed by CSIRO Plant Industry.Murray Fagg (murray@anbg.gov.au)

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