The survival of plants is affected by a number of factors, including sun, rain, wind, humidity, soil quality, soil type and temperature. Plant climate zones, also known as hardiness zones, help gardeners work out which species of plants will survive in a particular region of the world. However, plant climate zones should only be used as a guide, as many factors determine the success of the growth of a plant including micro-climates and other changes in climates within the same locality.
The American System of Plant Climate Zones in the East: USDA
Plant climate zone charts were established through the collection of data across the United States which showed the average lowest temperature of a particular region. This data formed the basis for a plant climate zone map that initially divided the United States into eleven plant climate zones.
The most used plant climate zone map in the Eastern United States of America (and Canada) was compiled by the United States Department of Agriculture (USDA). The first USDA plant hardiness map was compiled in 1960; the most recent version of the USDA hardiness zone map is 1990 and is compiled from weather records from 1974 to 1986.
However, the Arnold Arboretum of Harvard University also compiled a plant climate zone map in initial studies. The USDA plant hardiness map can be adapted and used by other countries in the world including Europe and Australia.
The American System of Plant Climate Zones in the West: Sunset
The Western United States of America has great variations in weather patterns and humidity which is affected by mountain ranges and storms that come in from the Pacific Ocean. Consequently, gardeners in the Western United States often refer to the Sunset plant climate zone map which was established by Sunset magazine. Sunset uses a twenty four climate zone system and concentrates on the Western states of:
- Alaska
- Arizona
- California
- Colorado
- Hawaii
- Idaho
- Montana
- Nevada
- New Mexico
- Oregon
- Texas
- Washington
- Wyoming
- Utah
Calculating Plant Climate Zones
A sample of the USDA cold hardiness zones shows temperature ranges as follows; be aware that this is just a guide and there might be slight deviations in temperature ranges. The Sunset plant climate zone map is more complex as it also factors in length of growing season, rainfall, humidity, winter lows and summer highs in temperature. USDA climate zones are:
- zone 1 – below - 50 F/- 45.6 C
- zone 2 – - 50 F to - 40 F/- 45.6 C to -40 C
- zone 3 - - 40 F to -30 F/- 40 C to -34.5 C
- zone 4 - - 30 F to – 20 F/-34.5 C to -28.9 C
- zone 5 - - 20 F to – 10 F/ - 28.9 C to -23.3 C
- zone 6 - - 10 F to 0 F/- 23.3 C to -17.8 C
- zone 7 – 0 to 10 F/-17.8 C to -12.3 C
- zone 8 – 10 F to 20 F/-12.3 C to -6.6 C
- zone 9 - 20 F to 30 F/-6.6 C to -1.1 C
- zone 10 – 30 F to 40 F/ -1.1 C to 4.4 C
- zone 11 – above 40 F/ above 4.4 C.
Disadvantages of the Plant Climate Zone System
The plant climate zone system of maps devised by the USDA only takes into account the average minimum temperature for a particular region. It does not take into account the average maximum temperature for a region or other factors such as humidity, rainfall, soil quality and heat. In addition, climate change factors in recent years are not accounted for in the 1990 USDA plant hardiness zone map which uses data from as far back as 1974.
Plant Hardiness Zone Maps
Although both the USDA plant hardiness zone map and the Sunset plant climate zone map are useful guides to determining the probability of the survival of a certain plant species in a particular region, remember that local factors may affect growing and survival rates for plants too. However, plant climate zone maps are a handy tool for those new to the world of botany and gardening.
References:
- The National Gardening Association web site, accessed July 19, 2010
- The United States National Arboretum web site, accessed July 19, 2010
- Oregon State University web site, Sunset's Climate Zones, accessed July 19, 2010
- Sunset Magazine web site, accessed July 19, 2010
Resources:
USDA Plant Hardiness Zone Map
Sharon Falsetto - Sharon Falsetto is a business owner, certified clinical aromatherapist & professional writer with a life-long interest in plants.
