The USDA Hardiness Zones

Hardiness zones are geographical area in which a given category of plants are able to grow and survive.

Hardiness zones are geographical area in which a given category of plants are able to grow and survive. The zone is defined by climatic conditions including temperature. Hardiness zone was a concept of the US Department of Agriculture, a department that develops and executes federal laws related to agriculture, forestry, farming, and food. It was first developed to guide in the landscaping gardening in the US. Today, most countries around the world also use the hardiness zones.

Overview of USDA Hardiness Zones

Hardiness zones are based on the average annual extreme minimum temperatures over a period of the past 30 years and not the lowest temperature ever recorded in the past or will be recorded in the future. A complex algorithm is used in determining the USDA hardiness zones. The method considers factors such as altitude change and proximity to water bodies. The current USDA Hardiness Zones divides a region or country into 13 separate planting zones, with each growing zone being 10 0F colder or warmer than the adjacent zone. Each growing zone is divided into “a” and “b” with 5 0F temperature difference.

Apart from hardiness zones, several other factors also contribute to the success and failure of crops. These factors include and not limited to light, humidity, soil, snow, and pollution. The way the plants are placed on a landscape and their health may also influence their survival.

The US Hardiness Zones

The warmer zones of the US, including zones 8, 10, 11, are in the southern half of the country and also the lower Pacific and Atlantic Coast while the cooler zones which include zones between 7 and 4 are on the northern and central parts of the country.

European Hardiness Zones

Britain and Ireland experience milder winter because of the moderating effect of the North Atlantic climate on the temperate maritime climate. The hardiness zones for Britain and Ireland are quite high, ranging from 7 to 10. Northern European countries are in different zones. Norway is in zone 3 while Sweden and Finland in zones ranging between 3 and 7. Central Europe transitions from oceanic to a continental climate. Thus, the hardiness zones in Central Europe tend to decrease eastwards from zone 8 on the North Sea coast of Belgium, Germany, and Dutch. Southern Europe is generally warmer than most parts of Europe except in the mountainous areas. Most of the Southern European countries are in the zones ranging from 8 to 10. However, the Southern Balkans which experience colder winters are in zones 6 to 7.

Criticism of the USDA Hardiness Zones

The concept of hardiness zones has several drawbacks if not used alongside other information. The heat levels during summer are not incorporated on the zone determination. Thus, regions with the same winter minima but different summer temperature will be assigned same hardiness zone. Colder zones (6-3) do not consider the fact that the snow cover can insulate against extreme cold and protects the roots of most of the hibernating plants. Finally, other factors that affect the growth of plants such as plant health, soil moisture, and humidity, among other factors are not considered in hardiness zones.

The USDA Hardiness Zones

ZoneAverage Annual Extreme Minimum Temperature Range
1a-60 to -55
1b-55 to -50
2a-50 to -45
2b-45 to -40
3a-40 to -35
3b-35 to -30
4a-30 to -25
4b-25 to -20
5a-20 to -15
5b-15 to -10
6a-10 to -5
6b-5 to 0
7a0 to 5
7b5 to 10
8a10 to 15
8b15 to 20
9a20 to 25
9b25 to 30
10a30 to 35
10b35 to 40
11a40 to 45
11b45 to 50
12a50 to 55
12b55 to 60
13a60 to 65
13b65 to 70

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