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How are the hardness zones in the United States defined? Do you know which zone your city is in?
A hardness zone is a geographical area defined by a specific average annual minimum temperature, which is critical to the growth and survival of many plants. This concept was originally developed by the United States Department of Agriculture (USDA) as a rough guide for gardening and landscaping, defining 13 zones based on long-term average annual extreme minimum temperatures.
In the United States, warmer areas are concentrated in the deep South, especially Florida, the Allegheny Coast, and Hawaii. In addition, some areas of Puerto Rico have even risen to the level of District 13. In contrast, the far north of Alaska is among the coldest areas, with temperatures as low as zone 1.
For example, if a plant is labeled "Zone 10," it means that the plant can withstand minimum temperatures ranging from 30 to 40 degrees Fahrenheit.
There is a degree of confusion surrounding the partitioning method of subdivisions or repeat subdivisions. Therefore, when discussing buildings and heating, ventilation and air conditioning systems, the term "climate zone" may cover the different areas mentioned in international energy efficiency codes. The distinction between these definitions is crucial for both gardening and construction professionals. Different hardness grade systems have been developed, such as those set by the Royal Horticultural Society in the United Kingdom and the Sunset Western Gardening Book system in the United States, as well as the hot zones defined by the American Horticultural Society (AHS).
Hardness zones in the United States
The U.S. Department of Agriculture's hardness zone system is established based on the lowest average annual extreme temperatures in a location. This system provides help and guidance to American gardening enthusiasts. Most of the warmer areas are classified in the southern United States, especially along the coast. For example, Key West, Florida, is the warmest place (Zone 11b) among the 48 U.S. states, while North Center in Minnesota is the coldest zone (Zone 2b).
In the 2012 update, the USDA updated their plant hardness map based on weather data from 1976 to 2005 and added two new zones (Zones 12 and 13), which combined the hardness of tropical and subtropical plants. The definition is clearer.
Historical evolution
The original hardness zone system was published in 1927 and 1938 by two researchers at the Arnold Arboretum in Boston. Over time, this system is continuously updated and revised as climate change and environmental data change.
Particularly in the United States, temperature measurement sites near airports show that the effects of climate change are causing hardness zones to shift northward in many areas, indicating a continued warming trend in the overall climate in the United States.
Specific cities and regions
According to the latest version of USDA in 2023, the hardness zones vary in different cities. Gardening enthusiasts should choose suitable plants according to the hardness zone of their location, which is crucial to ensure successful planting.
Limitations and Challenges
Although USDA hardness zones are based on the lowest extreme temperatures, they do not accurately describe other climate conditions in which plants grow. For example, frost dates and the frequency of snow cover can vary greatly from region to region. And the extreme minimum temperatures themselves may not be comparable across different climate conditions.
For example, areas with consistent snowfall coverage may be more protective of plants within the same hardness zone than those with less snowfall, and these are factors for gardeners to consider.
Alternatives
In addition to the USDA Hardness Zones, there are other ways to help understand a plant's growing conditions. For example, the use of "indicator plants" to aid the adaptability of these plants, where the range in which they grow is known, can provide another indicator. In addition, the sunset system also divides the climate zone into 45 more detailed areas to cover all climate factors.
Through these systems, different gardening enthusiasts can better understand the challenges they face. With so many options and methods out there, have you found the area that works best for your plants?
