What Is A Zone?

A zone is a system of seasons determined by latitude and longitude. Each zone corresponds to one degree or another of temperature variation between the equator and the freezing point at sea level (Fahrenheit). For example, the temperate zone extends from approximately 40°N latitude to 60°S latitude, while the tropical zone extends from 30°N to 70°W latitude. The difference between these two zones is called the seasonal temperature range (STR) and it varies depending upon where you live.

The term “zone” was first used in 1859 by a botanist named John Snow, who noted that plants growing near the equator were able to grow year round because they had access to relatively constant temperatures. Plants grown in colder climates, however, could not tolerate such conditions and would die during winter months if left out too much. Snow’s observation led him to propose the concept of a zone system; essentially, each zone corresponds with a different temperature range.

Snow’s original concept has since been refined and modernized by various horticultural societies. These societies have proposed three main types of zones, known as the ‘Hardiness Zone’, the ‘Annual Minimum Temperature’ (AMT), and the ‘Cooling Degree Days’ (CDD).

Each of these zones different in terms of its composition and use, but they all attempt to answer the same question: What is the ideal growing climate for my plants?

The Usa Hardiness Zone Map

Hardiness zones are the most basic and commonly used zones in North America. Invented by the US Department of Agriculture (USDA), the hardiness zone method uses the average annual minimum temperatures to predict whether an area can support certain plants. To find your hardiness zone, you must first find your city in reference to the map provided on the USDA website. Next, find your city’s coordinates by entering them onto google.

The last step is to enter your city’s coordinates into the hardiness zone calculator. The resulting number that the calculator spits out is your zone.

Annual Minimum Temperature (AMT) Zones

Annual minimum temperature zones attempt to provide an answer to a slightly different question: What is the coldest average temperature that a given area experiences over a period of several years?

AMT zones are most commonly used in more rural areas, where there is less data available. As a result, these zones tend to be less accurate than hardiness zones. That being said, they can still be quite useful in predicting the growing conditions of certain plants.

CDD Zones

Cooling degree days (CDD) is another method for determining whether a plant will thrive in a given environment.

Sources & references used in this article:

Ecoregions of North Carolina by JE Douglass, WT Swank – Gen. Tech. Rep. SE-13, Southeastern For. and Range …, 1975

Large-scale planting of North Carolina loblolly pine in Arkansas and Oklahoma: a case of gain versus risk by GE Griffith, JM Omernik, J Comstock, MP Schafale… – 2002 – ecologicalregions.info

Coexistence of monoecious and dioecious hydrilla in Lake Gaston, North Carolina and Virginia by CC Lambeth, PM Dougherty, WT Gladstone… – Journal of …, 1984 – academic.oup.com

Comparison of nematode communities in agricultural soils of North Carolina and Nebraska by FJ Ryan, CR Coley, SH Kay – Journal of Aquatic Plant Management, 1995 – apms.org



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