Weather varies significantly from one part of North Carolina to another. Plants that flourish in one part of the state may do poorly or fail in another area. The primary guide to determine plant hardiness is the USDA Hardiness Zone Map which is divided into ten zones based on average minimum temperatures. Each zone is subdivided into A and B sections in order to be more precise.
The A section is the colder portion of the zone and is often located north of the B section. In North Carolina the zones tend to be aligned more east and west instead of north and south. A plant is said to be hardy if it can tolerate the lowest average winter temperatures that usually occurs in a zone.
There is not a clear cut line between zones. A given location can be warmer or colder than the rest of a zone because of air drainage or elevation. Some plants can be grown in isolated areas north of their designated zone but may suffer from winter injury. A plant can often be grown in a warmer zone if growing conditions (rainfall, soil, summer heat) are comparable.
In some cases the hardiness zones listed by a reference book are conservative and are a full one half zone farther south than the plant is known to survive. Hardiness is affected by duration and intensity of sunlight, length of growing season, amount and timing of rainfall, length and severity of summer drought, soil characteristics, proximity to a large body of water, slope, frost occurrence, humidity, and cultural practices.
The USDA Hardiness Zone Map was revised in 1990. You will probably find older reference books that provide information on hardiness that differs from recent publications.
Plants can be classified as either hardy or nonhardy, depending upon their ability to withstand cold temperatures. Winter injury can occur to nonhardy plants if temperatures are too low, or if unseasonably low temperatures occur early in the fall or late in the spring.
© Erv Evans, Consumer