Planting Zones in America
- Planting zones are used to help gardeners choose appropriate plants for their geographic region. One of the most important criteria in choosing a plant for a specific location is its ability to withstand the winter season and its cold temperatures. The planting zones divide the nation into regions based on their average lowest temperatures. Gardeners can select plants that can withstand those climates and survive.
- The first hardiness map was designed by Alfred Rehder in 1927 and was included in his book "Manual of Cultivated Trees and Shrubs." The zones were based on the lowest mean temperature for the coldest month. It used eight zones divided by 5 degree (Fahrenheit) variations and was widely used until 1938. Donald Wyman, using information taken from the U.S. Weather Bureau between 1895 and 1935, developed a new map based the average annual cold temperatures for each region. His map was published in the book "Hedges, Screens and Windbreaks." In 1960, the USDA released its first hardiness zone map, building on the earlier work and using a standard 10-degree range to divide the zones.
- The planting zones are a guideline and do not take into account the factors that can influence a plant's ability to survive. Growing conditions such as soil type and condition, rainfall, exposure, humidity and shelter from cold winds all play an important role in a plant's ability to survive. Microclimates exist within larger planting ranges that might allow a particular plant to grow. This same plant might not thrive just a short distance away.
- Most garden centers, online stores and mail order catalogs will list the ideal planting zone information for each plant they sell. Refer to a planting zone map to understand your planting zone and its boundaries before making any plant selections. Purchase healthy plants suited for your zone. If you are on the edge of a planting zone, you may be able to grow plants suited to the neighboring zone. The further out of your planting zone you go with your plant choices, however, the higher the chance that the plants may not survive.
