Many plants have a specific zone in which they grow best. The United States Department of Agriculture has identified hardiness zones on a map by color and numbers. Zones are used as tools to help people make decisions about what they want to plant according to where they reside because certain plants need specific conditions to grow. Zones are based on temperature fluctuations and can vary not only from state to state but also within a state.
- There are 11 hardiness zones with the coldest, zone 1, having an annual average low of -50 degrees below zero F. The hottest is zone 11, which has an annual average low of above 40 degrees F. The zones go up with every temperature change of 10 degrees F. Zones 2-10 are subdivided into light and dark colors for easier map reading. The light color indicates the colder area in the zone. There is a five-degree difference between the light and dark shade. There are areas in the United States that have no zone designation. These areas are above a certain elevation and are considered unsuitable for planting.
- Between 1990 and 2006 the Arbor Day Foundation collected data from 5,000 cooperative climate stations, which showed a gradual warming across North America. Southern states saw slighter temperature increases than some northern states. For example, in 1990 Mississippi, Alabama, Georgia and South Carolina were zoned 7 and 8, with each state split almost in half. In 2006, these states were again zoned 7 and 8, but the majority of each state was zoned 8. Other states increased an entire zone. Missouri and Kansas jumped from being zoned 5/6 to 6/7; Iowa and Nebraska went from a zone 4 to a zone 5 in the majority of the state. Ohio, Indiana and Illinois went from zone 5 to 6.
- Zones are a guide to help with planting. Most seed packets and plants will state their hardiness zone, which means the plant should survive if planted in that area. However, this is not a guarantee. Hardiness zones can be affected by a number of things including the amount of rainfall it receives, severity of any drought, soil fertility, humidity, amount of frost, amount of sunlight, nearness to water and slope of the land. There are other natural stresses such as acid rain, pollution and toxic waste that can make a species grow insufficiently.
- Knowing the zone of an area helps with deciding what to plant. For instance, nuts need the warmer climates found in zones 6-11. California, which is largely zoned 9, is the country's largest producer of almonds and walnuts. Sugarcane thrives in the tropical heat of Hawaii. Large agricultural crops such as wheat, corn, soybeans and tomatoes thrive in zones 5 and 6.
- Crops can sometimes be grown in a warmer location than normally designated if other elements such as rainfall and summer temperatures are compatible. Some plants can be grown in colder climates than their regular hardiness designation.