Some zip codes are in small areas that are warmer or cooler than the surrounding area. These areas, called microclimates, are often associated with urbanization or steep elevation changes, and many are too small to show on our map.
If it looks on the map like your zip code should be in a different hardiness zone than the one we provide for you, your zip code is probably in a microclimate that is too small to represent on the map.
A good example of this is Chicago, Illinois. Several zip codes for the city of Chicago bring up zones 5-6.
Microclimates occur in most large cities due to the urbanization effect that buildings bring about. In addition, regions of the country with bodies of water or mountains may contain “pockets.” These climatic pockets mirror warmer or cooler zones than the surrounding region.
The Plant Hardiness Zones divide the United States and Canada into 11 areas based on a 10 degree Fahrenheit difference in the average annual minimum temperature. (The United States falls within Zones 2 through 10). For example, the lowest average temperature in Zone 2 is -50 to -40 degrees Fahrenheit, while the minimum average temperature in zone 10 is +30 to +40 degrees Fahrenheit. See more.
Suggested hardiness zones have been indicated for all trees and perennials available online from the Foundation. If a range of zones, for example, zones 4-9, is indicated, the tree or perennial is known to be hardy in zones 4, 5, 6, 7, 8, and 9. Suitable hardiness means a plant can be expected to grow in the zone’s temperature extremes, as determined by the lowest average annual temperature.
Keep in mind that local variations such as moisture, soil, winds, and other conditions might affect the viability of individual plants.
You may want to ask a local professional arborist or nursery about which trees to plant in your community.