What are plant hardiness zones and frost-free dates?
Plant Hardiness Zones
A hardiness zone is an area in which specific plants are capable of growing. Hardiness zones are particularly informative about the extremes of winter cold but do not take summer heat into account. The concept was developed by the U.S. Department of Agriculture (USDA) but has since been adapted by other organizations and countries.
The latest USDA (2012) Hardiness Zone Map is based on the average minimal temperature, divided into 10°F zones. The map is interactive and users may also type in a ZIP code and find the hardiness zone for that area. For example, this map shows that the New York Botanical Garden (Bronx, NY) is in 7a (0-5°F) whereas Manhattan is in zone 7b (5-10°F). A similar map is available from The Arbor Day Foundation. It lists Manhattan and the Bronx as being in Zone 7. Both these sites take into account recent warming trends that have occurred in the last decade or so.
It is important to remember that local variations in moisture, soil, wind, location etc. (microclimates) may make it difficult to assess the exact zone of a particular garden.
Labels on nursery plants usually carry information on the hardiness zone suitable for the plant.
Plant Heat Zones
The effects of heat damage are more subtle than those caused by cold but should nevertheless be taken into account when growing plants. This is a particularly important consideration for gardens in the southern United States and increasingly in the North as the climate changes. The American Horticulture Society has published aHeat Zone Map resembling the Hardiness Zone Map. It divides the country is divided into 12 zones, indicating the average number of days per year with temperature above 86°F (30°C). The Bronx, for example, is in Zone 5 (30-45 days above 86°F).
First Frost-Free Date
In the past, Mother’s Day (second Sunday in May) was considered to be the first frost-free date in the New York City area after which it was thought to be safe to plant tropical annuals outdoors. However, as the climate has warmed in recent years this date has now retreated to an earlier time. In the Bronx, NY, it is almost certain that you will not get any frost after May 7th. The precise date will vary according to your location. For example, Westchester County will have a later date than New York City. Even then, the date can only be given with a certain degree of probability. The National Climatic Data Center provides detailed state-by-state frost data.
For tips on a variety of gardening topics, see our Plant Information Guides.
- Courtesy of NYBG Plant Information Service