USDA Plant Hardiness Zone Map
Image source: http://planthardiness.ars.usda.gov
Interpreting the Plant Hardiness Zone Map
How does this Hardiness Zone Map differ from the old one?
- This map is based on 30 years of weather data, from 1976-2005. The previous map was based on data from 1974-1986.
- Maine’s temperatures now appear slightly warmer. Part of this “warming” may be a result of higher resolution due to the use of GIS techniques in the new map, rather than actual warming.
What do the Hardiness Zones mean?
- Maine spans Plant Hardiness Zones 3-6. Each zone is based on the 30-year average of the single coldest temperature recorded each winter. Zone 3 is 10 degrees F colder than Zone 4, etc. In addition, each zone is split in half. For example, sites in Zone 4a reach an average low temperature of -30 to -25 F, while sites in Zone 4b reach an average winter low temperature of -25 to -20 F.
What do the Hardiness Zones NOT mean?
- The map is a guideline, not a guarantee. A plant rated hardy to Zone 4b would likely thrive in Zone 5a, do well on good sites in Zone 4b, and might survive in quite protected sites in Zone 4a.
- The map does not consider other factors that affect plant survival: summer temperatures and heat accumulation, length of growing season, depth of frost in winter, precipitation, soil drainage, soil fertility, quality of plant specimens and management practices.
- The map is more accurate for trees and shrubs than for herbaceous perennials like iris and poppy, because snow cover greatly affects the winter survival of herbaceous perennials. An herbaceous perennial in a cold location with deep, reliable snow cover may survive better than one in a warmer zone where the ground is open.
How can you best use the map to select landscape plants?
- Be conservative when trying new plants. To be more certain about a new plant, buy a species rated one-half or even one full zone colder than the site is rated.
- Buy the best quality plant available. Follow good planting practices.
Follow good establishment and maintenance practices. Water well during the establishment period (this can be several years), fertilize as recommended, scout for problems and resolve them promptly.
From the GardenPro Answer Book; revised and updated by Lois Berg Stack, Ornamental Horticulture Specialist, University of Maine Cooperative Extension.