United States Dept. of Agriculture Hardiness Zones
There are several different schemes which attempt to categorise the varied climates of North America. The most widely used, and also the simplest, is the United States Dept. of Agriculture (USDA) Hardiness Zones.
The USDA zones are based on a single criteria: the average annual minimum temperature range. The USDA zones are therefore of limited value to gardeners because there is a lot more to growing fruit trees successfully than the cold-hardiness of the tree. However the USDA zones do have the merit of being very simple and very widely used. In addition the severity of the winter conditions can also be used to imply some other attributes - e.g. length of summer growing season. The USDA zones are therefore a userful starting point for deciding which fruit trees are suitable for your location.
The following table is reproduced from the National Arboretum USDA Plant Hardiness Zone Map:
|Zone||Temp. F||Temp. C||Example Cities|
|1||Below -50 F||Below -45.6 C||Fairbanks, Alaska; Resolute, Northwest Territories (Canada)|
|2a||-50 to -45 F||-42.8 to -45.5 C||Prudhoe Bay, Alaska; Flin Flon, Manitoba (Canada)|
|2b||-45 to -40 F||-40.0 to -42.7 C||Unalakleet, Alaska; Pinecreek, Minnesota|
|3a||-40 to -35 F||-37.3 to -39.9 C||International Falls, Minnesota; St. Michael, Alaska|
|3b||-35 to -30 F||-34.5 to -37.2 C||Tomahawk, Wisconsin; Sidney, Montana|
|4a||-30 to -25 F||-31.7 to -34.4 C||Minneapolis/St.Paul, Minnesota; Lewistown, Montana|
|4b||-25 to -20 F||-28.9 to -31.6 C||Northwood, Iowa; Nebraska|
|5a||-20 to -15 F||-26.2 to -28.8 C||Des Moines, Iowa; Illinois|
|5b||-15 to -10 F||-23.4 to -26.1 C||Columbia, Missouri; Mansfield, Pennsylvania|
|6a||-10 to -5 F||-20.6 to -23.3 C||St. Louis, Missouri; Lebanon, Pennsylvania|
|6b||-5 to 0 F||-17.8 to -20.5 C||McMinnville, Tennessee; Branson, Missouri|
|7a||0 to 5 F||-15.0 to -17.7 C||Oklahoma City, Oklahoma; South Boston, Virginia|
|7b||5 to 10 F||-12.3 to -14.9 C||Little Rock, Arkansas; Griffin, Georgia|
|8a||10 to 15 F||-9.5 to -12.2 C||Tifton, Georgia; Dallas, Texas|
|8b||15 to 20 F||-6.7 to -9.4 C||Austin, Texas; Gainesville, Florida|
|9a||20 to 25 F||-3.9 to -6.6 C||Houston, Texas; St. Augustine, Florida|
|9b||25 to 30 F||-1.2 to -3.8 C||Brownsville, Texas; Fort Pierce, Florida|
|10a||30 to 35 F||1.6 to -1.1 C||Naples, Florida; Victorville, California|
|10b||35 to 40 F||4.4 to 1.7 C||Miami, Florida; Coral Gables, Florida|
|11||above 40 F||above 4.5 C||Honolulu, Hawaii; Mazatlan, Mexico|
As a rule of thumb, few apple varieties will grow successfully in Zones north of Zone 4. Cold-hardiness in fruit trees results from a combination of the rootstock and the scion variety. Even cold-hardy varieties such as McIntosh and its offspring will benefit from using cold-hardy rootstocks such as Bud. 9 or the Geneva series of rootstocks. The M and MM series rootstocks which were developed in England are generally not as cold-hardy as more recent rootstocks; M26 is the most cold-hardy of the English rootstocks.
At the other end of the scale, few apple varieties enjoy the very hot summers that are associated with Zone 8 and above. Zones 4 - 7 are the best areas for growing apple trees in North America.
USDA Zones and Chilling Requirement
It is not just summer heat that makes it difficult to grow apples above Zone 8. Zones 9, 10 or 11 also rarely experience cold temperatures during winter. However the common orchard fruit species such as apples, pears, plums and cherries originated in mountainous areas of central Asia. As a result they expect a period of winter cold to induce dormancy. This "chilling requirement" is usually measured as the number of hours needed below about 45F / 6C. Whilst slightly higher temperatures up to 8C still contribute some chilling effect, perhaps surprisingly there is no additional benefit from lower temperatures.
Areas in Zones 9 - 10 usually receive from 600 hours down to 100 hours per year below 6C, and are considered "low-chill" areas.
Most apple varieties are high-chill and are not happy with less than 800 chill hours. Medium-chill varieties include Pink Lady and its close relative Sundowner, as well as the popular early-season variety Akane. These will grow in most of the warmer zones, but prefer at least 450 chill hours. A small number of varieties will tolerate less than 450 chill hours.
Whilst the USDA zones have the benefit of being very simple, by using only the average winter minimum temperature they do not take into account many other factors which determine whether a fruit tree variety will survive or thrive in an area. For example, the USDA zones put parts of Texas on a par with parts of the Pacific North West - two areas which may share the same winter minimum temperatures, but have far less in common in terms of sunlight, summer temperatures, and rainfall. Two other classifications help add to the picture. The Sunset Zones, popularised by Sunset magazine, use more wide-ranging factors to give a more precise estimation of whether a particular plant will thrive (as opposed to merely survive winter). The Sunset Zones originally only covered the western states (which is the target audience of the magazine) but have now been extended to the eastern states.
The American Horticultural Society has also developed a plant heat zone classification. This measures the number of days when the temperature is above 30C / 86F. This is also a valuable tool for determining which apple varieties will perform best in a location.