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Cold Hardiness Ratings for Some Additional Woody Plants 

 Zone 



Zone 



Abeliophyllum distichum (white forsythia) . 5b 



Acer platanoides (Norway maple) 4 



Aesculus carnea (red horsechesnut) 4 



Arctostaphylos uva-ursi (bearberry) 3 



Azalea Indian hybrid (Indian azalea) 9 



Azalea Kurume hybrid (Kurume azalea) .... 7 



Azalea Mollis hybrid (Mollis azalea) 5 



Bauhinia variegata (purple orchid tree) 10 



Berberis darwini (Darwin barberry) 8 



Betula pendula (European white birch) 2 



Bouvardia 'Coral' (Coral bouvardia) 9 



Butia capitata (Pindo palm) 8b 



Camellia reticulata (reticulata camellia) 9 



Camellia sasanqua (sasanqua camellia) 7b 



Carya pecan 'Major' (pecan) 5 (grows) 



6 (fruits) 



Ceanothus impressus (Santa Barbara ceano- 8 



thus). 



Cedrus deodara (deodar cedar) 7b 



Chamaecyparis pisifera (Sawara cypress) 4 



Cinnamomum camphora (camphor tree) 9 



Cistus laurifolius (laurel rock-rose) 7 



Cistus purpureas (purple rock-rose) 8 



Cornus alba (Tatarian dogwood) 3 



Cytisus praecox (Warminster broom) 6 



Elaeagnus multiflora (cherry multiflora) 5 



Elaeagnus pungens (thorny elaeagnus) 7 



Eriobotrva japonica (loquat) 8 



Euonymus alatus (winged euonymus) 4 



Euphorbia pulcherrima (poinsettia) 10 



Fremontia mexicana (San Diego f remontia) 9 



Ginkgo biloba (ginkgo, maidenhair-tree) 5 



Hibiscus rosa-sinensis (Chinese hibiscus) 9b 



Hibiscus syriacus (shrub althea) 6 



Hypericum patulum 'Hidcote' (Hidcote St. 7 



Johnswort). 



Iberis sempervirens (evergreen candytuft) 6 



Ilex crenata convexa (convexleaf Japanese 7 



holly). 



Jacaranda acutifolia (jacaranda) 10 



Juglans regia (English or Persian walnut) 6 



Juniperus horizontalis (creeping juniper) 3 



Koelreuteria paniculata (goldenrain-tree) 6 



Laburnum watereri (Waterer laburnum) 6 



Lagerstroemia indica (crapemyrtle)i. 7 



Mahonia aquifolium (Oregon hoUygrape) 6 



Malus arnoldiana (Arnold crabapple) 5 



Metasequoia glyptostroboides (Dawn red- 5b 

 wood). 



Nerium oleander (oleander) 8 



Olea europaea (common olive) 9 



Picea abies (Norway spruce) 2 



Pieris japonica (Japanese andromeda) 6 



Pinus mugo mughus (Mugho pine) 3 



Pinus radiata (Monterey pine) 7 



Pinus strobus (white pine) 4 



Raphiolepis indica rosea (pink raphiolepis) 8 



Rhododendron 'America" (hybrid rhododen- 5 



dron). 



Rhododendron loderi 'King George' (hybrid 8 



rhododendron). . 



Rhododendron 'Purple Splendor' (hybrid rho- 7 

 dodendron). 



Rosa rugosa (rugosa rose) 3 



Schinus terebinthifolius (Brazilian pepper 9 



tree). 



Sequoia sempervirens (redwood) 8 



Sequoiadendron giganteum (giant sequoia) 7 



Ulmus americana (American elm) 2 



Zelkova serrata (Japanese zelkova) 6 





'^Vi. \-<\j. 1*1 15 



Si4 



i\ i/( 



HARDINESS 



ZONE MAP 



.\ !/♦ 



Washington, D.C. Issued May 1960 



Miscellaneous Publication No. 814 



AGRICULTURAL RESEARCH SERVICE 



UNITED STATES DEPARTMENT OF AGRICULTURE 



PLANT HARDINESS ZONE MAP 



Prepared by the U.S. National Arboretum, Agricultural Research 



Service, U.S. Department of Agricidture in cooperation 



with the .\mcrican Horticultural Society ' 



This map shows in moderate detail the expected minimum temperatures 

 of most of the horticulturally important areas of the United States 

 (excluding Alaska and Hawaii) and Canada. It shows 10 different zones, 

 each of which represents an area of winter hardiness for certain orna- 

 mental plants. 



Cold hardiness zones for the United States area of this map are 

 based on isotherms of average minimum winter temperatures for the 

 years 1899 through 1938. Readjustments were made for 34 States on 

 the basis of January mean minimum temperatures for 1931 through 

 1952. as published by the U.S. Weather Bureau. 



Data for the adjacent area in Canada were provided by the Cana- 

 dian Meteorological Division, and are based on a 20- to 40-year period. 



Data from both .sources in the United States and Canada have been 

 modified or reinterpreted in many localities to conform with recent 

 and more detailed information provided by State experiment stations 

 and numerous individual cooperators. 



HOW TO USE THE MAP 



Each zone of the map on the reverse side has been subdivided into 

 darker-colored and lighter-colored sections that represent 5-degree dif- 

 ferentials within the 10-degree zone. The lighter color of each zone 

 represents the colder section; the darker color, the warmer section. The 

 accompanying table lists representative plants that normally survive in 

 each zone. 



The hardiness zones should be enough for most general reference 

 purposes. However, plant survival differences can be noted at smaller 

 north-south progressions than a full zone represents. When this addi- 

 tional detail is needed, use the 5-degree differentials within the 10-degree 

 zone. 



Some examples of these differences are as follows: Saucer mag- 

 nolia {Magnolia soulangeana) and wisteria ( Wisteria sinensis) are suit- 



' Based on studies conducted by a commission of the Society (formerly the 

 American Horticultural Council) upon recommendation of the American Associa- 

 tion of Nurserymen. Further background information may be found in an article 

 titled. The Geographic Charting of Plant Climatic Adaptability, by H. T. Skinner, 

 Report of the 15th International Horticultural Congress, 1958. 



For lale by the Superintendent of Documents, U.S. Government Printins Office, Washington 25, D.C. - Price 1 5 cents 



I 



able for zone 5b, but not for zone 5a; orchard peaches {Prunus persica) 

 are suitable in zone 6b, but will also succeed in those parts of zone 6a 

 that are relatively free of late frosts; Japanese camellia (Camellia 

 japonica), Chinese holly {Ilex cornuta), and Southern magnolia 

 {Magnolia grandiflora) are suitable in zone 7b but doubtful in 7a. 



In determining if a certain plant will survive in a given zone, it 

 is necessary to consider factors other than the minimum temperature 

 range of each zone. For example, temperatures of adjacent zones 

 become increasingly similar near their boundaries. Moreover, there 

 are innumerable island climates that may be considerably milder or 

 colder than the zone average. These islands are especially frequent in 

 hilly or mountainous areas. Mountainous areas on this map are not 

 shown to be as cold as might be expected. The reason for this is that 

 most weather stations from which records were obtained are located in 

 valleys where temperatures tend to be somewhat milder, and where 

 plants are most likely to be cultivated. 



Other plant-growth factors must also be considered. Frost occur- 

 rence, seasonal rainfall distribution, humidity, soil characteristics, and 

 duration and intensity of sunlight may bear little relationship to mean 

 winter temperatures. The combined effects of all factors determine 

 true plant adaptability. They would be difficult to depict geographically. 

 Minimum temperatures, on the other hand, can be readily depicted. 

 They are of prime importance in plant survival. Their effects can 

 seldom be changed by cultural practices. 



A plant species that flourishes in one part of a given zone is likely 

 to be adaptable in other parts of the same zone or in a warmer zone. 

 Other growth factors, such as rainfall, soil, and summer heat, have to 

 be reasonably comparable, however, or capable of being made compa- 

 rable through irrigation, soil correction, wind protection, partial shade, 

 or humidity control. Frost dates, length of growing season, and mini- 

 mum winter temperatures are among the least readily controlled of the 

 major factors that govern the geographic adaptability of plants. 



The zone in which a given plant may survive is not necessarily the 

 zone in which it should generally be recommended for planting. Abelia 

 grandiflora, for example, usually survives as a low-growing, winter- 

 retarded specimen in the colder areas of zone 6 or even in zone 5. It 

 develops and flowers normally, however, in zone 7. This species, there- 

 fore, should be properly recommended for zones 7 and above. 



Some gardeners may question a zone rating when a plant fails to 

 survive its first winter. A single test, however, is rarely reliable. A 

 small, young plant may be tender, whereas an older plant may become 

 quite hardy. Cultural conditions may affect the degree of hardiness. 

 Furthermore, no single winter is ever quite average: some may be more 

 severe than others in suddenness of freezing or in degrees of frost 

 registered. 



us, GOVERNMENT PRINTING OFFICE : I960 OF~537035 



