8i8 



Yearbook^ of Agriculture 1949 



-35; zone 3, -35 to -20; zone 

 4, -20 to -10; zone 5, -10 to 

 -5; zone 6, -5 to 5; zone 7, 5 

 to 10; zone 8, 10 to 20; zone 9, 20 

 to 30 ; zone 10, above 30. In moun- 

 tainous regions of western United 

 States the zones become crowded into 

 narrow bands according to altitude. To 

 conform to the base map, the narrow 

 zone 6 is omitted in the West, where 

 zone 5 extends from 10 to and 

 zone 7 from to 10. 



As factors other than latitude affect 

 the coldest temperature in winter, the 

 hardiness zones do not extend across 

 the United States from east to west uni- 

 formly in parallel strips. The zones 

 curve southward in the interior, where 

 extremes of temperature are great, and 

 shift to the south also in mountainous 

 regions, where higher altitudes have a 

 cooling effect. Zone 2, the coldest, is re- 

 stricted to the Canadian border region 

 from Wisconsin to Montana. Along 

 the coasts the southern zones project 

 farthest north, owing to the moderat- 

 ing influence of the oceans and ocean 

 currents. 



FOR EACH FOREIGN TREE SPECIES the 



northernmost zone of hardiness is 

 stated, or placed in parentheses if the 

 species is hardy only under favorable 

 conditions or in warmer parts of this 

 zone. Though southernmost zones are 

 not indicated, species hardy in the far 

 north generally will not thrive also in 

 the extreme south. Naturally many 

 trees from humid regions will not be 

 successful in drier portions of these 

 temperature zones unless watered or 

 irrigated. Zone 10 includes the nearly 

 frostless and almost tropical zones of 

 restricted, southern portions of Flor- 

 ida, Texas, Arizona, and California. 

 Subtropical trees can be grown also in 

 zone 9, which covers larger areas in 

 these four States and in other States 



along the Gulf, South Atlantic, and 

 Pacific coasts. 



To assist in identification, the 50 

 species have been combined into 11 

 artificial groups according to leaf 

 characters, such as whether evergreen 

 or deciduous (shedding leaves in fall), 

 whether borne singly or paired, and 

 whether or not divided into leaflets 

 (leaflets differ from leaves in usually 

 smaller size and in their attachment 

 on a common leafstalk which sheds 

 with them). These 11 groups are des- 

 ignated by letter, followed by the leaf 

 characters of the group. A specimen 

 being identified should first be placed 

 in the proper group and then compared 

 with the descriptions and drawings. 



Many additional shade and orna- 

 mental trees will be found among the 

 165 native forest tree species described 

 and illustrated in the article, "Impor- 

 tant Forest Trees of the United 

 States," on page 763, which also has 

 instructions for submitting specimens 

 to specialists for identification. The 

 six articles on shade trees for different 

 regions of the United States (pages 48- 

 85) contain notes on other species as 

 well as nearly all of the 50 selected here 

 and should be consulted for more de- 

 tailed information about the kinds 

 best suited to each geographic region. 



For further information about shade 

 and ornamental trees from foreign 

 lands and their identification, there is 

 included a selected list of general ref- 

 erences together with a few special 

 publications on the distinctive trees 

 of the subtropical regions. Most States, 

 through their agricultural experiment 

 stations or the State college extension 

 services, have issued bulletins or circu- 

 lars that describe the shade and orna- 

 mental trees, both native and foreign, 

 best suited to their regions and con- 

 taining instructions on planting and 

 care. 



