46 CIRCULAR 912, U. S. DEPARTMENT OF AGRICULTURE 



delay maturity of the wood and setting of terminal buds. Most re- 

 search workers agree that irrigation in late summer and early fall re- 

 duces the resistance of plants to frost injury. 6 Fall irrigation of im- 

 mature trees should be delayed until they are dormant. 



The form of growth and the comparative height and crown spread 

 given in table 5 for each of the recommended species are the normal 

 ones. Long dry periods or unfavorable soil types may prevent some 

 of the species that normally have a tree form of growth from ever 

 becoming more than medium or tall shrubs. Close planting such 

 as that often practiced in landscape groups may prevent full develop- 

 ment of the crown. Such planting frequently results also in either 

 weak, suppressed trees or forced height growth at the expense of 

 trunk diameter and crown-spread growth. Proper planting arrange- 

 ment and maintenance of the best possible growing conditions will 

 do much to assure the most satisfactory development of the trees. 

 Information on growing trees for windbreak and other purposes on 

 the northern Great Plains may be obtained by writing the County 

 Agricultural Agents or Extension Service Foresters of the respective 

 States; the Northern Great Plains Field Station, Mandan, N. Dak.; 

 or the Office of Information, United States Department of Agriculture, 

 Washington, D. C. 



6 Levitt, J. frost killing and hardiness of plants, a critical review. 

 Pp. 56-58. Univ. Minn. 1941. (Processed.) 



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