Shade Trees for the Rockies 



73 



rigation. Sometimes the soil in which 

 they are planted must be carefully 

 selected or specially prepared, either 

 with a view toward reducing alkalinity 

 or of penetrating underlying hardpan 

 to permit better drainage and root 

 growth. Under the most severe condi- 

 tions only the hardier kinds will live, 

 but even in the northern part in pro- 

 tected locations a number of the more 

 tender species that cannot be recom- 

 mended for the whole region can be 

 grown. 



Following are brief descriptions and 

 supplemental notes of the more com- 

 mon trees that have been successfully 

 planted in the region. Most of the 

 broadleaved evergreens suitable only 

 for the extremely mild winters of the 

 Gila River Basin have been omitted. 



THE DECIDUOUS trees are listed first. 



The tree-of-Heaven ailanthus, often 

 called simply ailanthus, is an aggres- 

 sive tree that can thrive where many 

 other kinds would perish. It grows 

 rapidly with some care, but it is rela- 

 tively short-lived and provides little 

 shade from its thin, loose crown. It is 

 recommended as a street tree where 

 low moisture, poor soil, and excessive 

 heat preclude the use of more attrac- 

 tive species. It is almost immune to 

 smoke and soot injury but is easily 

 broken by snow and high wind. Only 

 seed-bearing trees should be planted, 

 as the pollen-bearing form, the male, 

 has an offensive odor when it blooms. 

 It may be planted throughout the re- 

 gion except in the high mountains or 

 parts of the plateau where winter tem- 

 peratures are low. 



Green ash, although smaller than 



white ash, is the preferred street tree. 

 It has darker foliage, is more resistant 

 to drought, and seems to have fewer 

 enemies. It is suitable for the plateau 

 but it is not recommended for the 

 mountains. 



Velvet ash, especially the smooth or 

 Arizona form, is well adapted to the 

 Gila drainage area. It will not stand 

 protracted cold, but it resists drought 

 and can grow in strongly alkaline soil. 



White ash has a rather oval crown 

 and light-green and moderately dense 

 foliage. It will stand abuse but is sen- 

 sitive to drought. The oystershell scale 

 often attacks it. It is not recommended 

 for the mountains or the Gila Basin. 



Boxelder develops into a tree with a 

 ragged crown of fairly light-green foli- 

 age. Although the boxelder is extremely 

 drought-resistant and able to with- 

 stand abuse, it is recommended only 

 for places where more desirable trees 

 will not grow. It is the breeding place 

 of the boxelder bug, which in some 

 years becomes objectionable in the 

 houses nearby. Aphids commonly at- 

 tack boxelder, and in the cities the 

 honeydew that drops from them is 

 objectionable on pavements. It will 

 grow anywhere in the region. The 

 Arizona form is best adapted to the 

 Gila River drainage. 



Northern catalpa, a relatively small 

 tree, possesses a globe-shaped crown of 

 large, heart-shaped leaves. It stands 

 drought fairly well but not cold. Its 

 flowers are large and attractive, but 

 some persons object to it as a street 

 tree because of the litter of the fallen 

 flowers in early summer and the seed 

 pods in autumn. It may be planted 

 throughout the plateau, although it is 

 not recommended for the extreme 

 northern parts or for elevations above 

 5,000 feet. 



American elm has been planted ex- 

 tensively on the plateau for many years. 

 It is not very tolerant of alkali and 

 should have plenty of irrigation to 

 thrive. It is heavily attacked by the 

 European elm scale. It is subject also 

 to two serious epidemic diseases, the 

 Dutch elm disease, which recently was 



