74 



Yearbook^ of Agriculture 1949 



found in Denver, and phloem necrosis, 

 a killing virus disease that has not yet 

 been found in the region. American 

 elm should be used sparingly until con- 

 trols are available for the diseases. 



Siberian elm, which was introduced 

 as Chinese elm and is still often called 

 that, can persist despite drought, poor 

 soil, and abuse, and at the same time 

 provide shade and greenery. It is nat- 

 urally scrubby in habit, but it can be 

 trained while young into a fairly attrac- 

 tive street tree. It has been overplanted 

 in some cities. Its root system is prone 

 to invade tile sewers and to heave pave- 

 ments and curbs. It produces seed pro- 

 fusely and the seedlings may become 

 extremely noxious weeds in nearby 

 gardens. It is not subject to the epi- 

 demic diseases described for American 

 elm but is highly susceptible to the 

 cotton or Ozonium root rot. Often the 

 European elm scale attacks it. 



The common hackberry is usually a 

 small tree with a spreading, flat crown 

 of dense, light-green foliage. It is ex- 

 cellent for use under adverse growing 

 conditions in hot, dry climates. The 

 foliage does not tend to turn yellow 

 in alkaline soils. It is recommended for 

 all parts of the region except in the 

 mountains. 



Thornless honeylocust forms a thin, 

 irregular crown that provides light 

 shade. It is one of the last trees to leaf 

 out in the spring and one of the first 

 to lose its foliage in the fall a distinct 

 shortcoming in a region of long, hot 

 summers. But it is drought-resistant, 

 hardy, and capable of withstanding 

 abuse once it is established. It is not 

 recommended for the mountains. 



Linden is not extensively planted 

 here, although it is recommended as 

 a good street tree in some of the larger 

 cities for places where it will receive 

 fertilization and irrigation. It is sub- 

 ject to sunscald following transplant- 

 ing. American linden and the littleleaf 

 linden are the two preferred species 

 for the high plateau. Local authorities 

 should be consulted regarding suit- 

 ability of linden in parts of northwest- 

 ern Colorado. 



Black locust is a tree that will stand 

 the rigors of drought, poor soil, and 

 abuse. It produces showy, white, fra- 

 grant flowers in the spring, but some 

 persons object to its ragged appear- 

 ance, early leaf fall, and the litter of 

 its flowers and seed pods. In some 

 localities it is severely attacked by the 

 locust borer. The thornless variety is 

 preferable, but various pink-flowering 

 forms are gaining in popularity. 



Norway maple is a handsome tree 

 with a dense, globelike crown of dark- 

 green foliage. It is favored as a street 

 tree in the larger cities in places where 

 irrigation and fertilization are possible 

 and space is ample for root develop- 

 ment. It is less subject to snow damage 

 than silver maple and is moderately 

 resistant to smoke injury. The Schwed- 

 ler variety has bright red leaves in the 

 spring, which later turn deep green. 

 This variety is less likely to suffer from 

 yellow foliage in alkaline soil than 

 other maples. Norway maple is sub- 

 ject to sunscorch or leaf scald, especially 

 when the root system has not sufficient 

 room to develop properly. It is best 

 suited to the high plateau and is not 

 recommended for the Gila River drain- 

 age. It is best to consult local authori- 

 ties regarding the use of Norway maple 

 and its varieties, as the species has 

 proved to be tender in some localities. 



Red mulberry is a somewhat larger 

 tree than Russian mulberry but is less 

 resistant to drought and cold. 



Russian mulberry develops a crown 

 of dense, dark-green foliage. It is fairly 

 resistant to drought but is subject to 

 winter injury. The fruits attract birds 

 but also make a litter on walks and 

 streets. The male, or staminate, trees 

 do not produce fruit. This tree should 

 not be planted in the mountains or in 

 the colder parts of the high plateau. 



Oak may be grown successfully in a 

 few restricted areas in the region, not- 

 ably in the foothills of northern Colo- 

 rado. The limiting factor is soil alka- 

 linity, which, besides giving the foliage 

 a yellow cast, inhibits normal develop- 

 ment for a naturally slow-growing tree. 



Bur oak appears to be best adapted 



