SHADE TREES FOR THE PLAINS

ERNEST WRIGHT, T. W. BRETZ.

Good care is doubly important for shade trees in the Plains States.

Trees planted on shallow soil underlain with clay or other hardpans have little chance of survival unless watered artificially and, even then, growth is generally unsatisfactory. The best and deepest soil available should be chosen so tree roots can develop unhindered.

Cultivation, following planting, is necessary to help the trees compete with prairie grasses and other native vegetation. Cultivation should be shallow to avoid unnecessary injury to tree roots near the surface. After the crowns of the trees are well developed, particularly in group plantings, they tend to shade out competing vegetation, and cultivation may no longer be necessary. The tree also must be protected from injury by livestock and sometimes from damage by rodents.

At best, most of the trees planted in the Plains States are relatively short-lived as compared to the same species growing in more favorable regions.

Coarse and droughty gravels, clay-pan soils, the undrained alkaline basins (buffalo wallows), and shale-derived upland soils generally are unsuited for trees. Furthermore, the western third of the Plains States, from Texas to the Dakotas, have areas where the soil is deficient in iron or where iron or other essential nutrients are unavailable to growing trees.

Unavailability of iron causes chlorosis, or yellowing, of the leaves of some tree species, reduces growth, and frequently brings on premature death. Also, in Texas and Oklahoma along the Red River and southward east of the high plains, certain large areas are infested with the cotton root rot fungus, an indigenous soil fungus that infects the roots of many kinds of trees and eventually kills them. A few species, however, are highly resistant to the disease and will usually survive satisfactorily. The susceptibility of several of the more important tree species to cotton root rot is indicated later.

Adequate artificial watering of the shade and street trees is frequently not practicable in the Plains. In such cases their survival depends largely on rain and snow. Furthermore, isolated individual shade trees do not have the advantage of gaining additional moisture by stopping drifting snow, as do shelterbelt or block plantings. Because of the reduction in annual precipitation from east to west in the midcontinental section of the United States, there is a corresponding reduction in the choice of usable tree species. Survival and growth are usually poor, even for the hardiest species, where annual precipitation is less than 16 inches in the extreme northern part and 20 inches in the more southern part of the Plains States.

Low winter temperatures can likewise endanger the survival of trees. Even worse are freezes in early fall and late spring. Consequently, the choice of species becomes more restricted the farther north one goes.

In the following discussion of trees for the Great Plains, we have separated broadleaf species from the conifers and describe in greater detail the trees that can be used in all parts of the Plains.

The broadleaf species include the ash, cottonwood, elms, oaks, birch, poplars, and others.

GREEN ASH, a native species, develops spreading branches and makes a medium-sized, round-topped tree up to 50 feet in height. Its trunk may grow 1 1/2 feet thick. It makes a moderate growth and the light-green foliage, turning golden yellow in autumn, gives dense shade. The greenish flowers develop in late spring. Green ash develops best in deep, rich, lowland soil, but it also does well on heavier upland soil. It is one of the best broadleaf trees for streets and yards in the Plains States. It is moderately drought-resistant, but it will stand wet as well as dry sites. Borers damage it, especially on droughty sites. Several relatively unimportant leaf spot diseases and a rust affect it. Green ash is moderately susceptible to cotton root rot.

White, blue, or red ash are important locally, principally on the extreme eastern edge of the Plains.

THE PLAINS COTTONWOOD was widely planted by early settlers. This native tree quickly develops an open crown and it reaches a height of 80 or 90 feet. The trunk gets to be as large as 6 feet in diameter at the base. It affords light shade. Flower catkins appear in early spring before the leaves. The leaves become a colorful yellow in autumn. The trees may attain an age of 60 years or more on the better sites. Although they prefer rich, moist soil and thrive particularly well in the lowlands, they will also grow in drier, sandier locations of the uplands. They do poorly on soils with clay hardpan near the surface, and should not be planted on sand dunes. The life of the plains cottonwood on such soil is short, usually 10 years or less. The tree is susceptible to borers, especially on the drier sites.

Poplars and cottonwoods are highly susceptible to cotton root rot. Bacterial limb galls, not uncommon, cause considerable branch dying in older trees. Fungus stem cankers are destructive to young trees, especially on unfavorable sites where growth is slow. Leaf rusts are common but of little consequence to thrifty trees. Cottonwoods are subject to attack by gall insects, which make them unsightly but cause little harm. They are moderately drought-resistant when well established on good upland sites. They are also susceptible to chlorosis and grow poorly on alkaline soils. They are mainly usable as farmstead trees. When used for street plantings, only male trees should be selected, to avoid the nuisance from cottony seed. Roots of the cottonwood often clog sewer and drain pipes. The wood is weak and subject to breaking in storms. Large, old trees near houses are often dangerous. The cottonwood should not be planted on streets, except where more suitable species will not grow.

Other species, such as Lombardy poplar and the hybrid Carolina poplar, have also been used widely but are better suited to the eastern edge of the Plains area where rainfall is greatest.

THE AMERICAN ELM, perhaps the most beautiful broadleaf tree, sometimes reaches 90 feet in this region. This elm has a symmetrical, vase-shaped crown that frequently attains a width of 60 feet or more. It lives to an age of 75 to 100 years on the better sites, provides moderately dense shade, and grows moderately fast. Its small, brown flowers appear in early spring before the tree leafs out. It prefers deep, rich, moist loam, but it will stand adverse soil and weather conditions and does fairly well even on sandy exposed sites.

In the Plains States, American elm is often seriously attacked by the elm leaf beetle and European elm scale. The elm sawfly causes defoliation at times. The wooly elm aphid is also abundantly present, especially during wet seasons. A fungus leaf spot disease is common. Phloem necrosis has been found in eastern Nebraska, Kansas, and Oklahoma and is a serious menace to the existence of the American elm throughout the Plains. We fear that phloem necrosis will progress rapidly westward; until a method of control is discovered for this destructive disease, we advise against planting the American elm. Trunk rot due to a fungus is fairly common in planted street trees. American elm is susceptible to cotton root rot and should not be planted on soils infested by this fungus. Rabbits are another enemy.

THE SIBERIAN ELM, often sold under the name of Chinese elm, has been planted perhaps more widely than any of the other introduced species. It grows rapidly to a height of 50 to 60 feet. The main branches tend to grow upward and form sharp crotches that are easily broken by wind. Slime flux commonly develops in such wounds. The species provides moderately dense shade. Small purplish flowers appear in early spring before the leaves show. It does best on sandy loam soils, but it will also grow on sandy sites of low fertility. It lives about 50 years or less. It is drought-hardy, but, because it develops its leaves early in the spring and retains them later than most trees in the fall, it is in constant danger of severe damage by unseasonable freezes. Cytospora fungus cankers cause further damage to frost-injured trees. Siberian elm is highly susceptible to cotton root rot and cannot be used on soil infested by the fungus. Rabbits often damage it severely. This species is used for both street and yard planting, but is now in general disfavor because of its susceptibility to frost damage.

COMMON HACKBERRY makes a moderate growth up to 50 to 70 feet, and develops a large, rounded crown. Its spreading branches provide moderate shade. The small, greenish flowers appear in late spring. It prefers a rich, moist, and well-drained soil, but it will grow successfully on practically all types. The common hackberry is frost-hardy and withstands heat well. It attains an age of 60 to 70 years.

Webworms commonly cause defoliation, especially of young trees. This species is subject to rabbit damage. Witches' brooms are a distinguishing feature of older trees and are regarded as undesirable, but they do not seriously affect the health of the trees. Several minor leaf diseases occur on this species. It resists cotton root rot, and is drought-hardy when once established. It will not stand extensive flooding.

The common hackberry is a good street tree and should be used more extensively in this region.

THE BUR OAK, a large, slow-growing native tree, reaches a height of 80 feet on the best sites. Its life expectancy is 75 years or more. It has a short trunk and a broad, massive top that gives moderately dense shade. The slender flower catkins and the leaves appear in midspring. The bur oak does well on many soils but prefers rich bottom lands. Drought-resistant and frost-hardy, it should be planted more extensively despite its slow growth.

Twig galls are common, but few insects attack the tree. Leaf rollers are observed frequently and red spider occurs in dry years. Foliage diseases usually are not serious. Powdery mildew and leaf scorch sometimes do damage. Cronartium rust is common on oak leaves in some localities. Rabbit damage is severe to the young seedlings in the western part of the area. Bur oak is susceptible to the cotton root rot fungus.

Other species more limited in use are pin oak, black oak, and the chinquapin oak. In the uplands of Nebraska, the trunks of pin oaks often have vertical cracks that are the result of freezing injury.

Several broadleaf species have a more restricted use.