SHADE TREES FOR THE NORTHEAST

ALMA M. WATERMAN, R. U. SWINGLE, CLAYTON S. MOSES.

Throughout the Northeastern States, the maples, the elms, and the oaks have long been preferred for shade trees. The elms in this region, however, are threatened by two serious diseases. In the northwestern part a wilt disease impairs the value of red oaks for shade-tree planting. Fortunately, there are still many kinds of beautiful native trees and some introduced kinds that make satisfactory shade trees.

Some of the outstanding deciduous shade trees that can be recommended for residential and suburban sections, primarily because of their tolerance of city conditions, are : Sugar maple, Norway maple, red maple, white oak, pin oak, northern red oak, scarlet oak, Texas oak or Shumard oak, thornless common honeylocust, sweetgum, ginkgo, American sycamore, London plane-tree, common hackberry, black tupelo, green ash, silver linden, littleleaf linden, Kentucky coffeetree, yellow-poplar or tuliptree, the American yellow-wood, Japanese pagodatree, and Amur corktree.

In heavily congested and industrial areas the following species may be used : The ginkgo, the thornless common honeylocust, London planetree, ailanthus or tree-of-Heaven, and the Amur corktree.

In the Northeastern States, a large area, the climate and other conditions vary so much that not all the recommended kinds of shade trees will grow equally well throughout the region. The elevation above sea level, rainfall, the proximity of large bodies of water, river valleys, and other factors modify the natural distribution of plants and affect the growth of shade trees. For best growth, some kinds will be limited to the more northern or to the more southern sections. Some kinds that will grow in the southern border zone of the area may not grow at all in the most northerly parts. Some kinds of shade trees that grow best in New England may do well in the southern Appalachians, but very poorly on the Coastal Plain. On the other hand, some predominantly southern species may extend far north along the Atlantic coast.

THE MAPLES are widely planted as shade trees in the Northeast, but most of the native species are not entirely satisfactory for this purpose. They are short-lived, are subject to windbreak, and require moist, rich soil.

The Norway maple, introduced from Europe, and our native sugar maple are considered the most satisfactory for streets and lawns.

Sugar maple is one of the most common and attractive trees throughout the Northeast. It is a large tree, 50 to 90 feet in height. When it is grown in the open as a shade tree, it has a short trunk with a broadly egg-shaped or round-topped crown of stout, ascending branches. Horticultural varieties that have a narrow columnar head are especially adapted for planting along narrow streets. The leaves of the sugarmaple are thin, bright, rich green, and in the North usually develop in May together with a profusion of yellowish-green flowers, from which bees obtain pollen and nectar. In the autumn the brilliant yellow, orange, and scarlet coloration of its foliage is attractive.

Sugar maple is readily transplanted, its rate of growth is moderate, and it is relatively long-lived, with a possible life span of more than 100 years. It is injured by city smoke and gas fumes and therefore is not suitable for planting in industrial or congested residential areas. It is valuable, however, on lawns, along suburban streets, or on farmsteads. It attains its best development when it is grown in well-drained, moist, rich soil, but it will survive in less favorable sites in gravelly soil. It is the source of maple sugar.

A wilt disease is sometimes serious, and several leaf diseases caused by fungi are common on sugar maple. Brown dead areas in or along the edge of the leaf blade often develop when drying winds or bright sunlight and high temperatures immediately follow a period of moist weather.

Norway maple is grown extensively from central New England and New York southward. It is usually about 30 to 60 feet tall at maturity, with a short trunk and numerous stout, ascending branches that form a low, round, spreading head. The greenish-yellow flowers appear in abundance in April and May before the leaves develop. The leaves are slightly larger than those of the sugar maple, deeper green, and firmer in texture. The dense foliage remains on the tree late in autumn and the leaves turn bright yellow before falling. Norway maple is easily transplanted, its rate of growth is moderate, and it tolerates a wide range of soil conditions. It stands unfavorable soil and atmospheric conditions in cities and therefore is widely used as a street tree. Its low, dense head, however, requires considerable pruning to adjust it to street conditions, and it is not adapted for planting along narrow streets.

A horticultural variety with a narrower, more upright crown is sometimes grown successfully under such conditions. Because the thick shade and mass of fine feeding roots of the Norway maple make it hard for grass to grow under the tree, Norway maple frequently is considered undesirable as a lawn tree.

The Schwedler maple, a variety of Norway maple, has a similar type of growth. When young, it has bright-red leaves that change to dark red and finally to green. It is planted on lawns and sometimes along suburban streets for ornament and for shade.

The Norway maple is subject to about the same pests as the sugar maple, but is less subject to leaf scorch.

Red maple, a native, is less desirable for a shade tree than either the sugar maple or the Norway maple. It can be used when a fast-growing tree is needed. The red maple develops a conical or broad, rounded crown, with bright-green leaves that assume brilliant shades of orange, red, and scarlet in autumn. The foliage casts a moderately dense shade. In the spring, masses of red flowers make it attractive.

Red maple is easily transplanted. The wood is somewhat weak and subject to storm damage, and its roots often enter and clog sewers.

THE ELMS are outstanding trees, but unfortunately the American elm cannot be recommended now except for limited planting, because of phloem necrosis and the Dutch elm disease, both of which are spreading rapidly and causing heavy losses. New public plantings of American elm should be delayed therefore until satisfactory control measures for the diseases have been developed, and the home owner will do well to consider carefully whether some other kind of shade tree cannot be planted instead.

In the Northeast, the Dutch elm disease extends from the Atlantic seaboard westward to Indiana. An isolated outbreak has been found in Colorado. It has not been found in Maine, New Hampshire, Michigan, Wisconsin, Minnesota, Illinois, Iowa, Missouri, or Arkansas.

Phloem necrosis is not known to occur in Pennsylvania, States east of the Appalachians, or in Michigan, Wisconsin, and Minnesota. The American elm is subject to several other pests.

The American elm grows to a height of 50 to 100 feet and has a tall, branching trunk. It develops numerous ascending or drooping branches that form various types of crowns, such as the typical vase or umbrella forms. The beauty of its various forms of growth and the arching of its branches above the streets of New England towns have made the American elm an outstanding characteristic of the landscape. None of the many introduced species of elm can equal it for ornament or shade. The greenish flowers appear in drooping clusters in April or May before the leaf buds open. The leaves are 4 to 6 inches long, rough, dark green, unequally rounded at the base; they turn yellow in autumn and usually fall rather early.

The American elm is easily transplanted, grows rapidly, and often lives between 70 and 100 years under city conditions. The American elm is tolerant of a wide range of soil conditions, except dry, sandy locations, but its best growth is developed in moist, well-drained soils. In its natural habitat it is found along streams or in low, moist ground. It grows well on streets and in yards. The growth habit of the branches is such that the crotches of old trees often have to be braced in order to withstand heavy wind or ice storms.

The rock elm might be used more for streets and lawns, as it is a large, strong, narrow-headed tree. This elm is more suited to the northern than to the southern part of the region, and should be considered particularly for the Lake States. Rock elm grows more slowly than American elm.

In this region two European species of elm are grown sometimes as shade trees. They are susceptible to the Dutchelm disease and also are frequently affected by the elm leaf beetle.

The English elm (Ulmus procera) is a large tree, sometimes reaching 100 feet in height. It has a straight trunk that extends into the tree crown, and branches that spread or ascend to form an oblong, rounded crown more like the oaks than the American elm. The leaves, 2 to 3 inches long, remain on the tree later in the autumn than those of the American elm. The English elm can be transplanted quite easily and is adaptable to the same types of soils as the American elm. It has the tendency to produce numerous shoots or suckers from the roots.

For that reason, another European species, the Scotch or Wych elm, which is similar to the English elm in form and growth habit but does not produce suckers, has often been preferred, both for lawn and street planting. The leaves of the Scotch elm are about 3 to 6 inches long. Several horticultural varieties of both these species are in cultivation.

The Chinese elm (Ulmus parvifolia) has small leaves, which turn bright yellow in the autumn before they fall. Its flowers are formed in August or September. It is easily transplanted and grows rapidly. It is hardy in southern parts of the region. The Morris Arboretum in Philadelphia has a beautiful, large specimen of this tree.