SHADE TREES FOR CALIFORNIA

W. W. WAGENER.

Climate is the key to the trees that can be grown in a region. On the Pacific coast, the key to the climate is the Pacific Ocean, which imparts its relatively mild temperatures and its characteristic summer droughty period. The region embraces more than 16' of latitude and extends inland about 120 miles to the high barrier formed by the Sierra and Cascade Mountain chains. East of the barrier, the climate is and or semiarid and has a much greater yearly range in temperatures. Eastern Washington, eastern Oregon, and a part of eastern California share this interior type of climate. This article discusses shade trees in California; the next article is about trees in the northern Pacific area the western parts of Washington and Oregon.

No part of the United States presents a greater diversity in climate, topography, and soils than California. Relatively mild winter temperatures and a long summer dry season are common to all parts of the State except the higher mountains, but in other respects even a few miles may bring wide differences in the conditions that govern tree growth.

Few trees, consequently, have sufficient adaptability to be satisfactory in all parts of the region, and the planter must make his choice on the basis of the conditions prevailing in his own neighborhood. An important consideration in inland districts is whether the tree is to receive supplemental irrigation, directly or indirectly, or whether it must depend on moisture provided by the winter rains. Another is whether the ground contains appreciable quantities of soluble salts, commonly known as alkali. If so, the choice should be species known to be alkali-tolerant.

From the thousands of trees that will grow successfully in California, or parts of it, I shall discuss here a limited number that are generally suitable for shade and ornament or have specific qualities that fit them for use under conditions that are unfavorable for most species. In general, I omit trees used primarily for accent or specimen planting, the palms, nearly all of the eucalypts, and a few species of other types that once were popular but are not recommended now because of insects, diseases, or undesirable qualities. Among the last are the elms, Monterey cypress, and the black acacia.

Besides the trees here described, the owner who is considering planting around the home should not overlook the ornamental and shade value of our fruit and nut trees. The apricot, avocado, cherry, orange, kaki persimmon, mission fig, Persian walnut, and many another often serve a double utility.

THE CALIFORNIA LIVE OAK is a rather evenly rounded tree when it is young; it spreads broadly with age. It grows up to 30 to 75 feet rather slowly at first but faster when it is well established and supplied with moderate amounts of water in summer. Its leaves are small and oval, dark green and glossy above, paler below, and rather dense. It casts a fairly dense shade unless the crown is thinned by pruning. For yards, streets, and roadways it is satisfactory in the coastal districts, where it is native, and also in the less hot and dry parts of the interior. In some districts it is subject to defoliation by the larvae of the California oak moth which never kill the tree and are readily controlled by sprays. Some trees suffer from mildew in the coastal districts that have summer fogs. Because heavy pruning and heavy summer watering favor the development of mildew, the tree should not be planted on lawns or other areas that are constantly irrigated. Despite these disadvantages, the merits of the tree make it good for many districts.

SOUTHERN MAGNOLIA is a medium-sized or tall (25 to 60 feet) , round-topped or pyramidal evergreen with large, thick, glossy, dark-green leaves and rust-colored branchlets and buds. It is slow of growth, moderately long-lived, and relatively few insects and diseases bother it. The large, white; showy, fragrant flowers come in late summer and fall. It is hardy throughout the region except in the higher mountains, but does not tolerate alkali. Altogether, it is a satisfactory ornamental for home and street. It should be given additional moisture in summer in most parts of the region.

CAMPHOR-TREE is a handsome, compact, medium-sized and oval-crowned, evergreen tree, 20 to 40 feet in height when mature, with dense, glossy, light-green foliage, bronze-tinged in spring. The leaves, which have an odor of camphor when crushed, cast a fairly dense shade. The flowers are small, yellow, and inconspicuous; the growth rate is moderate, and the length of life is average. The trunk is rather heavy and enlarged at the base. Camphor-tree is satisfactory for planting around the home and as a street tree if parkways are wide enough. It is hardy in most of the region, including the central valleys, but it needs access to additional moisture in the drier situations in summer. It is fairly tolerant of alkali.

RED IRONBARK, a slender and open, medium-sized, evergreen tree of the eucalyptus family, eventually grows to 50 to 60 feet in height. It has rough, furrowed, dark, and persistent bark and small, gray-green leaves. The flowers are deep pink and are produced in profusion in late winter, spring, and into June. Its growth rate is moderate and its longevity is average. It is hardy to about 15 F. and stands drought well. Thus it is adapted to both coastal and inland situations. Moderately alkali-tolerant, it is a satisfactory tree for roadsides as well as for backgrounds or screen planting around the home.

THE CALIFORNIA PEPPERTREE is a medium to large and broadly round-topped, evergreen tree, and 30 to 50 feet high when mature. Its finely cut, light-green and drooping foliage casts a light shade. The small and yellowish-white flowers come in many-branched clusters and are followed in fall by pendent bunches of small, rose-colored fruits that persist through the winter. It grows fast and is of average longevity. It is somewhat tender, but it can stand temperatures of about 18 F.; consequently, it is usable in most of southern California and the milder parts of central and northern California. Although it is drought-resistant and somewhat tolerant of alkali, it has the reputation of harboring black scale and therefore is in disfavor among many citrus growers. It is also susceptible to Armillaria root rot, better known in the region as oak root fungus, and for that reason is uncertain on land formerly occupied by oak woodlands. It used to be planted often as a street tree, but for that purpose it has several faults. Nevertheless, the peppertree is so firmly identified with California, so attractive when it is properly used around the home, and fits so well with California architecture that it will remain popular.

CAPE CHESTNUT is a medium-sized, round-headed tree, 50 to 60 feet high when mature. Its medium-sized, elongated, somewhat sparse leaves cast a light shade. Growth rate is moderate; it is fairly long-lived. It is cultivated chiefly for its panicles of showy, lavender-rose flowers that appear in late May and June. Hardy in most of southern California and in warmer situations elsewhere in the region, it can endure temperatures to about 15 F. In the colder locations it is partly deciduous. It is suitable for planting around the home and as a street tree, but it requires watering in the summer.

THE GINKGO, an erect, rounded, and pyramidal tree, becomes somewhat spreading with age. The ginkgo is long-lived and rather slow growing to an ultimate height of 40 to 50 feet in California. The unique leaves are medium-sized, fan-shaped, and a clear green in color, changing to yellow in autumn. Because it is hardy and has practically no pests, it is useful both as an ornamental and shade tree, but it needs extra summer moisture in the drier places. Only male trees should be planted, because the fallen, mature fruits of the female tree have a disagreeable odor.

THE NORWAY MAPLE is a medium-sized, round-headed, spreading, deciduous tree, 25 to 60 feet high. It has moderately large, light-green leaves that form a dense crown and cast a rather heavy shade. It is relatively fast growing, of average length of life, and hardy. It has proved satisfactory as a lawn and street tree in the interior and mountain valleys of the region, except for a tendency of the roots to raise sidewalks. It is moderately tolerant of alkaline soils.

The silver maple is fast growing, large, spreading, and 60 to 100 feet in height. Its large leaves, bright green above and silvery below, form a rather open crown, which casts a medium to light shade. The flowers, greenish and in clusters, appear before the leaves. It is hardy, and its useful life is about average. It is similar to the Norway maple in uses and districts to which it is best adapted. Its roots sometimes raise sidewalks.

THE LONDON PLANETREE is rounded and pyramidal in habit, but becomes spreading with age. It grows to 30 to 70 feet, and has large, broad, lobed, green leaves that form a rather open crown and cast a light shade. Its growth rate is rapid; its longevity is about average. Its light-colored bark peels in thin plates. The brown, globular fruiting heads, about an inch in diameter, disintegrate when mature.

It is hardy throughout the region, but it is subject to the sycamore blight, which attacks the leaves, and the sycamore scale. Some strains of the tree are practically immune to the blight and resistant to the scale. It is a satisfactory shade and street tree, especially for inland valleys, when propagated from parent stock selected for freedom from blight and pests. The London planetree is alkali-tolerant.

THE SWEETGUM forms a rounded pyramidal tree, usually reaching not over 50 feet in height in California, with deeply furrowed bark and medium-large, deeply lobed, dark-green leaves, paler on the under side. These turn to a crimson or wine purple in the fall. The seeds are borne in spherical heads which are about an inch in diameter and rather prickly on the outside. The tree grows at a moderate rate and it produces a compact head, which casts a medium-dense shade. It is hardy and relatively long-lived, but not tolerant of alkali. The sweetgum is satisfactory as a street tree and for home planting in all parts of the region, except on alkaline soils. It is not adapted to locations exposed to dry winds, and it requires extra summer moisture in the drier localities.

OF THE VELVET ASH, the Modesto or Montebello forms make a spreading but rather compact tree, 30 to 40 feet high when mature. The willowlike, deep-green leaves cast a medium-dense shade. Fast in growth, it is hardy except at high elevations in the region. In longevity it is about average. It is drought-resistant, moderately tolerant of alkali, and more resistant to the red spider type of mite than the regular form. It is subject to occasional defoliation by insects in some districts, but these pests can be controlled readily by sprays. A popular shade and street tree for the interior valleys, it stands drying winds well and succeeds with little moisture. Recently a leaf disease has appeared in parts of the region which may make it less desirable as a shade tree in the future.

THE CAROLINA POPLAR, a tall, upright tree from 40 to 100 feet in height, is pyramidal to columnar in form and is fast growing. The medium-sized and rounded, bright-green leaves cast a medium shade. Of average longevity, it is hardy and slightly tolerant of alkali. It grows best in moist sites. Only the male trees should be used in order to avoid the disagreeable fuzz that is shed from the blooms of the female.

This and the other more spreading types of poplar are especially adapted as shade or roadside trees in mountain valleys or around irrigated pastures in the lower inland valleys. Suckers are sometimes troublesome. Poplars should not be planted close to sewer lines because of the penetrating roots.