Trees and shrubs classification
Both shrubs and trees have woody-fibered stems, the aerial parts persisting and continuing to grow from season to season after a resting period. These plants are correctly referred to as woody perennials in contrast to herbaceous perennials in which the aerial parts of the plants die down to the ground after a season's growth and the plant persists by means of its subterranean structures, the roots.
A woody plant with a single trunk or stem as its major axis is a tree. The trunk may be dwarfed to a few inches in height or tower toward the sly. Frequently suckers sprout from the root system, giving the tree a shrubby appearance, as is the case with the beech and even the apple.
A woody plant that has no major axis, no central, dominant trunk but a series of shoots and stems all arising from a root cluster, is a shrub. These stems also vary in height from a few inches to many feet, depending on the species.
Some plants classed as trees, such as cedar, hemlock, and laurel, may be trimmed and will grow as shrubs and hedges. Very few shrub species may be cultivated into trees, but it has been done as is shown in the tree rose and the heliotrope.
Deciduous and evergreen plants
Both trees and shrubs are classed as deciduous or evergreen. A deciduous plant is one whose leaves are never more than one season old, dying each autumn or early winter, although the dead leaves may not fall off until spring. Such trees as the beech, hawthorn, hickory, honey locust, linden, maple, oak, and walnut, and such shrubs as forsythia, calycanthus, deutzia, spirea, syringa, viburnum, weigela, rose, and beautybush are deciduous.
Evergreens retain their foliage, some until midwinter, some almost until spring, others until one or more crops of leaves have appeared, several crops frequently adorning a plant at the same time. Some evergreen species are known to have living leaves more than io years old.
Evergreens in turn are broadleaved, with flat blades such as most deciduous trees bear, or coniferous (cone-bearing) with scale and needle-like foliage. The broad-leaved ones are such plants as rhododendron, camellia, boxwoods, southern magnolia, abelia, and mountainlaurel. Among the conifers are hemlock, arborvitae, fir, pine, cedar, spruce, and yew. Although the cypress and the larches bear cones, they drop their needle foliage and consequently arc deciduous. Moreover, some plants, such as the California privet, are in the evergreen classification in southern locales but become deciduous farther north.
Special needs of permanent plants
Standing as they do throughout the years, trees and shrubs must be adapted to the conditions in which they are to grow-to continued seasonal alternations of cold and heat, to soil conditions and moisture or lack of it, to sooty, dusty air, to too much or too little sunlight, and to other unfavorable conditions. For at least three years after they are moved to a new location they should receive special attention, after which culture, feeding, moisture, and pruning should be provided as required.
Plants have various reactions of hardiness. Most deciduous shrubs of temperate regions need at least one bard winter freeze to succeed. Many plants may tolerate severe cold in dry areas but cannot stand it in wet regions.
Coniferous evergreens thrive in northern latitudes, but few broadleaved evergreens do well there. Few cone-bearing evergreens flourish in the extreme South. Evergreens in general like the moist eastern climate. Fewer deciduous plants are successful in dry western regions of severe freezing than in more humid regions. More species of deciduous shrubs flourish in the North than in the South.
Hardiness to area conditions
In choosing ornamental plants the gardener will simplify his work if he selects those that are hardy to the region in which he lives, thus sparing himself extra attention and, possibly, poor results. Many handsome native plants are available to him, and he may be confident that many introduced plants thrive over a wide area. A plant grown in a nursery in any region of this country may be transplanted with reasonable assurance that it will be hardy in other parts of that region.
Need for ample room
A cardinal rule in planting shrubs and trees is to remember that they grow, year after year, and that sufficient space must be provided for the development of both roots and tops. If an immediate scenic effect demands that plants be set close together, they must be thinned out as soon as they begin to crowd, lest they deform one another in their competition for room.
Avoid root competition
Root competition also must be avoided to assure sufficient nourishment. If several plants feed in the same soil layer, there may very well be too little food and moisture to go around. A soil layer may be regarded as a pantry and the roots as the mouths of a hungry horde of children raiding it. The pantry contains a certain amount of food, and if there are too many raiders, some will go hungry. The remedy is other pantries or more food in the one. Thus with plants: abundant food and moisture must be supplied, or plants must be selected whose roots feed chiefly in different layers of the soil.
Woody plants, like soft-stemmed plants, have varying soil and moisture needs and different responses to fertility. Some prefer clay, others like sandy soils; some must stand in water, others need completely dry earth; some want marked acidity; others require alkaline soil. Most of them prefer a medium of these characteristics and many plants are suited to a wide range of environmental aspects.