What are trees?
Trees may be broadly defined as long-lived, woody perennial plants, deciduous or evergreen, each usually with a single stem, although some, such as birches (Betula), may have 2 or 3 stems and still be regarded as trees. They are generally quite distinct from shrubs, which produce several or many stems that branch from or near soil level. As a group, trees are larger than shrubs, but show great variation in shape and height, ranging from dwarf cultivars only 3ft (lm) high to specimens of 300ft (90m). In horticulture, a trained or grafted shrub that is grown as a standard, even if only 6ft (2m) or so high, is often referred to as tree-like. Many conifers can withstand extreme climatic conditions and have distinctive, regular branching, often conical crowns, and linear, needle-like leaves. They are popular as specimen trees and for hedging and screening. Dwarf conifers are ideal in beds and containers.

Ornamental features
The leaves of trees are often highly decorative and vary greatly in size, shape, surface texture, and color. Occurring in many shades of red, green, as well as yellow, purple, and other hues, their dense mass of color can complement other plants throughout the year. Some deciduous trees, especially those originating from W. North America and parts of China, produce spectacular autumn color, including Acer, Cornus, Fraxinus, Ginkgo, Liquidambar, Liriodendron, Nyssa, Quercus, Stewartia, Tilia, and Zelkova. Leaf textures, whether smooth and glossy, or hairy or woolly, add further interest. Some trees bear aromatic foliage.

Many trees are also cultivated for their attractive, often scented flowers, which range from the small, clustered flowers of crabapple (.Malus) to the large single blooms of Magnolia. The berries, pods, or other fruits that follow the flowers, with their bold shapes and colors, often persist throughout autumn or winter. Some species bear fruit only in maturity, while dioecious trees, such as most hollies (Ilex), must be cross-pollinated by a second plant of the opposite sex before setting fruit. Bark can also provide fascinating patterns, textures, and colors (see panel, left). Some species may need to be pruned to the base or to major branch framework annually to stim­ulate vividly colored new growth.

Garden uses
Trees are most commonly grown in an open site as specimen plants, visible from all angles, generally on a lawn or underplanted with groundcover, or they may be grown in a large shrub border as a focal point.  Single trees can also be used to mark an entrance to or change of levels in a garden.  Ideally, a specimen tree will display one or more features at different times. In larger gardens, trees can be planted in groups.  Year-round interest is ensured if both evergreen and deciduous trees are included, since the branches of deciduous trees are bare for up to half the year in cold climates. Trees may also be used as hedging, as wind or sound barriers, to screen eyesores, to frame a view, or to line a pathway.  They can give shelter from sun or rain, as well as provide a home for wildlife.

A woodland of mature trees provides a naturalistic, shady, and sheltered environment in which to nurture shrubs, perennials, and bulbs that grow best in dappled shade.  Larger birches or oaks (Quercus) can provide an excellent canopy above smaller trees, such as some magnolias and maples (Acer). Many trees are also tolerant of cultivation in containers; bonsai is the primary example of this.  Since it is possible to move them around, they can be used in varied displays in areas such as patios and courtyard or rooftop gardens, or to flank steps or doorways.  Container-grown trees can also be underplanted with annuals and trailing plants for color and variety.  In cold climates, ensure that containers are frost-proof; tender trees can be moved under glass for protection in winter.

Trees can thrive for decades, some even for centuries, if they are grown in the right soil and climate, and have adequate shelter, levels of light, and rainfall.  Plant away from pipes, drains, cables, and usually walls, and buildings, although some tender trees are best grown against a sunny wall.  On slopes, plant trees halfway down, where it is warmer and less windy.  In coastal zones, select trees that tolerate salt winds and spray. Plant bare-root, usually deciduous trees in midautumn and midspring, but not in frosty weather; evergreen trees are best planted in autumn or midspring.  Plant hardy trees with fleshy roots, whether evergreen or deciduous, in midautumn or in mid- or late spring; in cold areas, less hardy and evergreen trees should be planted only in mid-spring.  Plant balled-and-burlapped trees in early or midautumn, or in early or midspring; deciduous balled-and-burlapped trees may be planted in winter when the weather is mild.  Plant out container-grown trees at any tie during the growing season, except during severe cold or drought.

Dig the planting hole 2 to 4 times as wide as the root ball, and 1½ times as deep, working organic matter into the base.  If necessary, drive a stake off-center into the hole.  Plant the tree, backfill with soil mixed with organic matter, tread it in firmly, and water well, mulching thickly or top-dressing with bark chips or a similar organic mulch.  Secure the tree to the stake and protect it with a stem guard.  Until established, water young trees regularly, especially those on light, sandy soils; keep them free of weeds to a diameter of 3ft (1m) around the trunk.  Provide a mulch. Remove suckers as they appear. Feed and water trees grown in containers regularly, watering freely in hot, dry weather.  In spring, replace the top 2in (5cm) of soil mix with fresh soil mix and apply a slow-release fertilizer.  Pot on every 3 to 5 years. Trees may be propagated by seed, cuttings, layering, or grafting (rarely used by amateur gardeners).  Species are often grown from seed, although they take a long time to establish.  Hybrids and cultivars rarely come true from seed and must be increased by vegetative means, usually from cuttings.

-Information obtained from The American Horticultural Society A-Z Encyclopedia of Garden Plants