Just consider one genus (Salix; known commonly as willow), for instance.
Common Name: Willow
Origin: Most parts of the world, except Australia; around 300 species.
Appearance: There is great variation in size from dwarf alpine creepers to large trees. Some have showy bark and weeping branches in winter. Others produce attractive male catkins which are of smooth appearance and silver grey in colour turning yellow with pollen. Female catkins are normally green and less conspicuous. Typically, willows are dioecious, that is the male and female catkins are borne on separate plants. Creeping species make good ground cover, whilst those with coloured bark contrast well with other trees noted for winter bark, such as silver birch and dogwoods.
Culture: Willows grow well in a sunny aspect in moist loamy soil. Some species will also grow in damp conditions. Almost all species prefer lowland sites. Weeping willows thrive in waterside locations. Larger species should be planted away from underground services due to their spreading root systems. Deadwood should be removed during winter. Those with coloured stems can be encouraged to produce more stems by pruning in late winter or early spring, or by hard pruning every other early spring.
Propagation: They may be propagated by hardwood cuttings taken during the winter months through to early spring. Potted cuttings can be planted out the following year.
Health: Generally hardy; willows are prone to caterpillars and beetle larvae. Gall mites can interfere with young growth causing stem distortion, whilst sawfly produce elongated galls on leaves. Aphids, especially the willow stem aphid, may infest plants causing sticky deposits. Scale insects may affect stems and new shoots. Willow scale can cause stems to look white if the infestation is severe. Diseases include anthracnose of willow which results in dark cankers on shoots and brown spotting of leaves. Bracket fungi may develop on stems and trunks. Dieback can occur as a result of various other fungi. Rust may present as orange or brown leaf spots. Willows may also be killed by honey fungus.
Uses: Shade tree, land rehabilitation, shelter belt, water garden plant, winter garden plant, specimen tree, colourful bark.
Problems: Most species of Salix are weeds in some places and useful plants in other places. Their root systems can be aggressive, damaging footpaths, drains and building foundations; and in the wrong place some species of willow trees can become very tall and wide
Pruning: Some species are routinely coppice pruned or pollarded, to control the size. This may be as severe as cutting back all growth to a stump only 10 to 30cm above the ground. This will encourage multiple stems and can create more of a shrub than tree like appearance. Pollarding may involve forming a trunk and a framework of short branches, to several metres above the ground and removing all growth from the framework annually.
These treatments can keep growth more controlled and healthy. Similarly, root pruning, or restriction of root growth with underground root barriers is sometimes used to manage willow growth.
S. alba: (White Willow): Originates from Europe and Northern Asia where it is often found along watersides and in meadows. It is a large tree of up to 12m with a conical habit and slim branches which droop towards the ends. Leaves are lanceolate, grey-green, with smooth hairs, and appear en masse making them look silver from a distance. Catkins occur with the leaves, the male ones being 5cm long.
S. alba ‘Britzensis’ (syn. S. alba ‘Chermesina’; Scarlet Willow): Has orange-red branches which are striking in winter.
S. babylonica (Weeping Willow): Native to China; reaches a height of up to 7.5m with a spread of 9m. It has slender lanceolate leaves which have green upper sides and greyish blue undersides. The smooth brown branches are long and pendulous. Catkins are 2.5cm long and yellowy green appearing with the spring foliage.
S. x sepulcralis ‘Chrysocoma’ (syn S. x chrysocoma; S. alba ‘Vitellina Pendula’; S. a. ‘Tristis’): Often regarded as the most beautiful of hardy weeping trees these hybrids have slender pale to mid green lanceolate leaves which are produced on long pendulous yellow stems. It is a wide spreading tree with many branches and drooping branchlets. Both male and female flowers are usually produced in the catkins, though they may occur as separate male and female catkins.