Willows, also called sallows and osiers, of the genus Salix, comprise around 350 species (plus numerous hybrids) of typically deciduous trees and shrubs. They are primarily found on moist soils in cold and temperate regions.
Most species are known as willow, but some narrow-leaved shrub species are called osier, and some broader-leaved species are referred to as sallow (from Old English sealh, related to the Latin word salix, willow).
Some willows (particularly arctic and alpine species) are low-growing or creeping shrubs; for example, the dwarf willow (Salix herbacea) rarely exceeds in height, though it spreads widely across the ground.
Description
Willows have watery bark sap rich in salicin, soft, usually pliant, tough wood, slender branches, and large, fibrous roots that are often stoloniferous. The roots are remarkable for their toughness, size, and tenacity to live, and roots readily sprout from aerial parts of the plant.
Leaves The leaves are typically elongated, but they might also be round to oval, frequently with serrated edges. Most species are deciduous; semi-evergreen willows with coriaceous leaves are rare, e.g. Salix micans and S. australior in the eastern Mediterranean.
All the buds are lateral; no absolutely terminal bud is ever formed. The buds are covered by a single scale. Usually, the bud scale is fused into a cap-like shape, but in some species it wraps around and the edges overlap.
The leaves are simple, feather-veined, and typically linear-lanceolate. Usually they are serrate, rounded at base, acute or acuminate. The leaf petioles are short, the stipules often very conspicuous, resembling tiny, round leaves, and sometimes remaining for half the summer. On some species, however, they are small, inconspicuous, and caducous (soon falling).
In color, the leaves show a great variety of greens, ranging from yellowish to bluish color.
Willows are among the earliest woody plants to leaf out in spring and among the last to drop their leaves in autumn. In the northern hemisphere, leafout may occur as early as February depending on the climate and is stimulated by air temperature. If daytime highs reach for a few consecutive days, a willow will attempt to put out leaves and flowers.
In the northern hemisphere, leaf drop in autumn occurs when day length shortens to approximately ten hours and 25 minutes, which varies by latitude (as early as the first week of October for boreal species such as S. alaxensis and as late as the third week of December for willows growing in far southern areas).
Flowers
With the exception of Salix martiana, willows are dioecious, with male and female flowers appearing as catkins on separate plants; the catkins are produced early in the spring, often before the leaves.
The staminate (male) flowers have neither calyx nor corolla; they consist simply of stamens, varying in number from two to 10, accompanied by a nectariferous gland and inserted on the base of a scale which is itself borne on the rachis of a drooping raceme called a catkin, or ament. This scale is square, entire, and very hairy. The anthers are rose-colored in the bud, but orange or purple after the flower opens; they are two-celled and the cells open latitudinally. The filaments are threadlike, usually pale brown, and often bald.
The pistillate (female) flowers are also without calyx or corolla, and consist of a single ovary accompanied by a small, flat nectar gland and inserted on the base of a scale which is likewise borne on the rachis of a catkin. The ovary is one-celled, the style two-lobed, and the ovules numerous.
Taxonomy The scientific use of the genus name Salix originates with Carl Linnaeus in 1753. The modern concept of types did not exist at the time, so types for Linnaeus's genera had to be designated later. The type species, i.e., the species on which the genus name is based, is Salix alba, based on a conserved type.
The generic name Salix comes from Latin and was already used by the Romans for various types of willow.Helmut Genaust: Etymologisches Wörterbuch der botanischen Pflanzennamen (German). 3rd ed., completely revised and expanded. Nikol, Hamburg 2005, (reprint from 1996). pg. 552 A theory is that the word is ultimately derived from a Celtic language, sal meaning 'near' and lis meaning 'water', alluding to their habitat.
Willows are classified into subgenera though what they should be is in flux. Morphological studies generally divide the species into 3 or 5 subgenera: Salix (though some split off subgenera Longifoliae and Protitae), Chamaetia, and Vetrix. Phylogenetic studies have suggested that Chamaetia and Vetrix be in one clade.
The oldest fossils of the genus are known from the early Eocene of North America, with the earliest occurrences in Europe during the Early Oligocene.
Selected species
The genus Salix is made up of around 350 species of deciduous trees and shrubs. They hybridise freely, and over 160 such hybrids have been named. Examples of well-known willows include:Mabberley, D.J. 1997. The Plant Book, Cambridge University Press #2: Cambridge.
Salix aegyptiaca L. – musk willow Salix alba L. – white willow Salix amygdaloides Andersson – peachleaf willow Salix arctica Pall. – Arctic willow Salix babylonica L. – Babylon willow, Peking willow or weeping willow Salix bebbiana Sarg. – beaked willow, long-beaked willow, or Bebb's willow Salix caprea L. – goat willow or pussy willow Salix cinerea L. – grey willow Salix discolor Muhl. – American pussy willow or glaucous willow Salix eleagnos – rosemary willow Salix euxina I.V.Belyaeva – eastern crack willow Salix exigua Nutt. – sandbar willow, narrowleaf willow, or coyote willow Salix × fragilis L. – common crack willow Salix glauca L. – gray willow, grayleaf willow, white willow, or glaucous willow Salix gooddingii C.R.Ball – Goodding's black willow Salix herbacea L. – dwarf willow, least willow or snowbed willow Salix humboldtiana Willd. – Humbolt's willow, native to central and South America Salix integra Thunb. Salix laevigata Bebb – red willow or polished willow Salix lasiolepis Benth. – arroyo willow Salix mucronata Andersson – Safsaf willow, a species from southern Africa Salix nigra Marshall – black willow Salix paradoxa Kunth Salix pierotii Miq. – Korean willow Salix purpurea L. – purple willow or purple osier Salix scouleriana Barratt ex Hook. – Scouler's willow Salix sepulcralis group – hybrid willows Salix tetrasperma Roxb. – Indian willow Salix triandra L. – almond willow or almond-leaved willow Salix viminalis L. – common osier
Ecology
Willows are shade tolerant and typically short-lived. They require disturbances to outcompete conifers or large deciduous species. The seeds are tiny, plentiful, carried by wind and water, and viable only for a few days; they require warm and moist conditions to take root. The plants can also reproduce vegetatively from decapitated stumps and branches.
Willows produce a modest amount of nectar from which bees can make honey, and are especially valued as a source of early pollen for bees. Various animals browse the foliage or shelter amongst the plants. Beavers use willows to build dams. The trees are used as food by the larvae of some species of Lepidoptera, such as the mourning cloak butterfly."Mourning Cloak". Study of Northern Virginia Ecology. Fairfax County Public Schools. Ants, such as wood ants, are common on willows inhabited by aphids, coming to collect aphid honeydew, as sometimes do wasps.
Pests and diseases Willow species are hosts to more than a hundred aphid species, belonging to Chaitophorus and other genera, forming large colonies to feed on plant juices, on the underside of leaves in particular. Corythucha elegans, the willow lace bug, is a bug species in the family Tingidae found on willows in North America. Rhabdophaga rosaria is a type of gall found on willows.
Rust, caused by fungi of genus Melampsora, is known to damage leaves of willows, covering them with orange spots.
Conservation Some Native Americans allowed wildfires to burn and set fires intentionally, allowing new stands to form.
A small number of willow species were widely planted in Australia, notably as erosion-control measures along watercourses. They are now regarded as invasive weeds which occupy extensive areas across southern Australia and are considered 'Weeds of National Significance'. Many catchment management authorities are removing and replacing them with native trees.
Cultivation Almost all willows take root very readily from cuttings or where broken branches lie on the ground (an exception is the peachleaf willow, Salix amygdaloides). One famous example of such growth from cuttings involves the poet Alexander Pope, who begged a twig from a parcel tied with twigs sent from Spain to Lady Suffolk. This twig was planted and thrived, and legend has it that all of England's weeping willows are descended from this first one.
Willows are extensively cultivated around the world. They are used in hedges and landscaping.
The high end shopping district of Ginza in Tokyo, Japan, has a long history of cultivating willow, and is well known for its willow lined streets.
Hybrids and cultivars
Willows are very cross-compatible, and numerous hybrids occur, both naturally and in cultivation. A well-known ornamental example is the weeping willow (Salix × sepulcralis), which is a hybrid of Peking willow (Salix babylonica) from China and white willow (Salix alba) from Europe. The widely planted Chinese willow Salix matsudana is now considered a synonym of S. babylonica.
Numerous cultivars of Salix have been developed and named over the centuries. New selections of cultivars with superior technical and ornamental characteristics have been chosen deliberately and applied to various purposes. Many cultivars and unmodified species of Salix have gained the Royal Horticultural Society's Award of Garden Merit. Most recently, Salix has become an important source for bioenergy production and for various ecosystem services.
Names of hybrids and cultivars were until recently (2021) compiled by a working party of the UN FAO, the International Cultivar Registration Authority (ICRAs) for the genus Salix (willows) , but it is no longer active.
Uses The Quinault people of Washington made the bark into a twine which sometimes served as harpoon line. The wood was used by some other Native American tribes to start fires by friction, the shoots to weave baskets, and both the branches and stems to build various items including fishing weirs.
History of use in folk medicine The leaves and bark of the willow have been mentioned in ancient texts from Assyria, Sumer and Egypt and in Ancient Greece the physician Hippocrates wrote about its properties as a folk medicine in the fifth century BC.
Interpreting Mesopotamian cuneiform texts is a challenge, especially when looking for something as specific as a species of plant being used to treat a recognisable condition. Some 5,000 medical prescriptions have been identified from Babylonian writings of the 7th to 3rd centuries BC, involving 1,300 drugs from 340 different plants. Whether any of these relate to willow is uncertain. The seeds of the Haluppu-tree were recommended in the Sumerian narrative of Gilgameš, Enkidu and the Nether World as treatment for infertility, but the "Haluppu-tree" could have been oak, poplar or willow.