The willow genus
Salix is a genus that will not hold still long enough to be counted, and it is the only one on this site whose main economic use is not the flower, the leaf or the fruit but the stem itself, cut young and used as a material.
somewhere between 350 and 500 species, and the count is genuinely disputed
Mostly the cold northern hemisphere, from the tropics to the high arctic
Everything in this guide that belongs to Salix
Nobody can tell you how many willows there are. Published counts run from around 350 to well over 500, and the disagreement is not carelessness. Willows hybridize with each other so freely and so persistently that the boundaries between species genuinely blur in the field.
Where two species grow together the hybrids are usually fertile, and they cross back with both parents and with each other, so what you find is not two kinds and a few intermediates but a continuous smear between them. Add that male and female flowers are on separate plants, that the flowers arrive before the leaves in many species so you rarely see both at once, and that the leaves change shape as a shoot ages, and identification becomes a specialist's job.
This is worth knowing rather than merely curious. A willow bought under a particular name is much more likely to be something approximate than a similarly labeled rose, and cultivated willows are propagated from cuttings precisely because seed will not come true.
The reason willow has been grown deliberately for thousands of years is not its timber. It is that cutting a willow hard makes it respond with a dense crop of long, straight, flexible one year rods, and it will do that indefinitely without being harmed. Cut it to the ground each winter and it will still be producing a century later.
Those rods are withies, and basketry ran on them across Europe and Asia for as long as there have been baskets, along with hurdles, fish traps, eel pots, coracles and fencing. Whole landscapes were managed for it, and some still are.
The color comes from the variety and from the processing rather than from dye. Cut and dried with the bark on gives brown. Stripped after being stood in water until the sap rises gives white. Boiled in its bark before stripping gives buff, where the tannin in the bark stains the wood underneath. A basket in three colors is one species handled three ways.
Willow wood is light, shock absorbent and does not splinter under impact, which is a rare combination and is why every cricket bat in the world is made of it. One particular variety of white willow, grown mainly in eastern England, supplies effectively the entire trade, and the trees are grown as single clear trunks and felled at about fifteen years.
The same wood burns to a fine even charcoal that leaves a soft mark and lifts cleanly, so the charcoal sticks in an art shop are willow, made by charring cut rods in a sealed tin.
And because willow strikes root from a bare cutting more readily than almost any other woody plant, it is the standard tool for holding riverbanks together. Live stakes driven into an eroding bank root and knit the soil, and the same trick builds living fences and tunnels that are planted rather than constructed. It is also grown as a short rotation biomass crop and used to draw contaminants out of polluted ground.
The genus reaches further north than almost any woody plant, and what it looks like when it gets there is nothing like a tree. The dwarf arctic and alpine willows are a few centimeters high, creeping flat over rock and gravel with leaves the size of a fingernail, and one of them is regularly described as the smallest tree on earth.
It is a tree in the sense that counts botanically: a woody perennial with a genuine trunk and branches, and if you dig one up you find a gnarled woody stem that may be older than the person holding it. They grow a fraction of a millimeter a year, and individual plants of a few inches across have been aged at well over a century.
Lying flat is the whole strategy. Anything standing more than a hand's width above the ground in that climate is outside the thin band of still warmer air near the surface, and is scoured by wind and ice. The same genus that supplies eighty foot trees beside a river solves the arctic by refusing to stand up.