Herb
Water-loving, quick to root, and the tree behind aspirin's family name.
These are associations the community holds, not settled fact — the more a reading is shared, the bolder it sits.
Willow takes root from a cut stick pushed into wet ground, which no doubt contributed to a reputation for uncanny vitality. It grows by water, bends rather than breaks, and has been coppiced and woven into baskets, hurdles, fish traps and coffins for as long as there have been people in northern Europe doing any of those things.
The tree is associated with grief in English usage — the willow garland worn by the forsaken lover is a Tudor commonplace, and Desdemona's willow song is the best-known instance. In craft it belongs to the Moon, to water, to intuition and to mourning. Wands of willow are cut for lunar and divinatory work, and its association with the dead is old and continental as well as English.
Its most consequential contribution is chemical. Willow bark contains salicin, known and used long before it was isolated, and the drug developed from that line of research became aspirin — a name that carries the old botanical name for meadowsweet inside it. It is one of the very few cases where a folk remedy led straight into a pharmacy.
Readings practitioners have offered, ordered by how many recognise them. Take what serves, leave the rest.
Carried forward
Nicholas Culpeper, The Complete Herbal (1653)
"The Moon owns it," says Culpeper, and every lunar attribution the willow carries in modern practice runs back to those four words. He also recommends the burnt ashes mixed with vinegar for warts, which is less often quoted.
Carried forward
English literary and folk usage, sixteenth century onward
The willow garland is worn by the forsaken lover, a convention common enough by the Tudor period to be understood without explanation. The tree of grief and the tree of the Moon are the same tree, and practice tends to keep both meanings in play.
Credit what you carry. Much of this comes from living traditions, and some of it from traditions that are closed — held by particular peoples and not offered for general use. Where you know whose practice a reading came from, name them; where you don't, say that too.