Herb
The spear-tree and the world-tree, still leafing late in every damp lane.
These are associations the community holds, not settled fact — the more a reading is shared, the bolder it sits.
Ash timber bends without breaking, which is why it made spear shafts, tool handles, cart frames and, later, the wing spars of aeroplanes. That practical quality sits underneath most of the folklore. Norse cosmology names the world tree Yggdrasil an ash, and the first man, Askr, is made from one; whether the ash of the poems is this species or a memory of some other tree is a live scholarly argument.
English folk practice is more domestic. Sick children were passed through split ash saplings, which were then bound up to heal, on the understanding that as the tree closed so would the illness — a custom recorded from the seventeenth century into the nineteenth. Ash keys were used for divination and against ill-wishing, and the tree shares with oak and thorn a reputation as a place where you should mind your manners.
Ash dieback has now killed a great many British ashes and will kill many more. Anything written about this tree at present carries an elegiac edge that has nothing to do with folklore.
Readings practitioners have offered, ordered by how many recognise them. Take what serves, leave the rest.
Carried forward
Nicholas Culpeper, The Complete Herbal (1653)
Culpeper writes that the ash "is governed by the Sun", and recommends the tender tops against the bites of adders — adding, with obvious relish, the old belief that a serpent will sooner go through fire than pass an ash bough. He does not endorse it, quite.
Carried forward
English folk healing, seventeenth to nineteenth century
A young ash was split, a sick child passed through the gap, and the tree bound up again to grow together. Folklorists collected the custom from people who had done it themselves, which makes it unusually well attested for a piece of sympathetic magic.
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.