The hazel, Corylus avellana, is a shrubby nut-tree of European hedgerow and woodland, and in Irish tradition it is the tree of concentrated wisdom. Nine hazels are said to grow around a sacred well (the Well of Segais or Connla's Well), dropping their nuts into the water where the Salmon of Knowledge eats them and so takes in all the wisdom of the world; the hero Fionn mac Cumhaill acquires that knowledge by accident, burning his thumb on the cooking salmon and sucking it. The hazelnut, compact and nourishing and long-storable, is a fitting emblem for wisdom gathered into a small hard shell.
Hazel has a long practical history in English woodland management distinct from its wisdom-lore: coppiced hazel, cut back to a stump every seven to ten years to send up a fresh crop of straight poles, supplied the wattle for wattle-and-daub building, hurdles for fencing sheep folds, and pea-sticks for the garden, a sustainable harvesting method that also, as a side effect, creates the open, sunlit woodland floor that supports Britain's richest woodland flora, so traditional coppicing and biodiversity turn out to depend on each other. Hazel carries a separate, more recent piece of British folk practice: a forked hazel rod is the traditional tool of dowsing, or water-witching, held to bend or twitch in the hand when passing over hidden underground water, a folk technique quite distinct from the older Irish lore of wisdom-bearing hazelnuts recorded above.