Oak
Quercus robur
The Druids did not worship in groves. They worshipped in oak groves. The distinction matters.
Overview
The oak is the axis around which British sacred tradition turns. The word Druid is almost certainly derived from the Proto-Celtic dru-wid — oak-knower, or those who know the oak — which means that the priestly caste of pre-Roman Britain defined itself by its relationship to a single tree. This is not metaphor. It is etymology, and it tells you something about how central the oak was to every dimension of life in northern Europe: as timber, as food in famine, as medicine, as the tree beneath which oaths were sworn, as the host of the rare and sacred mistletoe, as the home of a dozen species of moth, beetle, and bird that exist nowhere else. An old oak is not simply a large tree. It is an ecosystem, a record, and something that has been here long enough to have opinions.
Botanical Notes
A large, spreading, deciduous tree reaching 20–40 metres, with deeply fissured, grey-brown bark and the familiar lobed leaves. Male flowers are pendulous catkins; female flowers are small and inconspicuous, developing into acorns — the single-seeded fruit seated in a scaly cup — from September to October. The most abundant and widespread tree of the lowland British Isles, dominant in ancient woodland on clay and loam soils. A mature oak supports more species of invertebrate — over 280 — than any other native British tree. Oak galls, produced when gall wasps lay eggs in young tissue, were the primary source of iron gall ink for European manuscripts from antiquity into the nineteenth century.
Lore & History
The oak was sacred in Celtic, Germanic, Greek (to Zeus), and Roman (to Jupiter) traditions — the correlation of thunder gods and the tree most frequently struck by lightning (due to height, not any special affinity) is not accidental. In Norse tradition the oak belongs to Thor. The Royal Oak of British history — the tree in which the future Charles II hid after the Battle of Worcester — gave its name to hundreds of pubs, a ship of the line, and a public holiday. Oak Apple Day, the 29th of May, was a public holiday in England for over two centuries. The tannins in oak bark were the primary agent for converting hide to leather across the entire pre-industrial world. Oak timber built the ships that built the empires. The mistletoe found growing on oak was, in Druidic practice, the most powerful of all plants — because the oak itself was the most powerful of all trees, and what grew on it shared that power.
Warnings
Acorns contain tannins that are toxic to horses, cattle, and dogs in quantity — fallen acorns are a significant veterinary risk in autumn. Raw acorns are too bitter for human consumption but can be eaten after thorough leaching in water to remove tannins; processed acorn flour has been a famine food across Europe and North America. The oak is otherwise safe — bark tea and leaf preparations have a long medicinal history as astringents. No significant toxicity to humans at normal use.