The Oak Tree in Celtic Tradition

No tree in the European folk tradition carries more weight than the oak. From the Druidic groves of Gaul to the royal oak of Boscobel, from the Irish place-names that still preserve the word doire (“oak grove”) to the figure of the Oak King who fights the Holly King at the turning of the year, the oak has been the tree of endurance, of authority, and of the older gods. The Celts, in particular, wove the oak into the very vocabulary of their priesthood, and the trace of that older devotion is still detectable in the landscape and in the calendar.

The Druid and the Oak

The word druid — druí in Old Irish, druides in Latin — may mean no more than “oak-knower.” The two parts of the word, dru- and -wid, are often interpreted as referring to the oak tree and to knowledge, and the figure of the Druid is, in this reading, the person who knows the language of the tree. The etymology is not certain, and some scholars prefer to see the first element as referring to strength or to truth. But the older folk etymology was taken very seriously in the Celtic lands, and it shaped the way the oak was thought of for centuries: the Druid was the wise man of the oak, the priest of the tree, the reader of the leaves.

The Romans, who first encountered the Druids in Gaul and Britain, were impressed and disturbed by the oak groves in which they performed their rites. Pliny the Elder, writing in the first century, describes a great oak at the centre of a sacred grove in Gaul, on whose branches the mistletoe grew, and the ceremonies by which the mistletoe was cut with a golden sickle at a particular phase of the moon. The Romans did not, in most cases, cut down the groves; they simply noted them, and the Celtic custom continued into the Roman period and beyond, until the Christianisation of the western Celtic lands.

The Gathering of Mistletoe

The cutting of the mistletoe from the oak was the most sacred of the Druidic acts, and it has given the modern world one of its most familiar Christmas customs. Pliny describes the rite in detail: a priest, dressed in white, climbed the oak, and with a golden sickle cut the mistletoe, which was caught in a white cloth by other priests below. Two white bulls were then sacrificed, and the gathered mistletoe was distributed. The rite was performed when the mistletoe was in fruit, and the berries were understood to be a cure for infertility in animals and a protection against poisons.

The mistletoe, growing on a tree that was itself sacred, was felt to be doubly so. It was, in a sense, a gift of the oak to the gods, a small green sign of the oak’s own life. The custom of hanging mistletoe at Christmas, which spread to the rest of Europe from the Celtic and Anglo-Saxon traditions, is the modern echo of the old rite. The kiss under the mistletoe is a more recent addition — it appears in the late eighteenth and early nineteenth centuries, and may be a survival of an older licence of the midwinter season.

Sacred Groves of the Gauls

The Druid’s oak did not stand alone. It stood at the centre of a sacred grove, the nemeton in Celtic, the lucus in Latin. The grove was bounded, often by a circular ditch and bank, and within it no tree might be cut without the priest’s leave. The great nemeton of the Gauls was at Autun, in Burgundy, and there were major sacred groves at Dreux, at Bordeaux, at Chartres, at the edge of the Forest of Brocéliande. The groves were used for the assembly of the people, for the hearing of legal cases, for the inauguration of kings, and for the burial of the dead. The oak at the centre was the meeting-point of the worlds, the place where the gods were most easily reached.

The Oak in Irish Place-Names

The Irish landscape is thick with the memory of the older sacred tree. Doire is the Irish word for an oak grove, and it lies at the root of a great many place-names: Derry, where the old monastery was founded beneath a great oak; Kildare (Cill Dara, “the church of the oak”), where the sacred flame of Brigid was kept burning under the canopy of an oak; Kilkenny (Cill Chainnigh), named for a saint whose cult was associated with an older oak-figure; Durrow (Dairmhagh, “the plain of the oaks”), the site of an early monastery. The oak of Kildare is especially striking. The monastery was founded by St. Brigid in the late fifth or early sixth century, but the site had been sacred long before, and the sacred flame was, in some accounts, a continuation of the older fire of the oak.

The Royal Oak of England

The oak of the Celts was, in time, the oak of England, and the royal oak is its most famous political image. After the battle of Worcester in 1651, the young Charles II hid in an oak tree at Boscobel House in Shropshire to escape the Parliamentary soldiers. The tree became the “Royal Oak,” and its image was carved into pubs, inns, and churches across the kingdom. A later tree was planted in its place, and the descendants of the second Royal Oak are still growing. The oak had been the tree of the English monarchy, in folk imagination, since the ancient forests of the island: the wood of the great ships, the wood of the halls, the tree under which justice was done. The English king, in this older image, was the man of the oak.

The Oak King and the Holly King

The English folk tradition remembers, beneath the surface of the Christmas customs, a deeper story. The Oak King and the Holly King are the two figures who fight, twice a year, for the hand of the year. The Oak King rules from Midsummer to Yule, when the light is waning; the Holly King rules from Yule to Midsummer, when the light is waxing. The two meet and fight at the solstices, and the winner is the king of the coming half-year. The image is older than the Christian Christmas, and the green holly and the green oak of the midwinter ritual were, in some sense, the kings themselves.

The same story, in different forms, can be seen in the Celtic lands. The holly and the oak were the kings of the Welsh and Irish calendar, and the battle between them was the cycle of the year. The seasonal festivals of the Celtic year were the turning points of the cycle, and each of them was marked by a particular tree. The oak was the tree of the summer half, when the sun was strong; the holly was the tree of the winter half, when the green of the evergreen was the only colour.

Oak Veneration and the Festivals of the Celtic Year

The festivals of the Celtic year — Samhain, Imbolc, Beltane, Lughnasadh, and the two solstices — were each marked by the use of particular trees. The oak, as the great tree of the summer half, was the tree of Beltane, when the cattle were driven between two fires for purification, and the great bonfires were lit on the hill. The oak was the tree of the great meeting, the gathering of the people, the moment when the king of the year was set on his throne.

The same oak, at Samhain, was the place of the older offerings. The word Samhain means “the end of the summer,” and it was the moment when the cattle were brought in from the summer pastures, the last of the harvest was gathered, and the dead were remembered. The oak was the tree of this older cult, and the gathering of the acorns at Samhain was a sign of the cycle that was about to begin again.

Sources and Further Reading

Pliny the Elder. Natural History. Translated by H. Rackham et al. 10 vols. Loeb Classical Library, 1938–63.
Mac Cana, Proinsias. Celtic Mythology. Newnes, 1983.
Maier, Bernhard. Die Religion der Kelten: Götter, Mythen, Weltbild. C. H. Beck, 2001.
Ó hÓgáin, Dáithí. Sacred Trees of Ireland. Lilliput Press, 2002.