Almost everything that circulates today as Celtic tree wisdom descends from one book by one poet, published in 1948, who did not claim to be doing history.
The White Goddess
Robert Graves published The White Goddess: A Historical Grammar of Poetic Myth in 1948. It argues for a single suppressed European poetic religion of a triple moon-goddess, and it reads ogham as one of the keys to it.
From it comes the thirteen-month tree calendar, thirteen lunar months of twenty-eight days, each ruled by a tree, which is now printed on posters, sold as jewellery, and taught in a great many places as ancient Druidic practice.
There is no such calendar in any Irish source.
Where Graves got his material
Not from nowhere. He was working from Roderick O'Flaherty's Ogygia (1685), a learned but pre-modern history of Ireland containing a chapter on ogham, and from the letter-name and tree material in the medieval tracts.
The calendar's common name gives the borrowing away. It is called the Beth-Luis-Nion calendar, after what Graves took to be the first three letters. But the manuscript order is Beith, Luis, Fern, Sail, Nion, nion is fifth, not third. The name preserves an ordering error inherited from his source.
What the scholars said
R. A. S. Macalister, the leading ogham scholar of the day, told Graves directly that the tree alphabet was late and not to be relied on. Graves proceeded anyway.
Damian McManus's A Guide to Ogam (1991) is the standard modern treatment, and it is unambiguous. Only a minority of the twenty letter names are demonstrably trees at all, somewhere around eight. The rest are abstract nouns, or words whose meaning is uncertain, or words that later scholiasts fitted to trees because a tree alphabet was the scheme they had decided on.
What this does not mean
It does not mean the ogham entries in this atlas are worthless, and it does not mean Graves was a fraud. He described The White Goddess as a work of poetic argument and analeptic reasoning, not of philology. His readers made the historical claim on his behalf.
What it means is that a modern reader has to know which layer they are standing on:
- Fourth to sixth century, the letters, on stone, writing names.
- Early medieval, the letter names and the briatharogaim kennings.
- Medieval scholastic, the systematic tree scheme and the alternative oghams.
- 1685, O'Flaherty's ordering.
- 1948, the calendar, the goddess, and the divinatory system.
Every one of those layers is in the material sold today as a single ancient tradition.