Reeds (nGeatal) were very useful in Celt society. They were woven to make watertight roofing (lasting as long as 10 years), laid down as mats, used as candles, and so forth. The reed was associated with protection and harmony.
It may seem odd to think of reeds as trees but Celts treated any plant with a woody stalk as a tree. Reeds were also larger in those days than they are now.
The Ogham symbol for reed has no direct single letter counterpart in the English alphabet. The closest English equivalent is the NG combination.