The hawthorn, or May tree, is a particularly sacred and holy tree wherever it is found. In the Celtic Ogham tree alphabet, it represents the letter H, or Huath.
As well as having magical properties, the hawthorn is an incredibly useful tree. When it is cut back to make hedgerows its thorns become sharp, providing an excellent barrier for sheep and other livestock. The berries of the hawthorn provide food for birds and can be made into syrups, preserves, and wines. The leaves, too, are nutritious, giving it the nickname of the 'bread and cheese" tree.
The hawthorn is particularly tolerant of other plants growing close to it and so its presence encourages biodiversity. The hawthorn is a symbol of protection not only because of its thorns. There's an ancient belief that the tree will protect from fire, too. Every year a hawthorn "globe" would be woven and brought into the house as insurance against fire damage of any kind. The following year the globe would be replaced, the old one being burned and scattered on the fields to ensure a healthy harvest.
The white flowers of the hawthorn, which blossom in the late spring (hence the name May blossom), have five petals, with the matching sepals looking exactly like a star. Consequently, as with all members of the rose family to which it belongs, the flower is a natural example of the pentagram.
The hawthorn is symbolic of fertility and sensuality, underlined by the heady, strong, almost narcotic scent of its flowers that bloom during the time of Beltane, when the sap rises in both plant and animal life, a natural time for fertility rites and sexual congress.
Although the hawthorn is symbolic of frivolity and mirth, there's a legend which provides a contrasting imagery for the tree. Joseph of Arimethea, resting on Wearyall Hill in Glastonbury, leaned on his staff and in doing so pushed it into the ground. The staff sprouted into a thorn tree, and a Christian chapel was built at the site. Traditionally, this magical and holy tree, the "Glastonbury Thorn," blossoms at Christmas time and a sprig is sent to the Queen every year. The current tree is a very distant relative of its legendary parent.
From: Element Encyclopedia of Signs and Symbols