The consonants are representative of the different lunar months whilst the vowels represent the solar cycles. Five has always been seen as a sacred number traditionally associated with the goddesses and the changing cycles. It is seen as the number which represents the variety and colour of our three dimensional space representing our senses and the five elements: earth, air, fire,water and the soul, these in turn correspond with the seasons.
The five tree vowels are known as the "Sweet Cauldron of the Five Trees" which are represented in Robert Graves' poem.