Utilizing the evocative power of alliteration--the repetition of the initial /g/--and assonance--the short /o/, Philip Larkin emphasizes the resurgent powers of trees; yet ironically, in this resurgence there is also a measure of death as another ring is added to the tree's trunk that indicates age. For instance, Larkin writes,

Their greenness is a kind of grief
Is it that they are born again
And we grow old? No, they die too,
Their yearly trick of looking new
Is written down in rings of grain.

While there is rebirth each year in the trees, it is rebirth that is paradoxically recorded as age as the "yearly trick" is "written down." The trees, therefore, become symbolic of the regeneration of life in the respect that people, too, should "begin afresh, afresh, afresh." This final line is key to the theme of Larkin's poem, renewal of one's spirit. Clearly, then, there is a connection between the "rings of grain" and the years of a person's life.  These rings can provide a people with experience that can support new ideas, thus making a person wiser, "a castle" of thought, so to speak.