In Book 9 of the Analects, the flowers of the asian plum (Pound
translates 'prunus japonica') summon thoughts of a sweetheart far away.
So the unripe plums in "The River Merchant's Wife: A Letter" can be both
an archetypal symbol of innocent, inexperienced love as well as a
traditional emblem of separation and desire. As symbol of both innocence
and experience, the plum is an apt correlative for girl-becoming-woman.
Tim Romano

Hiroko Uno wrote:

> [...]
>
> According to Kumiko Kakehi, a Japanese scholar of Chinese ancient poetry,
> the line with "blue plum" refers to another ancient Chinese poem
> "Hyo-Yu-Bai" in Shikyo (I am sorry these are in Japanese
> pronunciation), in
> which very young people express their first pure love with unripe blue
> plums.