Well anon, it all depends - you didn't expect a definitive answer at a folk site did you?

probably the best known "language of flowers" song is "Let no man steal your thyme". My understanding of the symbolism here is:

Thyme - time in the sense of opportunities available in the present.

Lily - purity, chastity Violet - modesty Pink - Romantic love Red rose - passionate love ie sex Rue - rue, repentence Willow - sorrow, dispair

On the other hand Green willow in other songs especially those concerning blacksmiths, is more likely to be symbolic of fertility, cf the willow's ability to grow from a bare stick.

Of course the Victorian's went completely overboard on this and had meanings for just about anything that grew. Not too much relevance to folk song, but if you are interested there is a site here that gives meanings from various Victorian treatise on etiquete.

Pete M