I'm afraid I can't cite anything specific: these are things I picked up at some point that made the most sense to me.

The fennel for Claudius is a symbol of sorrow and of flattery. Claudius ought to be sorrowful about both his brother's and his chamberlain's death, and he is definitely a flatterer. The columbines are for ingratitude. Ophelia would be likely to feel that just about everyone was ungrateful to her father, but Claudius perhaps most of all.

The rue Ophelia gives Gertrude is a symbol sorrow for her husband's death and her own rue is for her father's death.

The daisies and violets for Laertes are about faithlessness and faithfulness, and the reason that the violets withered when their father died is that by killing her father and then leaving her, Hamlet proved faithless, just as Laertes said he would. Violets are symbols of just about everything, though: chastity, passion, romance, and death all fit.

I've always like the idea that Ophelia doesn't have any flowers with her--perhaps nameless weeds or nothing at all.