The plum tree is white today (9)
You know that white light at the end of the tunnel? Maybe it's a plum tree, shining in the sun. At least for this speaker, it sure seems to be. Plus, there are all those other deathly associations we have with white. First, there are the images of the dead that we have: bones and ghosts, the moon over the white tombstones in a graveyard. Then we might also make a connection with brides (wearing white), which, in the case of this poem, would make us think of her husband and then, yep, his death.
and fall into those flowersand sink into the marsh near them (27-28)
This is the moment in the poem when the depth of our speaker's sorrow is made most clear. Because of a death, her world has been changed (permanently, it seems) for the worse. And now she desires relief through – you guessed it – her own death. As we've mentioned before, the way she imagines this – falling into the flowers and sinking into the marsh – also has a way of making her imagined death seem like a reconnection with the natural world.