Posted: 20/05/2014 at 22:39
I find Symphytum officinale, comphrey does that job very well but maybe that's closer to a weed than you want to go. Iris pseudocorus is good as well. and Filipendula ulmaria, meadow sweet. One of the willowherbs, Epilobium hirsutum has a lovely pink flower and spreads nicely.
The comphrey and the willowherb are seeders as well which can be a disadvantage in the rest of the garden. Especially the willowherb that has windblown seeds
It you want something shrubby; willow, cut back regularly if you don't want anything tall, and the coloured stemmed dogwoods will cope with inundation.
I'd kill/remove the nettles for a start though