I don't know what to say. Some sites list F. ulmaria Flore Pleno as growing to 3-4 feet and other sites show F. vulgaris Flore Plena as a shorter plant, so there's at least a suggestion that they're two separate plants. I'll add an entry for the latter, but we might find out that they're the same plant someday.
Filipendula vulgaris 'Flore Plena'
