Gardening author Ed Hume suggests the aronia. Others have suggested Amaranthus. I sing "the pale amaranthus and the hyssop so blue" because it sounds good, seems to fit, and the pale amaranthus is a wild flower, and so is the hyssop which can be "so blue." However, the most probable origin of the aronatus is that it's a mondegreen or was simply made up to fit the song.