Thinking about Rhubarb it is the petioles we eat and so many people call those the 'stems' or 'stalks'. We would not want to encourage people to eat the wrong part. Yikes.
http://www.botanicalaccuracy.c...
What about something like:
Roots (and/or rhizomes or tuberous rhizomes)
Stem (and/or petioles)
and then use the comment section to make it more clear.
