“What’s the kindest vegetable?”/“A sweet potato.”

A popular food riddle is:
 
“What’s the kindest vegetable?”
“A sweet potato.”
 
The sweet potato riddle has been cited in print since at least 2006.
 
   
Wikipedia: Sweet potato
The sweet potato (Ipomoea batatas) is a dicotyledonous plant that belongs to the family Convolvulaceae. Its large, starchy, sweet-tasting, tuberous roots are a root vegetable. The young leaves and shoots are sometimes eaten as greens. Ipomoea batatas is native to the tropical regions in America. Of the approximately 50 genera and more than 1,000 species of Convolvulaceae, I. batatas is the only crop plant of major importance—some others are used locally, but many are poisonous. The sweet potato is only distantly related to the potato (Solanum tuberosum) and does not belong to the nightshade family.
   
Google Books
The Rainy Day Book:
Pick-Me-Ups for When You’re a Little Blue!
Nashville, TN: Thomas Nelson Inc.
2006
Pg. ?:
What’s the kindest vegetable? A sweet potato.
 
Google Groups: The Grif.net
11/12/07 Grif.Net - Retirees
Dr. Bob
11/12/07
(...)
6. What is the kindest vegetable? Sweet potatoes
   
Mickey’s Place
WEDNESDAY, FEBRUARY 27, 2008
Riddle
What’s the kindest vegetable?
sweet potato
 
Google Books
Blonde Walks into a Bar:
The 4,000 Most Hilarious, Gut-Busting Gags, One-liners and Jokes
By Jonathan Swan
Berkeley, CA: Ulysses Press
2008
Pg. ?:
What’s the kindest vegetable? A sweet potato.
 
Twitter
Wendy House
‏@Wendywitwoo
RT @FizzyDuck: What’s the kindest vegetable? A sweet potato! ~ For @Wendywitwoo :D
3:17 AM - 22 Oct 2009
   
Google Books
The Mammoth Book of Really Silly Jokes
Edited by Geoff Tibballs
Constable & Robinson Ltd.
2011
Pg. 109:
What’s the kindest vegetable? A sweet potato.
   
Twitter
Leigh-Ann Kilner
‏@leighannieexo
‘Whats the kindest vegetable?’ ‘A sweet potato’ you’re welcome.
2:42 PM - 25 Apr 2014