A popular Valentine’s Day riddle is:
 
Q: What kind of flowers do you never give on Valentine’s Day?
A: Cauliflowers.
 
The “flowers/cauliflowers” joke has been cited in print since at least 1880. The Valentine’s Day riddle has been cited since at least 2006.
 
   
Wikipedia: Cauliflower
Cauliflower is one of several vegetables in the species Brassica oleracea in the genus Brassica, which is in the family Brassicaceae. It is an annual plant that reproduces by seed. Typically, only the head (the white curd) is eaten. The cauliflower head is composed of a white inflorescence meristem. Cauliflower heads resemble those in broccoli, which differs in having flower buds. Brassica oleracea also includes broccoli, brussels sprouts, cabbage, collard greens, and kale, though they are of different cultivar groups.
 
Chronicling America
20 February 1880, Cincinnati (OH) Daily Star, pg. 4, col. 3:
“What kind of flowers shall we have on the table for dinner to-day, dear?” asked Mrs. Snooglefix. “Cauliflowers,” said the brute, and bolted for his stove-pipe hat and hickory stick.
     
31 May 1889, Daily Nebraska State Journal (Lincoln, NE), “Things That Do Happen,” pg. 4, col. 4:
A Memorial Day Episode.
“Did you decorate a grave yesterday?”
“Yes sir; I decorated the grave of my father in law. You know I married his daughter three years ago.”
“What kind of flowers did you use?”
“Cauliflowers.”
 
10 February 2006, Lincoln (NE) Journal Star, “For kids only,” pg. 3:
Q: What kind of flowers do you never give on Valentine’s Day?
A: Cauliflowers.
 
Google Groups: My Delhi
cool jokes
roops
3/8/06
(...)
Q. What kind of flowers do you never give on Valentine’s Day?
A. Cauliflowers!
 
Twitter
sahitya.illa
‏@sahityailla
@funnyoneliners What kind of flowers do you never give on Valentine’s Day?
Cauliflowers!
7:32 PM - 10 Feb 2010
 
Twitter
Utsab Bandopadhyay
‏@utsabnov
What kind of flowers do you never give on Valentine’s Day?
Cauliflowers!
10:55 PM - 12 Feb 2010
 
Google Books
A Year in the Life of a Total and Complete Genius
By Stacey Matson
Scholastic Canada
2014
Pg. ?:
What kind of flowers are the worst gifts on Valentine’s Day? Cauliflowers!
 
Google Books
Horrid Henry’s Jumbo Joke Book
By Francesca Simon
Illustrated by Tony Ross
New York, NY: Hachette Children’s
2015
Pg. ?:
What kind of flowers did Henry give Margaret on Valentine’s Day?
Cauliflowers.
   
Google Books
Wimpy Kid Jokes About Love & Valentines!:
The Ultimate Book of Valentines Jokes, Riddles, Fun Trivia, Facts, Puns and More!
By Marcus Stuart
LOL Kids Books
2016
Pg. ?:
What kind of flowers do you never give on Valentine’s Day? Cauliflowers!