“Romaine calm. Lettuce carrot on” (or simply “Romaine Calm and Carrot On”) is a food pun version of the British World War II “Keep Calm and Carry On” poster. A “Romaine calm” pun was posted to the newsgroup alt.humor.puns on January 29, 1999. “Lettuce all romaine calm” was posted on Twitter on February 26, 2011.
 
“Romaine calm and lettuce carrot on” was posted on Twitter on August 14, 2014. The food pun saying has been printed on many images.
 
   
Wikipedia: Keep Calm and Carry On
Keep Calm and Carry On was a motivational poster produced by the British government in 1939 in preparation for the Second World War. The poster was intended to raise the morale of the British public, threatened with widely predicted mass air attacks on major cities. Although 2.45 million copies were printed, and although the Blitz did in fact take place, the poster was hardly ever publicly displayed and was little known until a copy was rediscovered in 2000 at Barter Books, a bookshop in Alnwick. It has since been re-issued by a number of private companies, and has been used as the decorative theme for a range of products.
 
Google Groups: alt.humor.puns
Veg
Jenni Saqua
1/29/99
(...)
> Lettuce beet him to the punch
 
Romaine calm. (Then, beet him to a pulp: we all need more fiber)
     
Twitter
L/\wry Dene/\u‏
@LawryDeneau
Lettuce all romaine calm. @ilmercatosunny #halifax #restaurant
12:40 PM - 26 Feb 2011
 
Twitter
Pepperonipizzach‏
@Musclesmadden
lettuce romaine calm
1:25 PM - 29 Nov 2011
 
Twitter 
Olivia Wojtowicz‏
@oliviawojtowicz
I carrot bay leaf volleyball marathon is Friday. Everyone peas romaine calm. Lettuce Turnip the Beet will kale you on the courts.
3:32 PM - 4 Mar 2014
 
Twitter
Jess ✌️‏
@itsjess_3
Romaine calm and lettuce carrot on 😂😂
7:50 PM - 14 Aug 2014
 
Twitter
Carrot Dirt Organics‏
@CarrotDirt
Overindulge a little yesterday? It is ok! We can help you romaine calm lettuce carrot on 😊 come grab… http://instagram.com/p/xEp80BEFKM/
8:08 AM - 26 Dec 2014
 
Twitter
Jennifer Thompson‏
@rawfoodbliss
Romaine Calm. Lettuce Carrot On. 😊
3:02 AM - 29 Dec 2014
 
Google Books
The Frame:
Hollywood Heroes, Book 1
By Monique Martin
Published by author
2015
Pg. ?:
Her mother flitted about the kitchen like a deranged Martha Stewart wearing an apron that said “Romaine Calm and Carrot On.”
 
20 June 2015, Orlando (FL) Sentinel, “Jumble” puzzle, pg. D9, col. 3:
Answer: When they ran out of lettuce for salads, he told the kitchen staff to—“ROMAINE” CALM
 
Twitter
Sandra Aphrodita‏
@1s22Y95XGS3R3hY
Romaine Calm Lettuce Carrot On Vegetarian Humor Ladies T-Shirt Large Sport Grey http://goo.gl/I2YBR2
3:41 PM - 7 Jul 2015
   
SYCMU
Top 10 Funniest Aprons You Can Buy
July 28, 2016
(...)
9. Romaine Calm, Lettuce Carrot On
Apron for the clever one in the house
 
Bustle
22 Clever Gifts For Grandparents With A Great Sense Of Humor
By MARIA CASSANO Oct 28 2016
(...)
21. This Punny Apron For The Cooking Grandparent
Romaine Calm Lettuce Carrot On Funny Apron, $19, Amazon
 
Whether they’re a vegetable enthusiast or just a huge fan of puns, this Romaine Calm Lettuce Carrot On funny apron puts a hilarious spin on a slightly-outdated phrase, which is what old people do best.