“It’s not rocket science” means “it’s not complicated.” “It’s not rocket salad” is a jocular variation with no special meaning.
 
“It’s not rocket salad, surely?” was posted on the newsgroup alt.fan.cecil-adams on January 1, 2003. “It’s not rocket salad” was popularized on the British television comedy-drama Love Soup in October 2005.
 
“I waited an hour for my starter so I complained: ‘It’s not rocket salad” is a joke from Lou Sanders that was featured in the August 2012 Edinburgh Festival Fringe.
 
“I always thought growing lettuce would be hard. Turns out it’s not rocket science” is a joke on both “rocket salad” and “rocket science.”
     
   
Wikipedia: Eruca sativa
Eruca sativa (syn. E. vesicaria subsp. sativa (Miller) Thell., Brassica eruca L.) is an edible annual plant, commonly known as rocket salad or arugula; other names include rucola, rucoli, rugula, colewort, and roquette.
   
Wikipedia: Love Soup
Love Soup is a British television comedy-drama produced by the BBC and first screened on BBC One in the autumn of 2005.
           
Google Groups: alt.fan.cecil-adams
Protest Coverage
John Dean
1/1/03
(...)
Or - the moving mass of the march occupied at each instant in time x acres of ground and we use a theoretical density of z marchers per acres therefore etc.
 
It’s not rocket salad, surely?
 
1 October 2005, The Spectator (London, UK), “Uphill struggle” by Simon Hoggart:
There are also some very funny lines. I loved Ms Greig’s nice but dim colleague trying to explain that something is easy:
 
‘It’s not rocket salad.’
       
2 October 2005, Sunday Herald (Glasgow, Scotland), “Recipe for success . . . or just a load of bouillon?” by Graeme Virtue, pg. 36:
The downside is that Love Soup can sometimes feel like a pair of different sitcoms cut awkwardly together instead of two storylines complementing one another. Still, there are enough decent one-liners - “Come on, it’s not rocket salad!” - and flashes of pitch-black humour to keep things interesting.
 
2 November 2005, Daily Mail (London, UK), “Love? Don’t be absurd!” by Peter Paterson, pg. 57:
Cleo’s catchphrase - ‘It’s not rocket salad’ - will probably outlive Love Soup, while Milly reversed centuries of prejudice by giving us a really dumb brunette.
 
Google Groups: uk.media.radio.archers
OT: Air travel & conspiracy theories
BrritSk
8/29/06
(...)
Yes of course everything you say is true, but it’s not whether there are people who can do the work, there are - it’s not rocket salad.
     
Google Books
Dim Wit:
The Funniest, Stupidest Things Ever Said
By Rosemarie Jarski
New York, NY: Random House
2008
Pg. 62:
It doesn’t take a lot to figure that out. It’s not rocket salad.
Bill Green
       
Twitter
Jamie Burgoyne
@White667
“It’s not rocket salad.” hehe… Love Soup is all sorts of fun, kinda… Not sure if I should be enjoying it so much. =P
4:36 PM - 13 Dec 2008
 
Twitter
Hannah
@musicallover2
I have now officially heard the Love Soup clip containing “It’s not Rocket Salad” and know where @ChloeJPugh is coming from!! 😊 xx
5:23 PM - 22 Jun 2011
 
Google Books
5,000 Great One Liners
Compiled by Grant Tucker
London, UK: Biteback Publishing Ltd
2012
Pg. ?:
I waited an hour for my salad so I complained: ‘It’s not rocket salad.’
         
BBC News
Stewart Francis’s Posh and Becks joke Fringe ‘funniest’
21 August 2012
(...)
9.Lou Sanders - “I waited an hour for my starter so I complained: ‘It’s not rocket salad.”
 
Twitter
Mark Bisson
@MA_Bisson
Best 1-liners from Edinburgh Fringe. My pick: “I waited an hour for my starter so I complained: ‘It’s not rocket salad” http://www.bbc.co.uk/news/uk-scotland-19316443 …
4:10 AM - 21 Aug 2012
 
Reddit—Malaphor
It’s not rocket salad (self.malaphor)
submitted December 16, 2017 by victorylaps_
 
Twitter
John
@JFD1960
I saw this bloke struggling to cut up a lettuce. “Come on mate it’s not rocket salad?” I said
5:28 PM - 13 Mar 2018