Something that is “like watching (the) grass grow” is something that is very boring. “Slow motion pictures are to be used in the teaching of golf and if they even film Chet Smith teeing off the world will have an interesting time watching grass grow at comparable speed” was cited in June 1926.
 
American newspaperman and author Damon Runyon (1880-1946) wrote in August 1926 that “Watching an athlete at his training always struck me as about as thrilling as watching the grass grow.” Runyon made the expression famous when he used it at the America’s Cup from Newport in September 1934:
 
“There is nothing more unexciting than watching a yacht race unless, perhaps, it is watching the grass grow.”
 
“Watch paint dry” is a similar expression.
     
 
The Free Dictionary
be like watching grass grow  (humorous) also be as interesting as watching grass grow (humorous)
if you say that watching an activity is like watching grass grow, you mean that it is very boring
   
25 May 1924, The Sunday Oregonian (Portland, OR) “Ain’t They Gonna Stop Talking About Prohibition?” by Sam Hellman, sec. 5, pg. 4, col. 1:
“They’ll be chewing the fat about he second coming of booze long after your grandchildren is watching grass grow from the roots up.”
 
18 June 1926, Rockford (IL) Register-Gazette, “The Toreador,” pg. 22, col. 2:
SLOW MOTION PICTURES ARE TO BE USED IN THE TEACHING OF GOLF AND IF THEY EVER FILM CHET SMITH TEEING OFF THE WORLD WILL HAVE AN INTERESTING TIME WATCHING GRASS GROW AT COMPARABLE SPEED.
 
31 August 1926, Clearfield (PA) Progress, “Runyon Says” by Damon Runyon, pg. 5, col. 2:
Watching an athlete at his training always struck me as about as thrilling as watching the grass grow, and in only one solitary instance was I ever able to glean any valuable information from the process and then it wasn’t my fault.
 
10 July 1927, Winston-Salem (NC) Journal, “Dave Shade Says Jack Dempsey Is In Fine Shape” by Damon Runyon, pg. 6-C, col. 3:
I fear, however, that watching the terrible Sharkey man at his training is about as thrilling as watching the grass grow, for the terrible Sharkey man presents no elements of mystery to the experts, nor is he coming back.
 
28 August 1929, Tampa (FL) Morning Tribune, “Kid Chocolate Picked to Win Unless Stopped” by Damon Runyon, pg. 9, col. 2:
Watching blokes like Kid Chocolate and Al Singer in training is about as thrilling as watching the grass grow.
 
16 September 1934, The Times-Picayune (New Orleans, LA), “Old England ‘Saved by Bell’ as She Receives Another Star-Spangled Licking” by Damon Runyon, sec. 4, pg. 8, col. 3:
There is nothing more unexciting than watching a yacht race unless, perhaps, it is watching the grass grow.
 
22 November 1934, The Daily Northwestern (Evanston, IL), pg. 6, col. 2:
There is nothing more unexciting than a yacht race unless it is watching grass grow under ones feet.—Damon Runyon.
 
19 March 1938, Canton (OH) Repository, “Batter Up!” by Damon Runyon, pg. 4, col. 7:
You can remember when you thought watching a baseball club in training was about as thrilling as watching grass grow, but you also remember that in general a spell in a training camp was a lot of fun.
   
22 September 1958, Washington (DC) Post, pg. A14:
NEWPORT, R. I., Sept. 21 - Damon Runyon’s Guys and Dolls was merely one of America’s legacies from that fine impressionist. He once wrote into a ten-word sentence a description of yacht racing that has gone ringing down through the years as the model of scorn for that esoteric sport with its spinnakers, liffs, jibs, jibes and complete reliance on windy drafts.
 
Runyon was reporting his first America’s Cup race. It was a switch from his world of the violence of the fight camps, the crash of the World Series home runs, the race track whirl of fast horses and whipping riders, and the body-assaults of football. To the slow moving yacht races he reacted with impatience. “Watching an America’s Cup race,” he wrote, “is like watching grass grow.”
 
19 February 1962, Boston (MA) Traveler, “Tradition Says Boats Should Have Lines Like Debbie Drake” by Tim Horgan, pg. 23, col. 1:
A sportswriter consigned to the America’s Cup races off Newport a few years ago produced the classic definition of yacht racing as a spectator sport. After an afternoon spent goggling through binoculars at two distant wisps, he wrote:
 
“Watching a yacht race is like watching grass grow.”
 
Old Fulton Post Cards
17 September 1964, The Times Record (Troy, NY), “The Yankee Racehorse” (editorial), pg. 24, col. 1:
Ring Lardner once wrote the America’s Cup races are about as exciting as watching grass grow.
 
OCLC WorldCat record
With folks keeping machines longer, waiting for a rebound will be “like watching grass grow”
Edition/Format: Article Article : English
Publication: BUSINESS WEEK -NEW YORK- no. 3765, (January 14, 2002): 98
Database: British Library Serials