Green’s Dictionary of Slang

(US) the suburbs or outskirts of a large town.

1877		Grange Advance (Red Wing, MN) 4 Apr. 7/4: I was taking a stroll in that part of town Mrs Partington called the ‘Rhubarbs’.
1881		Wkly Miner (Butte, MT) 17 May 1/3: A dear old lady [...] said she did not like to live in the crowded streets of a city, but preferred the ‘rhubarbs’.
1896		Roanoke Dly Times (VA) 27 Mar. 6/3: I led him to the rhubarbs of the village.
1907		L.A. Herald 17 Feb. 14/2: The [stage] company put together to play the ‘rhubarbs’ and the provinces.
1910		Hawaiian Star (HI) 8 Apr. 1/3: The ‘Farmers Weekly Blade’ [is] circulating extensively through the rhubarbs of Kansas.