The Library of Congress labels this photo “Agriculture Department Dahlia Show,” 1911, but I’m sure it’s from the USDA’s annual Chrysanthemum show, which was held in one of the Department’s greenhouses in Washington, D.C.
The first of the annual exhibitions opened in October of 1902. I haven’t been able to find out anything more about them, but they were still being held in 1937.
All the photos here are by Harris & Ewing, via Library of Congress Prints and Photographs Division.
My advice to the women of America is to raise more hell and fewer dahlias.
— William Allen White (1868 – 1944)
I’m sure the same applied to mums.
enclos*ure 
Definitely mums! Did you look at the flower names: “War Mother” and “Gen. Pershing”. Makes sense for 1917 but must be new varities to get those au courant names.