In the June 1945 issue of Screenland Magazine costume designer Bonnie Cashin, in her column "Notes from a Designer's Diary" comments "If the average American girl could be the heroine of her own life story, and dress accordingly! This thought struck me more forcibly than it ever had before while I was fitting Dorothy McGuire for the part of Katie in "A Tree Grows in Brooklyn." Most of the girls want to look a little glamorous on screen (and off) whether the story calls for rags or riches. Not Dorothy. A stickler for characterization, she stood for hours in her old rags and ravels, suggesting a patch here, a droop there, deliberately deglamorizing herself in order to make sure that not a single bright thread should give the lie to Katie's threadbare life. Dorothy was playing a heroine of poverty and she dressed accordingly. So should we all, in the parts we play, in make believe, or in life. Joan Blondell didn't complain, either, when as Aunt Sissy, she had to wear the sort of ugly-period-of-1914 clothes, the high-topped shoes, the blousy blouses, the too-tight corset. "Oh, Bonnie," little Peggy Ann Garner said to me when we were making Francie's clothes, "oh, Bonnie, every picture they put me in I have to wear poor girls' clothes. Can't I have one good dress?" So we gave her the white graduation dress and the red roses and Peggy Ann accepted poverty and trouped through the picture, patiently ironing her one faded cotton (and she did iron it) and well content.
According to the Sunset Garden Book, the tree that grew in Brooklyn was an Ailanthus tree, or Tree of Heaven. It has naturalized itself over much of the U.S., to the point of being considered a weed tree, but it is still invaluable as an attractive windbreak and shade tree, adaptable under the harshest conditions.
Nicholas Ray came out from New York with Elia Kazan for Kazan's directorial debut. Besides his brief appearance in the cast as a bakery clerk, various sources have Ray working as assistant director and assisting Alfred Newman with the score, but studio records officially list him as dialogue director.
Besides Gene Tierney, Mary Anderson and Jeanne Crain were rumored for the Katie Nolan role, while it was reported that Phil Regan was the leading candidate for the role of Johnny. Alice Faye was the first actress considered for the role of Katie, not Aunt Sissie, according to the Hollywood Reporter in 1943.