People are happier if they stay with their own people in their own communities.
The conflict through this whole play is Racism. The house Mama bought is in a white neighborhood. Mama didn't see a problem but everyone else did.
Mr. Lindner was subtle in being racist but he still let them know they weren't wanted.
Mama's plant represents the family's love. Through everything they stayed together and loved each other. Just like the plant lived in the sad place it was in.