In The Possibility Of Evil, the roses symbolize the deceptiveness of outward appearances of beauty. A rose is a beautiful flower, but its thorns can cause both discomfort and injury.
In the story, Miss Strangeworth is an elegant older lady with pretty dimples. She is easily recognizable 'with her dainty walk and her rustling skirts.' Her red, pink, and white roses are the talk of the town and the envy of tourists. She is a woman of deep tradition; Miss Strangeworth never gives her roses away, believing that the roses belong on Pleasant Street. The townspeople accord her deep respect and courtesy.
Yet, Miss Strangeworth masks a petty and spiteful spirit (thorns) behind her sophisticated and graceful manner (beauty of the rose). She secretly writes unkind letters to those who she believes are in need of her vindictive observations and her self-righteous advice.
The Roman tradition of the Rosalia includes the practice of offering roses to the deceased (who are viewed as the protectors of homes). Similarly, the imagery of Miss Strangeworth's preoccupation with her roses and her town is apt; she imagines herself the spiritual conscience and the preserver of good moral order in her town. The fact that her maliciously written messages on nondescript colored paper (she does not utilize the elegant Strangeworth stationary for these missives) may prove hurtful to her audience thoroughly escapes Miss Strangeworth's calculations.