Sister Aloysius points out that the gardener "pruned this bush, which was the right thing to do, but he neglected to protect it from the frost." How does this statement apply to Sister Aloysius?

Sister Aloysius' actions in tending the rosebush can be interpreted as a representation of her actions in attacking Father Flynn. In the same way she protects the delicate rosebush from the physically destructive frost, she acts out of a desire to protect Donald Muller, whom she has already indicated is "delicate," from what she believes is the physically, spiritually and destructive attack of Father Flynn.