Having a haircut has no effect on the rate of hair growth. Whether you get a hair trim or not, hair grows at the rate of one inch per month. The only proven factors that affect the rate of hair growth are genetics, hormones, and diet. However, it is important to note that the chopping of the split ends (which impede healthy hair growth) may promote better hair growth. Still the best way to promote healthy hair growth is to reduce the likelihood of split ends by avoiding over-styling.