Cotton is an perennial plant that we force to be an annual. If left alone and not harvested/stalks cut, cotton will grow into a tree. In the northern hemisphere (where 90% of the world’s cotton is grown), we don’t allow cotton to continue growing into a tree growing because:

1. The plant’s energy will go into growing branches/leaves instead of producing fruit. We want the maximum number of bolls per plant in order to maximize the yield.
2. Cotton is a heat-loving plant and is not likely to survive frost

The cotton is harvested in the fall, when it is about thigh-high. The stalks are then chopped down and rotated into the soil, and the cotton is planted again in the spring.