While harvesting is one of the final steps in the production of cotton crops, it is one of the most important. The crop must be harvested before weather can damage or completely ruin its quality and reduce yield. In about 140 days after planting or 45 days after bolls appear, cotton bolls will begin to naturally split open along the bolls segments and dry out. When enough bolls have opened naturally, harvest aids are applied to the plant to help speed up the maturation process. This, also, is done either by ground or air application. Defoliation helps the leaves to dry and fall off and to help any of the remaining unopened cotton bolls to open. This practice enables growers to hasten the opening of the cotton bolls which can then be gathered quickly, in a short period of time. Most cotton in the United States, Europe and Australia is harvested mechanically, either by a cotton picker, a machine that removes the cotton from the boll without damaging the cotton plant, or by a cotton stripper, which strips the entire boll off the plant. Cotton strippers are used in regions where it is too windy to grow picker varieties of cotton, and usually after application of a chemical defoliant or the natural defoliation that occurs after a freeze. Cotton is a perennial crop in the tropics, and without defoliation or freezing, the plant will continue to grow. In developing countries cotton continues to be picked by hand.