Normal growth patterns of cool season grass produce most leaf growth in the spring. Therefore fertilization in spring tends to stimulate even more leaf growth which may in turn decrease long-term stress tolerance in the summer months. On the other hand, cool season grasses tend to slow down leaf growth in the fall producing relatively more rhizomes or stolons and more tillers in the fall, resulting in improved density. Fertilizing in September encourages leaf growth only slightly while tremendously enhancing rhizome or stolon, tiller and root production. Therefore, fall fertilization results in denser and healthier turf.