Fall is the most important time of year to fertilize your lawn. The greatest amount of root growth occurs in the fall. A fall specific fertilizer product helps the grass plant build a strong, healthy root system. The grass uses the nutrients to prepare and protect itself during the harsh winter months. The grass plant will also use fertilizer at this time of year to make food reserves that would be stored until spring. At that time, the food reserves would supply the plant with the energy to promote active leaf growth. Lastly, fall fertilization also encourages an early spring green-up of your lawn.