One may think that spring and late summer fertilizer applications are the most important but for cool season turf grass it is the late fall application. A late fall application is typically made after the 3rd hard frost of the season. At this point the turf is no longer growing and preparing for winter. The higher the level of nitrogen the better the fertilizer will be for root production and assisting in the storing of energy. The plant is able to take up the nitrogen and help store reserve energy (carbohydrates) in the plant for overwintering and earlier green up in the spring. If a plant is lacking carbohydrates in the fall it will not recover from the summer stress nearly as good as a plant with a larger reserve of carbohydrates. Another key factor is to help the turf by increasing the root depth and growth in the spring, in return establishing much stronger root system.
Timing is a major factor in determining when you make the application. If you apply too early the plant shoots (leaves) will grow excessively which can increase disease pressure during the winter. Late winter application have no negative effects to the turfgrass whereas a spring application can inhibit root growth. With spring applications the top half of the plant uses all of the carbohydrates to increase shoot growth, all while stealing the carbohydrates from the roots which are also trying to grow and increase at the same time. Better carbohydrate storage and increased roots produce an overall healthier and a more stress tolerant plant.
Check boards are used often in the turf industry to test new and old products. We use them with almost every application that we make on the course. A check is basically a small piece of plywood that covers the turf and doesn't allow the product to be taken up by the plant. As you can see, the turf that has been fertilized in the late fall has remarkable color compared to the check, which is still yellow and not growing nearly as much.