Labor Day is the optimum time for fertilizing lawns in Wisconsin. Regardless of the maintenance level program followed on a lawn, it will benefit from an early fall application of nitrogen.
Nitrogen is the primary nutrient lawn grasses need on an annual basis. Nitrogen not only contributes to green color but promotes grass plant development that helps maintain a thick stand of grass. Not only do thicker lawns look better and hold up to foot traffic, but they also resist encroachment of weeds. Thus, sound lawn care practices, including timely nitrogen fertilizing and proper mowing, can reduce the need to use herbicides.
When fertilizing lawns, there are three key decisions to make: when to apply, what kind of fertilizer to purchase, and how much is needed. Early September should always be included in the annual nitrogen fertilizer schedule for lawns. For minimal maintenance lawns this might be the only application made all year. Nitrogen applied in this period is used for key fall growth and development, including root systems.
Focus on the nitrogen content, specifically the type of nitrogen, when choosing fertilizers. Nitrogen fertilizer categories include fast (quick) and controlled (slow) release. This refers to the rate of conversion from nitrogen stored in fertilizer products to nitrogen available for plant uptake through roots. Providing a small amount of available nitrogen over an extended time is much better for lawns than having a large amount available in a brief time frame.
Source : wisc.edu