The seasonal window to fertilize your lawn is beginning to close. North Dakota State University Extension recommends the fall fertilizer application should take place around Labor Day. The fall application is the most important because it helps your cool season lawn recover from summer stresses. The nitrogen is useful in building a deep and robust root system which will pay dividends next year.
Exact timing over the Labor Day weekend is not required but fertilizing beyond the end of September is not recommended. Why so early?
University research conducted in Minnesota and Wisconsin shows that the lawn must be actively growing to take up the nitrogen fertilizer. In the experiment, soluble nitrogen was applied on the 15th of either September, October or November. For both states, the vast majority of nitrogen that was applied in September was taken up by the plant. October and November applications were wasteful and had the potential to either leach into the soil profile or into waterways with melting snow in spring.
Calendar recommendations are not entirely accurate because weather conditions fluctuate from year to year. However, the lawn is less likely to be actively growing once temperatures dip below 50 degrees Fahrenheit. To be on the safe side, NDSU Extension recommends applying a nitrogen fertilizer that is 50% slow release before September 15th.
Applying lawn fertilizer in October can be risky in some years because it can stimulate the lawn to actively grow rather than acclimate for winter. In years when cold weather comes early, this can result in winterkill.
Source: ndsu.edu
Photo Credit: North Dakota State University
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