The seasonal window to fertilize your garden is starting to close. North Dakota State University recommends fall fertilizer applications around Labor Day. Fall application is most important because it helps your cool-season lawn recover from summer stresses. Nitrogen is useful in building a deep, strong root system that will bear fruit next year.
Exact timing over Labor Day weekend is not required but fertilization is not recommended after the end of September. Why so early?
University research conducted in Minnesota and Wisconsin shows that grass must be actively growing in order for it to absorb nitrogen fertilizers. In the experiment, dissolved nitrogen was applied on September 15, October, or November. For both states, plants absorbed the vast majority of the nitrogen applied in September. October and November applications were wasteful and could have seeped into the soil or into waterways as the snow melted in the spring.
Calendar recommendations are not completely accurate because weather conditions fluctuate from year to year. However, the grass is unlikely to actively grow once temperatures drop below 50 degrees Fahrenheit. To be on the safe side, the NDSU Extension recommends using a 50% slow-release nitrogen fertilizer before September 15.
Applying lawn fertilizer in October can be risky in some years because it can encourage the grass to actively grow instead of acclimate to the winter. In years when cold weather comes early, this can kill the winter.
During my first spring on the job, I visited a completely dead garden. After asking several questions, I learned that a lawn service company had used a rapid-release nitrogen fertilizer in October. The homeowner was interested in how lush the grass would be in the fall. This was the evidence we needed to diagnose the problem. Late fertilizer application caused the grass to grow instead of shutting down for the winter. I’m not saying that applying fertilizer in October will always kill your lawn but it does increase the risk of winterkill when winter suddenly comes.
Finally, is it worth abandoning the “winter” fertilizer product? The winter product is a lawn fertilizer that contains high levels of potassium as well as nitrogen. The jury is still out on whether potassium applied in the fall is beneficial in the North Country. Research in southern states has shown that potassium can help warm-season grasses such as bermudagrass survive low winter temperatures. The winterizers you see on the market were developed for southern states.
More research is needed to study the effect of potassium on northern grasses such as Kentucky bluegrass.
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