Cool-season turfgrass species should be fertilized mainly in the autumn. September and November are the two best times to fertilize a lawn in Indiana. Fall nitrogen promotes good root development, enhances storage of energy reserves, and extends color retention in cool-season lawns. Most of the benefits from late fall nitrogen will be seen next spring and summer with earlier green-up, improved turf density, and improved tolerance to spring diseases such as red thread and pink patch, and reduced weeds.
There are many fertilizer choices available to the homeowner. Organic, inorganic, and synthetic organic products are all available. As with all plants, turfgrasses cannot tell the difference between the sources of nutrients. Some products contain high amounts of slow-release N while others contain none. Although there are exceptions to the rule, it is good practice to use products with a greater percentage of slow-release nitrogen sources during spring and summer months and a greater percentage of quick-release nitrogen sources in the fall.
For the September application, pick a product that contains some quick and slow-release nitrogen. The timing of the September application is anytime of the month after the daytime high temperatures are no longer in the 90s °F. The target application rate for this fertilization should be 1.0 lbs. N/1000 square feet.
The late-fall or November application timing should be near or after the last mowing of the year, but while lawn is still green. Typically, there may be a month or more between your last mowing and the time the grass turns brown or goes under snow cover. Generally the first few weeks of November are when to apply. Earlier Purdue research suggests that the nitrogen must be taken-up by the plant before winter to be most effective. Therefore, a quick-release (or soluble nitrogen source) such as urea, ammonium nitrate, calcium nitrate, or ammonium sulfate is most effective. The target application rate for this November fertilization should be 0.5 to 1.0 lbs. N/1000 square feet.
More fertilizer program information is available in AY-22: Fertilizing Home lawns at www.agry.purdue.edu/turf/pubs/ay-22.pdf
If you are confused on how much of a particular product to apply to achieve a particular N-rate, use our fertilizer calculator to help determine exactly how much product to use: www.agry.purdue.edu/turf/fertcalc/Fertilization%20calc.html.
Aaron Patton, Turfgrass Extension Specialist