3/27/2018 - By Dr. Jeff Atkinson

If I asked you as a golf course superintendent, what time of the year is the most important time for fertilizer applications, what would your answer be?

My guess would be that most superintendents would say spring or summer depending on the type of turfgrass you are growing. 

The correct answer is Fall.  It may seem strange, but fall is by far the most important time to apply fertilizer.  Obviously, more than one fertilizer application per year is recommended, but if I were limited to only one per year, it would be in the fall. 

The fall timing is critical as that is when plants are manufacturing carbohydrate reserves to support their survival.  With the onset of winter, plants are genetically programmed to process their food production into energy reserves that are stored in their roots.  These energy reserves are the primary food source that promotes growth and regeneration the following spring.   

The primary trigger for this is temperature.  Ideally, there is a point in time in the fall when temperatures drop to a point where shoot growth is reduced, yet root growth is still active. 

Table 1.  Optimum temperatures for cool-season and warm-season turfgrass growth.

So, the optimum time to make this fall fertilizer application is when clipping production is minimal, yet soil temperatures are still above 50 F. 

Many agronomists will recommend the use of fast release or water soluble nitrogen sources for this fall fertilizer application such as ammonium sulfate or urea.  This will work if the application timing coincides with the turfgrass growth response to allow maximum use of the applied nitrogen to be utilized for food reserves. 

I personally like to “cover the bases” with this application and use a slow release nitrogen source such as a polymer coated, sulfur coated urea like XCU during this time period.  What I mean by this is that the release mechanism for XCU is controlled by water and complete release will occur within 6 to 8 weeks after application.  So, by leveraging the strengths of XCU (release mechanism and longevity of release), we can guarantee the turfgrass plants a supply of nitrogen during this critical time to insure we have produced the maximum amount of carbohydrate reserves as possible and not limited this production with the use of a fast release fertilizer that releases completely within one week after application. 

It is also recommended that the mowing heights be increased this time of year to allow for more leaf area to develop to increase the photosynthetic capacity of the turfgrass plants.  This is very important for supporting the manufacture of food reserves as an increase in photosynthesis equals an increase in carbohydrate production.

This fall fertilization is needed to supply the nutrient sources (raw materials) required to manufacture the food reserves in the plant.  We must supply the turfgrass with sufficient nutrients to manufacture as many carbohydrates as possible during this special time.  As the old saying goes, “we have to make hay while the sun is shining”.

As a reminder, this fall fertilization should also include potassium (K) as a component of the fertilizer blend that is applied as K is very important for improved stress tolerance such as cold temperatures. 

Potassium also regulates the water content of plants. Turfgrass plants with sufficient K contain less water and are therefore, more tolerant of stress such as cold temperature exposure (winterkill), disease infection, traffic, etc.   

Tissue testing is the best means to determine how effective your K fertilization programs are working.  Ideally, you would like to see tissue test results showing K levels above 1.5% on a dry weight basis. 

If budgets are such that your fall fertility programs are limited, then apply K to the high traffic areas and other areas that are under the most severe stress; however, please realize that your turfgrass needs the nitrogen for food reserves so don’t skimp on it!