September is the most important time to fertilize a cool season bluegrass and tall fescue lawn. A healthy dose of fertilizer prepares the grass for its rapid growth during fall. The plant’s internal processes use this application to build strong roots and food reserves.
November is the second most important time to fertilize. This application should be done about the time of the final mowing of the season, usually mid-month. This enables the lawn to green up earlier without encouraging the excessive shoot growth that often accompanies early spring applications.
Nitrogen, the first number listed on a bag of fertilizer, is the foundation of a nice lawn. This nutrient can quickly leach from the soil, requiring additional applications for best growth. How often you fertilize over a year depends on how much maintenance you want to do. But even lawns that don’t get much care will benefit from the September application.
Phosphorus, the second number listed on the bag, is needed for growth of roots and shoots. Adequate quantities are often present in the soils of established grass.
Potassium, the third number listed, is essential for the overall health, stress resistance and cold hardiness of lawns. Like phosphorus it is usually found naturally in our soils. Therefore, fertilizers with phosphorus and potassium are not recommended unless a soil test indicates the need for those nutrients. A starter-type fertilizer higher in phosphorus can be applied only when overseeding to help encourage the seed to establish quickly.
Based on the need of the grass and local soil conditions, higher concentrations of nitrogen should be used in fall. Look for bags with numbers such as 30-0-0, 27-3-3 or 25-5-5.
This emphasis on fall fertilization differs from those who advocate spring applications. Cool season lawns experience a flush of shoot growth in the spring. Applying fertilizer before this flush is over can cause the grass to grow too fast. The rapid growth exhausts the plant’s food stock and leaves it with little reserves for the stressful summer ahead. Therefore, it is best to wait until the flush is over, usually in early May, before making any spring applications.
This summer has been one for the books, with ample rainfall, and many lawns look better than they usually do following our usual drought months. Still, the September application is vital to help them remain vigorous.
The fertilizer must be watered into the soil by rainfall or irrigation in order to be used by the grass. Be sure to sweep or blow all fertilizer pellets back onto the lawn. Pellets that land on hard surfaces wash into our water supply, which can harm water quality.
Dennis Patton is a horticulture agent with Kansas State University Research and Extension. Got a question for him or other university extension experts? Email them to firstname.lastname@example.org or visit KCGardens.KansasCity.com.