It’s all about the numbers: Inspect fertilizer bag before all-important application

Consider this your yearly reminder. Now is the optimal time to fertilize the lawn. September is the most crucial month for fertilizing cool-season bluegrass and tall fescue lawns. Fertilizer applied now prepares the lawn for rapid growth during the fall and helps overcome summer stress by naturally thickening up thin stands.

Growing a healthy lawn is more about root development than top growth. The September application is used internally to build strong roots, create food reserves, and thicken the lawn. If this is achieved, a lush green carpet of grass is within reach.

November is the second most important time to fertilize. This application is timed around the final mowing of the season, ordinarily mid-month. This enables the lawn to green up earlier in the spring without encouraging excessive shoot growth from early spring applications. This results in a greener lawn with less mowing.

Bags of fertilizers always have three numbers displayed. The numbers stand for the percentage of nitrogen, phosphorus and potassium, in that order. Based on our area’s grass growth habits and soil conditions, fertilizers with higher nitrogen concentrations should be used.

Nitrogen, the first number listed on a bag of fertilizer, is the foundation of a thick lawn. Nitrogen is used in the greatest quantity and leaches from the soil quickly, requiring additional applications.

Phosphorus, the second number, supports the growth of roots and shoots. Adequate quantities are often present in local soils. Potassium, the third number listed, is essential for overall health, stress resistance and cold hardiness. Like phosphorus, it is found naturally in our soils.

Examples of fertilizers to apply include 30-0-0, 27-3-3 or 25-5-5. Fertilizers with slightly different numbers are good, but the nitrogen should be around 30%, and phosphorus and potassium should be close to zero. Brand names are not as important as the numbers.

Applications of fertilizers containing phosphorus and potassium are unnecessary unless a soil test indicates the need. Apply a starter-type fertilizer when overseeding, one higher in phosphorus, to help encourage the quick establishment of the new seed.

The total number of yearly applications depends on your preferred level of maintenance. A low-maintenance lawn will benefit from this September application. A thicker lawn will need additional applications, which means fewer weeds and reduced soil erosion.

The emphasis on fall fertilization may conflict with programs promoting spring applications. Cool-season lawns experience a flush of rapid spring shoot growth. Fertilizer timed around the flush pushes shoot development. This rapid growth exhausts the plant’s food supply and leaves it with few reserves for the stressful summer ahead.

It’s best to wait until the flush is over, ordinarily in early May, before making any spring applications. The only exception would be crabgrass control, which often contains fertilizer. No other early spring fertilizer should be applied. Fertilizers should always be watered into the soil either by rainfall or irrigation.

One more tip. Sweep or blow all stray fertilizer pellets back onto the lawn. Pellets landing on hard surfaces quickly wash into our water supply, leading to poor water quality.

Dennis Patton is a horticulture agent with Kansas State University Research and Extension. Need help? Contact the Johnson County Extension gardening hotline at 913-715-7050 or email garden.help@jocogov.org.