September is the most crucial time to fertilize a cool season bluegrass and tall fescue lawn. Fertilizer applied now prepares the lawn for rapid growth during the fall and helps to overcome summer stress.

Growing a healthy lawn is about root and shoot development. The September application is used internally to help thicken up a lawn by building strong roots and food reserves. If this is achieved, a dark 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, normally mid-month. This enables the lawn to green up earlier in the spring, without encouraging the excessive shoot growth that often accompanies early spring applications. Simply, this means a green 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 grass growth and soil conditions, use fertilizers containing higher concentrations of nitrogen.

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 quickly leaches from the soil, requiring additional applications.

Phosphorus, the second number, supports the growth of roots and shoots. Generally, 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 native 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 somewhere at 30% and phosphorus and potassium close to zero.

Applications of fertilizers containing phosphorus and potassium are not needed unless a soil test indicates the need. Apply a starter type fertilizer, one higher in phosphorus, when overseeding a lawn 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 but also 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 spring shoot growth. Fertilizer timed around the flush pushes shoot development. This rapid growth exhausts the plant’s food supply, leaving it with few reserves for the stressful summer ahead.

It is best to wait until the flush is over, normally early May, before making any spring applications. The only exception would be the use of crabgrass control, which often contains fertilizer. No other early spring fertilizer should be applied.

September is the most important application for a healthy lawn. Water the fertilizer into the soil either by rainfall or irrigation.

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

Nelson is Oklahoma Cooperative Extension Service ag educator for Garfield County.

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