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Is it Too Late to Apply Nitrogen Fertilizer to Warm Season Grasses?

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  nitrogen fertilizer The simple answer if you are reading this in late August or early September is “No.”  This decision is based on several different factors.  A general rule is to apply the last nitrogen fertilizer to a lawn that contains warm-season turfgrasses two months before the first frost. Unless you live in the deep south, the last application of a fertilizer that contains a high amount of nitrogen would be September 15 at the latest. If you live in the more northern areas, you run the risk of turf damage and lawn disease development if nitrogen is applied after September 1st.  Of course, these are based on averages, so there is a little “wiggle” room, but not much.  

Applying Nitrogen Fertilizer to Warm Season Grasses

Centipede, St. Augustine, Hybrid Bermuda and Zoysia grass are the most common warm season grasses and they usually go dormant in the late fall.  Applying a high rate of nitrogen after the middle of September for the more moderate warm-season areas will increase the shoot and leaf growth while the plant is slowing its growth. It is important that these grasses have a chance of “harden-off” before going dormant.  If pushed to grow, the tender new growth is more susceptible to freeze damage.  These succulent shoots are prone to being attacked by a common cool-season disease called Large Patch. Another problem with fertilizing later in the fall is that it may increase the number of cool-season weeds that germinate. Weeds like henbit, common chickweed and Shepherd’s Purse are considered winter annuals and will grow and spread while the desired grasses have slowed down growing.

Determining Nitrogen, Phosphorus and Potassium in Fertilizer

The three numbers on a bag of fertilizer indicate the percentage by weight of nitrogen, phosphorus and potassium in the bag. The number one nutrient needed by turf is nitrogen. Here is a chart that provides the amount of nitrogen each turf type needs for the entire growing season.
Warm Season Grass #’s of Nitrogen Per Year
Hybrid Bermuda 5 to 6 lbs.
Common Bermuda 4 to 5 lbs.
Centipede Bermuda 1 to 2 lbs.
St. Augustine grass (sun) 3 to 4 lbs.
St. Augustine grass (shade) 1 to 2 lbs.
Zoysia grass 3 to 4 lbs.
  If you haven’t applied the recommended amount of nitrogen to your lawn this year, don’t try to catch up now.  In regards to the other two nutrients, phosphorus and potassium, they are required, but they are usually found in an abundant amount in the soil. The only way to tell if the lawn needs either of these two nutrients is to have your soil tested.  Most cooperative extension services offer this service at a low fee of less than $25.00 per sample. If you live within the area that has a first frost date of mid-October, try to fertilize between Labor Day and the end of September, depending whether you are in the northern or southern parts of that zone. If you are not sure, contact your neighborhood lawn care professional at Spring-Green.