Late Season Fertilization
of Cool Season Grasses
bluegrass, perennial ryegrass and tall fescue

Curtis E. Swift, Ph.D.
Colorado State University Extension
Tri River Area

Introduction:

The cool-season turf grasses - Kentucky bluegrass, tall fescue and perennial ryegrass - need nitrogen, phosphorus, potassium and other fertilizer elements for proper growth, for the development of a deeper more extensive root system and for the enhanced resistance to insect and disease problems. The quantity of nutrients needed is quite specific. Excessive quantities of nutrients are often as detrimental as deficiencies. Adding more in excess of what is needed may adversely affect the availability of other nutrients that were previously in sufficient supply. For example, adding an excess amount of phosphorus (excess of what is needed by the plant) may result in a deficiency of available iron both within the soil and within the plant.

To determine what nutrients are needed, a soil test should be conducted by a reputable soil testing facility (laboratory) to determine the quantity of available nutrients. Soil test results indicate that in many areas of the Tri River Area (Mesa, Delta, Montrose and Ouray counties), with the exception of nitrogen, most of the necessary fertilizer elements already are in sufficient supply in our soils. This can only be determined, however, by a soil test for your specific site.

Providing the Nitrogen Needs of turf:

Nitrogen is necessary for leaf production. Without adequate leaf growth, roots do not receive the carbohydrates (starches and sugars) necessary for continued development. Without adequate root growth, water and nutrient uptake for the top of the plant is restricted and the plant is more susceptible to drought stress, as well as insect and disease problems.

There are several ways to correct a nitrogen deficiency. Many homeowners and commercial lawn care companies fertilize turf grass every six to eight weeks, applying one pound of nitrogen per 1000 square foot area of lawn during each application. Others apply two pounds of nitrogen in the late season and a couple pounds of nitrogen scattered throughout the spring and summer months to provide the four pounds of nitrogen per 1000 square foot area typically required during the growing season by cool-season grasses.

Gardeners applying manure as a top dressing assume (incorrectly) that this provides the nutrient needs of the turf. Manures are very low in nitrogen with several inches of manure being necessary for each pound of nitrogen needed by the turf.

Depending on what level of maintenance you wish to devote to your lawn, the annual needs of the cool-season turf grasses may be from two to six pounds of nitrogen per year per one-thousand square foot area of lawn.

When over-fertilized with nitrogen, excessive growth results. This succulent growth increases insect and disease problems. If a heavy application of nitrogen fertilizer is applied just prior to the heat of mid-summer, severe stress can result and increased disease problems may occur.

The Late Season Fertilization Program:

Many gardeners and professional turf managers are switching to a late season fertilization (LSF) program to avoid these problems. The LSF program is not to be confused with the `Winterizing' fertilizer program that recommends the application of high levels of phosphorus (P) and potassium (K) and low levels of nitrogen (N). This `Winterizing' program is the opposite of what research shows is preferred by turf. There is no evidence to suggest that extra P or K in the fall is beneficial to bluegrasses, ryegrasses, or fescues.

The Late Season Fertilization program involves the application of up to two pounds of nitrogen per one-thousand square feet sufficiently late in the growing season so that excessive shoot growth is avoided. In Colorado this fertilizer application should be made sometime between October 15 and December 15 based on the growth phase of the turf. This nitrogen enhances late fall color retention and the corresponding high chlorophyll content. The resulting higher level of carbohydrate reserves enhances late winter survival and spring recovery. LSF also has been shown to result in less disease and insect problems during the spring, and a healthier less stressed turf at the start of the growing season. LSF may even reduce the incidence and/or severity of some winter diseases, and help heal disease damaged turf.

The nitrogen source for this late-season application should be independent of microbial activity for release. At this time of year soil temperatures are cool and microbial activity is reduced. Consequently, applying a fertilizer requiring microbial activity to release the nitrogen will not produce the desired results. Fertilizers containing higher percentages of quickly available nitrogen would be preferred for the late fall treatment in the LSF program. The quickly available nitrogen will cause a rapid green up of the turf increasing the time in which the turf can manufacture and store carbohydrates. While sulfur-coated urea (SCU) and isobutylidine diurea (IBDU) are referred to as controlled-release fertilizers, the rate at which nitrogen is released from these materials is based mainly on soil moisture and not microbial activity thereby making these fertilizers appropriate for the LSF program.

Note: In areas where turf is growing on very sandy soils fertilizer should not be applied after late September, because nitrogen can leach into groundwater during the winter months. Slowly-available nitrogen fertilizers (sulfur-coated urea, IBDU, and natural organic-based fertilizers) should be used on sandy soils throughout the year to reduce the potential of leaching.

The timing of the late-season fertilizer application is dependent on the availability of irrigation water as the fertilizer should be watered in. If your irrigation water is supplied by ditch, the fertilizer will need to be applied prior to your ditch company turning off the water supply for the season. If you use domestic water for irrigation, apply the fertilizer and water it in as late in the fall as possible after the final mowing but before the grass becomes dormant and turns brown.

Please note that only nitrogen (N) is being applied. Only in those areas where low phosphorus (P) and potassium (K) levels are a problem are these nutrients needed. The majority of the soil tests for the Colorado's Tri River Area have shown high levels of P and K. The numbers on a fertilizer label indicates the percentage by weight of nitrogen (N), available phosphate (P₂O₅) and soluble potash (K₂O). For example a fertilizer labeled as 20-10-5 would contain 20% N, 10% P₂0₅, and 5% K₂0 by weight. Thus a 20 pound bag of a 20-10-5 fertilizer, would contain:

• 4 pound of nitrogen (20% of 20 pounds),
• 2 pounds of phosphate (P₂0₅) (10% of 20 pounds), and
• 1 pound of potash (K₂0) (5% of 20 pounds).

With the LSF program the lawn is usually not fertilized until late May - early June when three-quarter to one pound of N can be applied. An additional pound of N is applied in August in September to green up the turf following the stressful summer period. This latter application also prepares the turf to receive the late fall application.

Note: A March-April application may be necessary if the late-season application the pervious fall did not provide the desired rate or degree of spring greening. This may have been due to improper timing of the nitrogen application or if rapid or unexpected changes in temperature and/or moisture occurred during the fall and winter. The use of a slow acting (slow-release) fertilizer may also result in poor results.

Lawn fertilizers are typically labeled with directions on how to set various types of lawn fertilizer spreaders so that one pound of nitrogen is applied per one-thousand square foot area of turf. To apply a half pound of nitrogen reduce the fertilizer setting by one-half. To apply two pounds of N per one-thousand square foot area, double the spreader setting or make two applications at the recommended one pound application rate.

If you are on a commercial fertilizer application program your lawn may be receiving one pound of nitrogen every six to eight weeks during the growing season. While research shows that the late season fertilization program discussed above is probably best for the turf, the commercial program is much better than no fertilizer at all. If interested in a late season fertilization program, talk to your commercial lawn care professional.

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References Used:

Beard, J.B. June, 1987. Nitrogen: Apply less more often? Ground Maintenance.
Beard, J.B. August,1987. Late season fertilization. Ground Maintenance.
Koski, A.J. & J.R. Street. November, 1986. Late Season Nitrogen Fertilization. ALA.
Koski, A.J. January/February 1988. Late-season nitrogen fertilization: What we do and do not know. USDA Green Section Record.
Koski, A.J. 1995. Turfgrass Management - Master Gardener Training. Colorado State University Extension.

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