Late-season fertilization (LSF) applied to sports fields offers many benefits. The application of nitrogen fertilizers in late fall results in turfgrass that is visibly greener in color through spring/summer of the following year, but without the excessive shoot growth that typically occurs with nitrogen fertilization in the spring. More importantly, overall plant health is improved for the following season.

The turf on the left did not receive a late-season fertilizer application, the turf on the right did.

The effects of late-season nitrogen applications on cool-season turf were documented by research conducted in Virginia during the late ’60s and early ’70s. In those studies, it was shown that the quality and root production of cool-season turf grown in the transition zone could be enhanced by using late-season nitrogen applications and avoiding early aggressive spring nitrogen fertilization.

While researchers and turfgrass practitioners alike have demonstrated that the year-round quality of cool-season grasses like Kentucky bluegrass can be enhanced by late-season nitrogen fertilization, research efforts concerning the effects on root growth and plant carbohydrate status have been lacking.

This article will review past studies that have examined late-season nitrogen fertilization and relate those findings to the results of research at Ohio State University (OSU) that assessed how the timing of nitrogen applications can influence the quality, carbohydrate status and root growth of cool-season grasses like Kentucky bluegrass.

Turfgrass quality

Turf fertilized in September and again during the late-season window is generally shown to possess better fall and winter color than turf that was not fertilized at that time. In addition, signs of spring green-up have been shown to occur two to six weeks earlier if the turf was fertilized during the previous fall. Most importantly, the enhanced rate of spring greening is realized without stimulating excessive shoot growth that accompanies the early spring nitrogen applications called for in most turf fertility programs.

For field managers, the longer greening into the late fall and the earlier green-up in the spring, especially, are beneficial when scheduling sports like baseball. Early spring green-up for all cool-season turf is not easy to initiate from a traditional spring fertilized turf that is attempting to green up from winter dormancy. Turf fertilized only during the spring and summer is decidedly inferior to turfgrass that receives fertilizer during the previous fall.

The rate of spring green-up is often slow as well, with acceptable color being attained only after nitrogen is applied during March or April. Although turf color then becomes equal to that of turf that receives the LSF application, the excessive shoot growth that sometimes accompanies spring fertilization is undesirable. OSU research found that the spring color of late-season-fertilized turf remained quite good until late May or early June, when the effects of nitrogen applied the previous fall began to “wear off.” A .75 to 1-pound follow-up application of nitrogen is usually recommended at that time (late May-June) to maintain an acceptable level of turf quality throughout the summer period where turf managers are on a six to eight-week programming schedule. For sand-based field managers, that is also the ideal time to initiate spoon/liquid feeding when a late-season program is in place.

Nitrogen fertilizer sources

It is important to remember that the nitrogen sources for late-season applications be relatively independent of microbial activity to ensure adequate nitrogen release due to colder air and soil temperatures. This means that urea, sulfur-coated urea (SCU), IBDU, ammonium sulfate and the more highly active methylene ureas are the most efficient nitrogen sources for late-season applications.

Although SCU and IBDU are referred to as controlled-release fertilizers, the rate at which nitrogen is released from these fertilizers is mainly dependent upon soil moisture level and not on degree of microbial activity. Microbially, temperature-dependent nitrogen sources (e.g., UF-types, polymer-coated ureas and natural organics) for late-season nitrogen applications may not elicit the desired fall/winter color response because they do not provide enough available nitrogen for plant uptake when soil temperatures are low. However, these latter slow-release nitrogen sources are ideal for spring and summer use.

Carbohydrate relations: the key to plant health

Plant carbohydrate levels during early fall do not appear to be greatly affected by the timing of nitrogen application. From December to February, however, the carbohydrate content of late-season fertilized turf may be lower than turf fertilized only during the spring and summer. This probably occurs for two reasons: because energy must be expended to take up and assimilate fall-applied nitrogen; and nitrogen applied during the fall and winter has been shown to increase respiration during the winter months.

Regardless of the timing of nitrogen applications, carbohydrates are accumulated by the slowly growing turf plant during the fall and winter months, reaching a peak sometime during the December to February period. The early spring (March, April and May) carbohydrate content of turfgrass plants fertilized the previous fall is often higher than plants that did not receive late-season nitrogen.

The ability to store carbohydrates (energy) at this time is a result of the earlier greening (two to six weeks) realized through the use of late-season nitrogen fertilization. Photosynthesis occurs in the slowly growing turf plant at this time, thus allowing it to accumulate carbohydrates. In fact, green turfgrass tissue will continue to photosynthesize at air temperatures close to freezing, but at a much slower rate. Greening without growing is a key cultural and management strategy to improve overall plant health.

As root and shoot activity and plant respiration rates increase during the late winter and early spring, plant carbohydrate content generally decreases. This decline may be quite significant when the turf receives an early season (February to April) nitrogen application, as compared to grass that has not been fertilized since the previous fall. The rapid decline occurs because carbohydrates are needed to support the increased shoot growth resulting from nitrogen applications made early in the season.

Conversely, the more slowly growing, late-season-fertilized turfgrass plants may develop a larger carbohydrate pool during the spring period. As will be discussed later, the process of spring root production can benefit from this greater concentration of carbohydrates from a late-season nitrogen application.

Another possible advantage resulting from late-season fertilization is that the levels of stored carbohydrates are higher than those found in spring-fertilized turf as summer approaches. The higher levels of carbohydrates are desirable at this time of the year since greater stress tolerance and/or the increased ability to recover from pests, traffic or stress-induced damage may be realized. (Key in summer stress enhancement.)

Root growth

For years, researchers have claimed that fall and winter root growth of cool-season turfgrass species should be stimulated by late-season and/or winter nitrogen applications. This stimulation should occur as fall temperatures decline to the point that root growth is favored over shoot growth.

Previous research at OSU has shown that root growth of cool-season turfgrass species does indeed occur during the fall after shoot growth has slowed or ceased. This situation develops because roots grow quite well when soil temperatures are between 40 to 65 degrees Fahrenheit, while shoot growth is favored at air temperatures in the 60 to 75-degree range. In fact, based on our OSU rhizotron research (underground root observation lab) conducted by R. Kuharski and A. Koski in the 1980s, some root growth will occur as long as the soil remains unfrozen.

In the Virginia study, no significant stimulation of winter root production by late-season nitrogen applications was observed. In fact, heavy and/or frequent nitrogen applications during the winter months (December and February) appeared to reduce the amount of roots produced during the winter.

Research at OSU has similarly revealed no noticeable stimulation of fall or winter root growth in response to late-season nitrogen applications. Three possible reasons for the absence of increased root production can be offered here. The first is that soil temperatures optimal for root growth may not be sustained for a long enough period of time in the fall and early winter, thus preventing any noticeable stimulation by late nitrogen applications from occurring. Secondly, it is possible that the fall hardening off process – during which rapid accumulation and storage of carbohydrates occurs – effectively competes with the roots for available energy stores. Finally, uptake and assimilation of fall-applied nitrogen is an energy-consuming process that may also compete for carbohydrates.

It is conceivable that singly or in combination these factors may prevent the stimulation of root activity that many thought would occur with late-season nitrogen fertilization.

The true advantage that LSF provides to turfgrass root growth is realized during the following spring. It has been shown that the root growth of turf fertilized during the late winter/early spring declines soon after the nitrogen application. Conversely, turf fertilized using the LSF concept becomes green early and rapidly, without the need for an early spring nitrogen application, and root growth continues at a maximum rate. It appears that the excessive shoot growth encouraged by early spring nitrogen applications utilizes carbohydrates that may otherwise be used for growing roots.

Why timing is important

It has been claimed that LSF reduces turfgrass cold hardiness and may increase the risk of winter damage by snow mold diseases. Research has shown that LSF applications cause neither problem, and observations over two winters at OSU detected no damage caused by either disease or cold injury. However, both types of injury can occur when high nitrogen rates are used and/or applications are not timed properly, resulting in excessive growth going into the late fall or winter.

For the LSF concept to work successfully, it is essential that the turf be green when the application is made. In central Ohio, this means that .75 to 1 pound of nitrogen be applied per 1,000 square feet in September (in a single or equivalent multiple feed applications/spoon feeding) to enhance fall recovery and sustain greening for the late-season fertilization. This will ensure that the turf remains green late into the fall when the actual LSF application is made.

The late-season nitrogen application should be applied with top growth stoppage (but the turf still green) at .75 to 1 pound of nitrogen per 1,000 square feet using a nitrogen source that is not highly dependent on soil temperature for nitrogen release, such as urea, ammonium sulfate, urea/PCSCU, urea/IBDU, highly-active MU, etc.).

It is also important, however, that excessive shoot growth not be encouraged by overapplication of nitrogen during September. The production of lush, succulent growth then may decrease cold tolerance and increase the incidence of snow mold during the winter and following spring. For the same reasons, the LSF application should be delayed if extended periods of unusually warm weather with average daily temperatures greater than 55 to 60 degrees are being experienced.

The higher nitrogen fertilizer rate for late-season fertilization may raise a red flag for field managers where spoon-feeding at light rates is the standard program. However, the late-season fertilization program will truly benefit the field manager’s number one strategy: plant health.

Pam Sherratt is a sports turf specialist at Ohio State University and served on the STMA board of directors from 2010-2011. Dr. John Street has been a professor in turfgrass science at Ohio State University for the last 30 years. Dr. T. Karl Danneberger is a professor of horticulture and crop science at Ohio State University.