The Benefits of Late Fall Fertilization
Professors of Ohio and Colorado State Universities detail below the benefits of a fall fertilization program for lawns. They emphasize that the application of nitrogen fertilizers in late fall results in lawns that are visibly greener in color through summer of the following year, and without the excessive shoot growth of lawns fertilized in spring.
Late fall nitrogen fertilization
Late season nitrogen fertilization has been practiced by professionals for some time, but new evidence shows the wisdom of this technique.
Late season nitrogen fertilization, sometimes referred to as fall fertilization, has been utilized by turf managers for years. This type of fertility program involves the application of much of the season’s nitrogen during the late season months of September through December. It is important that late season fertilization not be confused with dormant and/or winter fertilization. The latter method implies that fertilizer applications are made after the turf has lost most or all of its green color and is not actively growing. This differs notably from the late season concept, which requires that nitrogen be applied before the turf loses its green color in the late fall.
Late season fertilization has become popular because many of the agronomic and aesthetic advantages attributed to its use supposedly are not realized when spring and/or summer fertilization are practiced. Purported advantages of the late season concept include: better fall and winter color; earlier spring green-up; increased shoot density; improved fall, winter, and spring root growth; and enhanced storage of energy reserves (carbohydrates) within the turf plant. Some claimed disadvantages include: increased chance of snow mold injury and decreased cold tolerance.
The effects of late-season nitrogen application on creeping bentgrass (Agrostis palustrus Huds.) were documented by Powell et al. 2 3 and Snyder and Schmidt 4 in research conducted in Virginia during the late 60s and early 70s. In those studies, it was clearly shown that the quality and root production of creeping bentgrass grown in the transition zone could be enhanced by using late-season nitrogen applications and avoiding early spring nitrogen fertilization.
While researchers 1 and turfgrass practioners alike have demonstrated that the year-around quality of Kentucky bluegrass can also be enhanced by late season nitrogen fertilization, research efforts concerning effects on root growth and plant carbohydrate status have been lacking. This article will review past studies which have examined late-season nitrogen fertilization and relate those findings to the results of work recently completed at Ohio State University that assessed how the timing of nitrogen application can influence the quality, carbohydrate status, and root growth of Kentucky bluegrass.
Turf fertilized in September and again during October, November, or December is generally shown to possess better fall and winter color than a turf which was not fertilized at that time 1 2 4. In addition, signs of spring green-up have been shown to occur two to six weeks earlier if the turf has been 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.
The winter color of turf fertilized only during the spring and summer is decidedly inferior to that of grass which 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 which receives late-season nitrogen applications, the excessive shoot growth which accompanies spring fertilization is undesirable.
At Ohio State, we 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 0.75 to 1.0 lb. Application of nitrogen is recommended at this time (late May-June) to maintain an acceptable level of turf quality throughout the summer period.
It is important to remember that the nitrogen source used for the fall applications be of the type that is not dependent on microbial activity to effect nitrogen release. This means that urea, sulfur-coated urea (SCU), IBDU, and ammonium sulfate are ideal nitrogen sources for the 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 the degree of microbial activity. The use of microbially-dependent (i.e., UF-type) nitrogen sources 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 temperatures are low. However, these slow-release nitrogen sources would be ideal for spring and summer use.
Plant carbohydrate levels during early fall do not appear to be greatly affected by timing of nitrogen application. From December to February, however, the carbohydrate content of late season-fertilized may be lower than that of turf fertilized only during the spring and summer. This probably occurs because: (1) energy must be expended to take up and assimilate fall-applied nitrogen; and (2) nitrogen applied during the fall and winter has been shown to increase respiration during the winter months 2. Regardless of timing of nitrogen application, carbohydrates are accumulated by the slowly-growing turf plant during the fall and winter months, reaching a peak sometime during the December-February period.
The early-spring (March-April) carbohydrate content of turfgrass plants fertilized the previous fall is often higher than that of those plants which did not receive late season nitrogen. The ability to store energy at this time is a result of the earlier greening realized through the use of late-season nitrogen fertilization. Photosynthesis occurs in the still slowly-growing turf plant, thus allowing it to accumulate carbohydrates.
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-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 possess 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.
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 increased ability to recover from pest-, traffic-, or stress-induced damage may be realized.
For years, researchers have claimed that fall and winter 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 Ohio State 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 and 65 degrees Fahrenheit, while shoot growth is favored at temperatures over 55 degrees. In fact, some root growth will occur as long as the soil remains unfrozen.
In the Virginia study conduced by Powell and his associates 3 on creeping bentgrass, 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 Ohio State 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 are energy-consuming processes 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 late-season fertilization 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 nitrogen application 3 4. Conversely, turf fertilized using the late season 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 by growing roots.
Disease and winter injury
It has been claimed that late-season fertilization reduces turfgrass cold hardiness and may increase the risk of winter damage by the snow mold diseases. Ledeboer and Skogley 1 and Powell et al. 2 reported that late-season nitrogen applications caused neither problem in their respective studies. Observations over two winters at Ohio State detected no damage caused by either disease or cold injury. However, both types of injury potentially can occur when high rates (more than two pounds of nitrogen per 1,000 square feet) of a quickly-available nitrogen source are used at one or more application dates and/or applications are not timed properly.
Why timing is important
For the late-season concept to work successfully, it is essential that the turf be green when the late (October or November) nitrogen application is made. In central Ohio, this means that 0.75 to 1.0 lb of quickly available nitrogen (such as urea) should be applied during the latter half of September. This will ensure that the grass will remain green late into the fall when the other application will be made. It is also important, however, that excessive shoot growth not be encouraged by over-application of nitrogen during September. The production of lush, succulent growth may decrease cold tolerance and increase the incidence of the snow mold diseases during the winter and following spring. For the same reasons, the October/November application should be delayed if extended periods of unusually warm weather (average daily temperatures greater than 55 degrees) are being experienced, or are forecast.
Late-season fertilization lengthens the fall/winter green period and enhances the rate of spring green-up without stimulating excessive shoot growth, thus allowing the turf plant to maintain higher levels of carbohydrates than when spring/summer fertilization is used. Nitrogen applied during early spring increases shoot growth rates and decreases the levels of available carbohydrates in the plant, resulting in depressed root growth rates. Late-season nitrogen applications have no similar negative effects on root growth. No winter damage or snow mold injury occurred as a result of late-season nitrogen applications.
The authors of this paper are A.J. Koski and J.R. Street. In 1986, when it was written, Koski was a research associate at Ohio State University. Dr. Koski is now at Colorado State University. Dr. John Street remains a Professor of Agronomy at Ohio State University. One item of note, high-activity methylene ureas were not yet popular when this article was written. Although they do require microbial activity for their release, they are much more available in cooler temperatures than the UF-type fertilizers mentioned in this article.