Late-Season Nitrogen Fertilization:What WeDo and Do Not Knowby ANTHONY J. KOSKIThe Ohio State University

Figure 1.

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8 USGA GREEN SECTION RECORD

ETE-SEASON fertilization (LSF)of cool-season turfgrasses has been

discussed often in recent years.This practice, sometimes referred to asfall fertilization, implies most nitrogenis applied from August through Decem-ber. Many superintendents have longappreciated the advantages of using thisfertilization philosophy in the manage-ment of roughs, fairways, tees, and pos-sibly even greens. They have recognizedthe positive effects of LSF on someaspects of turf quality, including betterfall and winter color, increased turfdensity, and enhanced spring green-uprate. These desirable effects generallycan be experienced without noticeablyincreasing shoot growth during the fallor winter. Additionally, LSF can reducethe dependence on early spring appli-cations (which often stimulate excessiveshoot growth) to enhance the rate anddegree of spring green-up.

For all of the talk about LSF, rela-tively little university research has beenconducted on this subject. One purposeof this article is to discuss briefly whathas been revealed about late-seasonfertilization from a research perspective,and to point to areas where further re-search should be directed. A secondpurpose is to define the how to's, whento's, and why's oflate-season fertilizationfor those superintendents who are un-familiar with the practice and wish toexperiment with it, possibly with theintent of integrating it into their overallfertilization programs.

SINCE the early 1960s, research con-ducted at universities in Virginia,

Rhode Island, Illinois, Michigan, Minne-sota, and Ohio has provided substantialevidence to support the positive effectsof LSF on turf quality. In order toachieve maximum benefits with regardto late-season color and early springgreening, follow a program similar tothat outlined in Figure 1.

The program is most effective if it isbegun with the August-September appli-cation of nitrogen described in thattable. Proper fertilization at this timeis important, because it greens up theturf following the stressful summerperiod, and prepares it to receive the latefall (October-December) application. Itis important that the turf be green butnot actively growing when this late fallapplication is made. The August-Sep-tember application assures this. Propertiming of the late N application resultsin at least part of that N being taken upby the plant to enhance fall I winter color.It is not certain whether any N absorbedby the turf plant at this time plays a partin the enhanced greening the followingspring, or if some of that N remains inthe soil over the winter and becomesabsorbed by roots during late winter Iearly spring. This uncertainty has impor-tant implications with regard to winterN leaching, especially on sandy soils,unfrozen soils, or in areas with highwinter precipitation.

The March-April application is neces-sary only if 1) no late-season N appli-cations were made the previous year, or2) late-season N applied the previousyear has not provided the desired rate Idegree of spring greening. Late-seasonfertilization may produce poor resultsifN applications are not timed properly,or if rapid or unexpected changes intemperature andl or moisture occurduring the fall or winter. Proper timingand normal temperatures and moistureare especially important to those nitro-gen sources highly dependent on tem-perature and lor moisture to effect Nrelease.

Research at Ohio State Universityhas shown that fertilizer sources con-taining higher percentages of quicklyavailable N may be somewhat easier touse for the late fall treatment in the LSFprogram. One reason is that the quicklyavailable N source can cause a rapidgreening response (seven days), thuswidening the window during which thesefertilizer sources can be used success-fully. The temperature- or moisture-dependent N sources may require asmuch as two to three weeks lead time togenerate the same degree of greening asa quick-release source. An unexpectedor rapid decrease in temperature ormoisture availability may reduce 'Nrelease from these sources at a crucialpoint in the program. Secondly, a generaldependence upon adequate moisture to

trigger N release from certain fertilizersources may limit their use to irrigatedareas of the golf course. The fertilizersource used may even dictate how farinto the fall the irrigation system mustremain operational.

The May-June application is necessaryto maintain adequate quality during thesummer. The positive effects ofN appliedthe previous fall begin to wear off atthis time.

It is important to recognize thatoptimal N application dates will vary

Figure 3.

Figure 4.

with location in the country. For example,the proper timing for the spring appli-cation in Ohio is from mid to late May,while in Michigan, Wisconsin, or Minne-sota it might be in mid to late June.Similarly, Ohio locations may receive Nin early to mid September and again inearly to mid November. In the morenorthern states, these application datesmay translate into August and mid tolate October, respectively. Considera-tion must also be given to the type ofN source for each application.

JANUARY/FEBRUARY 1988 9

IT HAS LONG been hypothesized that LSF would promote fall/winter

root growth because it occurs during the fall and winter at temperatures below which shoots are inactive (Figure 2). Nitrogen fertilization during the fall or winter, it was reasoned, would stimulate root growth without affecting shoot growth.

Research at Ohio State, however, has detected no such stimulation of late-season root growth by LSF (Figure 3). Root growth benefits significantly during the spring with LSF. This benefit is derived from the fact that spring green­ing takes place without requiring stimu­lation from early spring N applications. Nitrogen applied during March and/ or April appears to depress root growth. This probably occurs because N-stimu-lated shoot activity (growth and respi­ration) effectively outcompetes growing roots for energy produced and stored in

the plant in the form of carbohydrates (Figure 4). Thus, LSF does not actually stimulate winter or spring root growth, but instead allows spring root produc­tion to occur at a maximum by forgoing the dependence on spring fertilization to promote spring green-up.

Although LSF may slightly lower the total non-structural carbohydrate (TNC) levels during fall and early winter, the enhancement of winter color and earlier spring greening allow the plant to accumulate more carbohydrate (via photosynthesis) than turfgrass plants that are not under an LSF program. This small but detectable surplus in TNC is carried into the summer. It has not been proven that the higher TNC content confers any advantage to turf­grass plants managed using LSF, but it certainly cannot be considered disadvan­tageous, either.

For many years it has been suggested that LSF would lower a turfgrass plant's

resistance to low-temperature injury. The previously mentioned decrease in fall/winter TNC content with LSF was considered to support this contention, since concentration of carbohydrate in plants during the hardening-off process is considered of importance in confer­ring resistance to low-temperature injury. However, there is little research evidence to support this contention where LSF is properly implemented.

Similarly, there is little evidence to suggest that LSF increases the occur­rence of cold-weather diseases, such as the snow molds. In fact, published findings from Minnesota, Virginia, and Rhode Island indicate that LSF may even reduce the incidence and/or severity of some winter diseases, and may help heal turf damaged by disease.

Low-temperature injury and disease may become problems where LSF is not practiced properly, either as a result of

Figure 5. LSF study on perennial ryegrass at Ohio State showing response to 41-0-0 and SCU (left) compared to unfertilized check plots (right) at 30 days after application of 2 lbs. N/1000 sq.ft.

over-application of N, or where appli-cations are not timed to allow for naturalhardening-off. Recent (November) obser-vations of an LSF study at Ohio Stateon mixed bentgrass/ Poa annua puttinggreen turf showed that Poa annua inplots fertilized in September and/ orOctober continued to grow and remainsucculent. At the same time, the bent-grass in those plots retained excellentcolor, and appeared to harden-off andcease growing. This would suggest thatthe effects of LSF on resistance to low-temperature injury and disease incidencemay pose more of a concern otl annualbluegrass turf. It is difficult to findinformation regarding the effects ofLSF on annual bluegrass. This deficiencypoints to just one area where knowledgeof certain aspects of late-season fertili-zation is lacking, and research should bepursued.

A number of other questions remainunanswered regarding the response of

some ofthe cool-season turfgrass speciesand cultivars to late-season fertilization.Late-season fertilization studies cur-rently under way at Ohio State are evalu-ating the effects of various N sources, Nrates, and application timings on thequality of tall fescue, perennial ryegrass(Figure 5), and putting green heightcreeping bentgrass (Figure 6). Otherresearch should examine the relation-ship between LSF and the quality ofKentucky bluegrass cultivars. The culti-vars of this species tend to vary greatlyin fall/ winter color and rate of springgreenmg.

As new fertilizer technologies aredeveloped, the suitability of theseproducts for use in LSF programmingmust be evaluated. Another projectrecently initiated at Ohio State onKentucky bluegrass is comparing newand experimental fertilizers to some ofthe standard N sources. These fertilizers

should be evaluated for their ability toperform well in situations where waterfrom either irrigation or precipitationis limited or unpredictable in availa-bility.

While university research and thepractical experiences of professionalturfgrass managers both have revealedthe advantages of late-season fertili-zation, it is obvious many questionsremain about this concept. While con-tinued research is important, it is justas essential to encourage the exchangeof results, ideas, and concerns betweenresearchers and superintendents. Theseexchanges will help stimulate productiveinvestigations by the university re-searcher. They will also allow super-intendents to refine LSF programsalready in use, and better inform super-intendents interested in incorporatingthe LSF concept into their current turfmanagement programs.

Figure 6. LSF study on putting-green-height creeping bentgrass at Ohio State .. ~