Creative Uses forPlant Growth RegulatorsThey offer more advantages than growth reduction.

by NICK CHRISTIANS, Ph.D.

My POSITION as a universityeducator has led to manyopportunities over the years

to speak to the general public aboutlawns. Whenever I speak to peoplewho have little knowledge of turfmanagement, I can always expect onequestion: "Is there anything that I canspray on my lawn so that I won't haveto mow?"

Not having to mow has long been adream of both the Saturday morningnovice and the professional turfgrassmanager. While the answer to theabove question is a simple "no," thereare a number of compounds that havethe ability to slow the growth of grassesand consequently reduce the mowingrequirements. This becomes particu-larly important on the golf courseduring periods of rapid growth such asin the spring for cool-season grassesand summer for warm-season grasses.

PGR Classification and OverviewTable 1 contains a list of both past

and current plant growth regulatingcompounds (PGRs) that have beenlabeled for use on turf. The system bywhich PG Rs are classified is under-going change. The original systemdivided the compounds into two cate-gories, Type I and Type Ipo

Type I compounds are foliarly ab-sorbed and inhibit cell division in theplant meristem.

Type II materials are usually crownand root absorbed. They suppressgrowth through the inhibition of gib-berallic acid (GA), a naturally occur-ring plant hormone that reduces cellelongation. The Type II materials,which are also known as the GAinhibitors, include flurprimidol,paclobutrazol, and trinexapac-ethyl.

Most of the Type I PGRs are excel-lent seedhead inhibitors. Mefluidide isparticularly well known for its abilityto stop seedhead formation. Maleichydrazide is also very effective atstopping seedhead formation. Bothcompounds tend to be somewhatphytotoxic and have limited use on

Table 1Plant growth regulators that have

been labeled for use on turf.Common Name Trade NameAmidochlor LimitChlorflurenol Maintain CF125Endothal EndothalEthephon Ethrel, ProxyFlurprimidol CutlessMaleic hydrazide Royal Slo-GroMefluidide EmbarkPaclobutrazol TGR, Turf-

Enhancer, TrimmitTrinexapac-ethyl Primo MAXX

high-maintenance turf. However, theyare quite useful on low-maintenanceturf such as roadsides. Mefluidide alsois used to inhibit Paa annua seedheadformation in golf course turf. Paaannua seedhead suppression is difficultbecause it requires very precise appli-cations of mefluidide and a thoroughunderstanding of how the grass willreact.

Type II compounds are usually lessphytotoxic, although they also cancause some grass discoloration. TheType II compounds are not as effectivein stopping seedhead formation as arethe Type I materials, although they arequite effective at slowing growth andcan be used to reduce the need formowing if properly used. In the golfindustry, one of the primary uses offlurprimidol and paclobutrazol hasbeen the gradual removal of Paaannua. These GA inhibitors are knownto have a greater inhibitory effect onPaa annua than on creeping bentgrass.With careful application and propermanagement techniques designed todiscourage Paa annua, these materialsmay help increase the amount of bent-grass in the stand.

Trinexapac-ethyl is the newest of theType II materials. Its advantage over thetwo older compounds is that it can be

taken up through the foliage, whereasflurprimidol and paclobutrazol areprimarily root absorbed. It has beenused extensively on golf course fairwaysand to a limited extent on lawns toinhibit tissue growth and reduce theneed for mowing. Trinexapac-ethyl hasrecently been labeled for Paa annuaconversion programs.

The new classification system dividesPGRs into classes A, B, C, and D.31 ClassA materials are GA inhibitors thatinterfere with GA production late in thebiosynthetic pathway. Trinexapac-ethylis the only Class A material at this time.Class B materials are those that inhibitGA early in the biosynthetic pathway.Flurprimidol and paclobutrazol areincluded in this class. Class C materialsare mitotic inhibitors like maleic hydra-zide, mefluidide, and amidochlor.Finally, Class D materials are PG Rs thatproduce a phytotoxic growth regulatingresponse at low levels and act as herbi-cides at higher levels. Two herbicides,chlorsulfuron (Telar) and glyphosate(Roundup), are examples of Class Dcompounds.

The newest material to reach the turfmarket is Proxy (ethephon), although ithas been available for years in thefloriculture and crop production mar-kets. This material affects the growth ofplants by releasing the plant hormoneethylene. It does not fit into any of theexisting categories. The most strikingeffect is on Kentucky bluegrass, whichundergoes some very unusual struc-tural changes when treated with thisproduct. Ethephon-treated bluegrassdevelops elongated internodes from thecrown area and shortened leaves. Thenet effect is a stoloniferous Kentuckybluegrass that looks more like ber-mudagrass than bluegrass.5, 6 As is thecase with other PGRs, the effect of theethephon varies with species. Work ispresently being conducted at IowaState University to characterize theseresponses on cool- and warm-seasongrasses.14 The effect of Proxy on creep-ing bentgrass fairways has been variablein recent studies, and more work will be

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Paa annua contamination is a problem for golf course superintendents around the world.Embark is one plant growth regulator that effectively suppresses Paa annua seedheads,but there is potential for turf discoloration.

required to fully evaluate this productfor fairway use.n, 22

Growth reduction is generally thegoal in the use of PGRs, but a numberof other creative uses have been devel-oped for these useful compounds inrecent years. Some of these uses havebeen the result of studies in the scien-tific community, but others have comeabout as the result of observationsmade by turf professionals in the field.

Poa annua ControlPoa annua control remains a serious

problem for golf course superinten-dents around the world. One of thecreative uses of PG Rs has been to usethem as part of a carefully structuredintegrated program to reduce Poaannua in golf turf. As was mentionedearlier, this generally involves the GA-inhibiting (Type II) materials and hasbeen most effective on bentgrass/ Poaannua fairways. The GA inhibitors donot kill the Poa annua, but slow itsgrowth more than that of the bentgrass.Over time, this results in an advantageto the bentgrass and reduction of thePoa annua. While this program be-came widely used in the 1990s, resultshave been quite variable by location.Success depends on the skill of thesuperintendents in adapting the pro-gram to their particular situation. Re-sults also may vary with the Poa annuabiotype in the region.2,24

Flurprimidol (Cutless) was the firstmaterial to be used in this way, andpaclobutrazol (TGR, Turf Enhancer,

Trimmit) became the most widely usedin the 1990s. In the spring of 2001, anew program that involves applicationsof paclobutrazol (Trimmit) in springand fall and trinexapac-ethyl (Primo-MAXX) during the summer was alsointroduced to the market.

Seedhead suppression of Poa annuamay also be a goal in the use of PG Rs.The Type II materials are only moder-ately effective in reducing seedheads.Mefluidide (Embark) is by far the bestseedhead suppressor, but its use isdifficult and discoloration of the turfcan easily occur. Ethephon (Proxy) hasrecently been tested as a seedheadsuppressor. It has proven to be quiteeffective on some Poa annua biotypesin California, but results have beenmore variable in other regions of thecountry.

Color Enhancement

From the beginning of PG R use onthe golf course, superintendents haveobserved color changes when theseproducts are used. With the earlier TypeI materials, there was often a negativeeffect and turf discoloration was com-mon. With the GA inhibitors, however,improvements in turf color are oftenobserved. This is particularly true withtrinexapac-ethyl (Primo), which oftenresults in a darker green color of treatedturf.21,18

Reduced growth and improved colorare a very beneficial combination onhighly maintained turf. As is usually the

case with PG Rs, this response can behighly variable.

OverseedingPG Rs have been employed as a tool

to improve oversee ding of cool-seasongrasses into warm-season turf. The goalis to slow the growth of the warm-season grass without inhibiting theestablishment of the cool-season seed-lings. 1, 11 Timing is critical to prevent in-hibition of the cool-season seedlings16,29

and results may be quite variable.12

Trinexapac-ethyl (Primo) tends to beone of the best PG Rs for this purposebecause of its foliar absorption and itsreduced likelihood of inhibiting thecool-season seed germination.8 A criti-cal factor in using trinexapac-ethyl forthis purpose is that it be allowed todry on the bermudagrass tissue beforeoverseeding takes place. 17 Current labelrecommendations suggest applyingPrimo one to five days before seeding.

Water UsePG Rs reduce growth, but does this

translate into a reduction of water use?Research in Australia 15 showed a 25%to 30% reduction in water use rate ontall fescue treated with trinexapac-ethyl. There is a great deal of interestin this subject, particularly in aridregions, and more work is needed on avariety of species.

Freezing DamageIn northern regions, freezing damage

can be a serious problem. PGRs slowgrowth, thicken cell sap, and mayprovide an antifreeze-like effect. Rossiand Buelow (1995) observed enhancedfreeze tolerance of annual bluegrasstreated with low rates of trinexapac-ethyl. However, Dunn et ai. (1999)found no reduction in freezing damageon zoysiagrass treated with thisproduct. Northern superintendentswho often experience Poa annua lossduring the winter may want to experi-ment with this idea.

FungicidesOne of the factors that limits fungi-

cide efficacy is plant growth, or whenthe contacts are mowed off soon afterapplication. PGRs tank-mixed withfungicides show promise in extendingefficacy and in reducing the fungiciderates needed for disease control.4,2S,28,32

Some PG Rs may even directly suppressdollar spot on treated turf.3

Other ObservationsResearch has shown that PG Rs can

improve shade tolerance of certain spe-

12 USGA GREEN SECTION RECORD

des, particularly zoysiagrass.lO, 20,21,26,27Trinexapac-ethyl is now being widelyused for this purpose in the transitionzone of the United States and through-out the Orient.

Finally, trinexapac-ethyl has beenshown to reduce clippings, preventscalping, improve establishment ofnew sod,23 and stimulate tillering ofKentucky bluegrass being grown forsod.21

These are only a few of the potentialuses for PG Rs in the turf industry, andother innovative ideas are likely tofollow. A number of these uses hadtheir origin from observations made bygolf course superintendents and otherturf professionals working with thematerials in the field. Those with othercreative ideas are encouraged to sharethem at meetings or on-line so that theycan be further developed and tested.References1Baker, B. 1997. New tools for oversee dingsuccess. Turf Grass Irends. 6(9):14.2Bingaman, B. R., N. E. Christians, and M.B. Faust. 1998. Effects of Primo and Beaconon Poa annua populations in creepingbentgrass maintained at green height. 1998Iowa Turfgrass Res. Rep. p. 54-56.3Burpee, L. 1995. Dollar spot managementin fairways: PGR-fungicide interactions.Proceedings of the 66th International GolfCourse Conference. p. 16-17.4Demoeden, P. H., J. M. Krouse, Y Feng.1998. Brown patch control with Novartisfungicides, 1998. Fungicide and NematicideTest. 54:483.5Diesburg, K. L., N. E. Christians, and R. J.Gladon. 1989a. A continuous air-exchangeroomette and gas-metering system. Crop Sci.29:344-348.6Diesburg, K. L., and N. E. Christians.1989b. Seasonal application of ethephon,flurprimidol, mefluidide, paclobutrazol, andamidochlor as they affect Kentucky blue-grass shoot morphogenesis. Crop Sci. 29:841-847.7Diesburg, K. 2000. Growth regulators boostdensity in different ways: More tillers vs.more leaves per tiller: Products offer distinctresults. Golf Course Management. 68(4) :61-63.8DiPaola, J. M., D. P. Shepard, L. D. House-worth. 1997. Regulation of turf-impact ongrowth and performance. Proceedings ofthe 8th Inter. Turfgrasses Res. Conf. 8:146.9Dunn, J. H., R. H. Ervin, M. R. Warmund,B. S. Fresenburg. 1999. Cold tolerance ofzoysiagrass as influenced by cutting heightand Primo. Missouri Turfgrass Res. Report.p. 11.lOErvin,E. H., C. Ok, B. S. Fresenburg, J.Dunn, S. Dunn. 1999. Primo for sustainingzoysiagrass quality in shade. 1999 MissouriResearch &> Information Rep. p. 2-3.

The product works. A Poa annua-infestedsod strip was untreated before it was useddown the center of a golf course fairway.The strip stands out amongst fairway turfthat had been treated to suppress Poaannua seedheads.

llGreen, R. L. 1999. Improvement of thespring transition of overseeded bermuda-grass putting greens: A two-year projectfunded by the Hi-Lo Desert GCSA. Cali-fornia Fairways. 8:12.12Henry,M. J. 2000. An evaluation of plantgrowth regulator and contact herbicidepre-treatments during initial oversee ding ofbermudagrass with perennial ryegrass. Calif.Turfgrass Culture. 50(1-4):3-6.13Howieson, M. J., N. E. Christians. 2000a.Effect of mower setup and plant growthregulators on mowing quality. 2000 IowaTurfgrass Res. Rep. 21:1-5.14Howieson, M. J., N. E. Christians. 2000b.Bermudagrass, creeping bentgrass, finefescue, Kentucky bluegrass, manilagrass,perennial ryegrass, tall fescue, and zoysia-grass responses to ethephon. Agron. Abs.160.15King,R. W, C. Blundell, L. T. Evans, L. N.Mander, and J. T. Wood. 1997. Modifiedgibberellins retard growth of cool-seasonturfgrasses. Crop Science. 37:1878-1883.16Menn,W G., R. H. White, and M. H. Hall.1998 to 2000. Effect of Primo on perennialryegrass (Lolium perenne) oversee dingestablishment in Tifway bermudagrass(Cynodon sp.). http://dallas.tamu.edu/-pub/ttrr98/turf -98-20 .html.17Miller, G., and S. Killingsworth. 1997.Primo influences: On overseeded perennialryegrass and Poa trivialis. Florida TurfDigest. 14:24.

18Pound,W 1993. Primo growth regulatorevaluation on creeping bentgrass. OhioTurfgrass Res. Rep. p. 115-119.19Rossi,R, and E. Buelow. 1995. A prelimi-nary report on the effect of plant growthregulators on freezing stress tolerance ofcool-season turfgrasses. Wisconsin Turf Res.13:83-85.2°Nus, J. 2000. Improving shade toleranceof zoysiagrass. Golf Course Management.68(10):80.21Shepard, D. P., and J. M. DiPaola. 1999.Using trinexapac-ethyl to enhance turfgrassstress tolerance. Agron. Abstracts. 91:123.22Stier,J., Z. Reicher, and G. Hardebeck.2000. Effect of the growth regulator Proxyon creeping bentgrass fairway turf. J. ofEnvir. Hort. 18(1):53-58.23Stowell, L. 1997. Primo-treated sod: har-vest, establishment, and transplant (fieldvalidation). PTRI Turfgrass Res. Report.p.78-79.24Street, J. R., and P. Sherratt. 1998. Plantgrowth regulator effect on creeping bent-grass/annual bluegrass sward. Ohio Turf-grass Res. Rep. p. 92-94.25'fredway,L. P., B. B. Clarke, G. W Towers,E. N. Weibel, and P. R. Majumdar. 1999.Evaluation of fungicides for control ofbrown patch, 1998. Fungicide and Nemati-cide Tests. 54:484-485.26Qian, Y L., and M. C. Engelke. 1999a.Zoysiagrass in shade: Influence of trinexa-pac-ethyL Turf Grass Irends. 8:12-14.27Qian, Y L., and M. C. Engelke. 1999b.Influence of trinexapac-ethyl on "Diamond"zoysiagrass in shade environment. Crop Sci.39:202-208.28Uddin,W, and M. D. Soika. 1999. Evalu-ation of fungicides for control of gray leafspot (blast) on perennial ryegrass fairway.Fungicide and Nematicide Tests. 55:526.29Waltz, c., M. Blalock, and T. Whitwell.1997. Growth regulators as overseedingaids. Clemson Univ. Turfgrass Prog. p. 98-104.3°Watschke, T. L., M. G. Prinster, and J. M.Breuninger. 1992. Plant growth regulatorsand turfgrass management. Turfgrass-Agronomy Monograph no. 32 of the ASA-CSSA-SSSA. p. 557-565.31Watschke, T. L., and J. M. DiPaola. 1995.Plant growth regulators. Golf Course Man-agement. 63(3) :59-62.32Wilkinson, H. T., J. M. McMeans, T. WFermanian. 1998. Plant growth regulatorsand disease management on creeping bent-grass. 1998 Illinois Turfgrass ResearchReport. p. 23-25.

DR. NICK CHRISTIANS is a professor inthe Iowa State University HorticultureDepartment. His major duties on campusinclude turfgrass research and graduateand undergraduate student instruction.

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