Ethephon Forces Plumeria for Winter Flowering
A plumeria lei is a traditional greeting among bothresidents and tourists in Hawaii. As a lei flower,plumeria is much underestimated in terms of its valuein the floriculture industry, because many flowers aregathered from backyards and roadside plantings and thusare not counted in the annual census conducted by theHawaii Agricultural Statistical Service, which has re-ported on average about 10 commercial producers overthe past decade. In 1998, nine farms reported totalplumeria sales of $601,000 from 23.8 million blooms(HASS 2000). But the “image value” of plumeria flow-ers to Hawaii’s tourist industry is probably many timesthat amount.
During winter, plumeria flower production nearly ceasesat a time when visitor counts are high. Other flowers areimported to meet the needs for lei flowers. Plumeriagrowers have tried different strategies in an effort to pro-duce flowers for the winter market, ranging from handdefoliation to planting many acres so that at least a fewflowers can be gathered, or choosing warm parts of thestate in which to grow plumeria trees.
Inflorescences are largely produced in the spring andmay continue to bear flowers for six months, althoughthe last flowers are small and infrequent. Murashige(1966) reported that leaf retention and abscission werecontrolled by daylength. Lawton and Akpan (1968)reached the same conclusion and added that stem growthand leaf production continued under long days. Sheehanand Murashige (1962) reported that plants treated withlong days or gibberellic acid continued to produce leaves and did not go dormant until natural short days wereimposed. Criley (1995) reported success in preventingfall dormancy by chemical defoliation of the trees withethephon. The result was early development of inflores-cences already initiated in the summer.
This report summarizes additional research, includ-ing an experiment involving cultivars in addition to thecommon yellow plumeria, ‘Celadine’.
Research was conducted at the University of Hawaii’sWaimanalo Research Station during fall-winter 1994–1995. Eighteen trees of ‘Celadine’ planted in 1986 wereused. Ethephon was applied as a foliar spray of 800 mgai/liter of water plus 2 ml of X-77 spreader (after Criley1995). Three trees were treated at each spray date. Com-parable control trees were not sprayed. Dates of ethep-hon application were September 13 and 26, and Octo-ber 10. The data consisted of counts of inflorescences atthe emerging bud, elongation of the peduncle, and openflower stages of development. Data were collected ev-ery two weeks between October 10, 1994, and April 7,1994.
Because defoliation and inflorescence-forcing weresuccessful with ‘Celadine’ (the common yellowplumeria), the extension of this work to other cultivarswas of interest. Foliar applications of 800 ppm ethep-hon were made to ‘Donald Angus’, ‘Kimo’, ‘Lurline’,‘Scott Pratt’, and an unnamed white hybrid designated18-41. Trees of ‘Celadine’ were included in this plot.Single ethephon applications were made to one tree ofeach cultivar on September 13, 24, or October 10. Observational and flower count data were taken every twoweeks from October 10 through March 24, 1995.
It should be noted that the trees used for cultivar-effects were water-stressed and had not been fertilizedfor many years. For this reason, perhaps, defoliation wasnot as uniform as on the ‘Celadine’ plants in the otherplanting. For ethephon applications made in Septem-ber, nearly all cultivars still retained the youngest leavesnear the branch tip when observed on October 10. Thetreatment of September 24 was somewhat more success-ful than that of September 13 in achieving defoliation.
The controls responded similarly for each treatment date,and only one set is presented (Figure 1). Although oldinflorescences were still in bloom into December, nonew flower buds were evident until late January. Elon-gation of the peduncle took four to six weeks until openflowers wre produced, with peak flowering commenc-ing in April 1995. In contrast (Figure 1), ethephon-treatedtrees, which defoliated within two weeks of treatment,began showing inflorescence buds about four weeks af-ter treatment, and produced open flowers before Christ-mas in the case of the September 13 treatment. Both theSeptember 26 and October 10 treatments also producedopen flowers well in advance of the controls. It shouldbe noted that the majority of the bud and flower produc-tion on ethephon-treated plants followed the same tim-ing as on control trees, but a small percentage were somuch earlier that the efforts made to stimulate themwould have been worthwhile.
Except for very early treatments (Criley 1995), ethe-phon-treated trees have not shown any adverse effects inthe same or subsequent years. However, once the inflo-rescence is initiated, sometime in July, I believe, defo-liation may force elongation of a poorly developed in-florescence stump that fails to produce flowers. It is myopinion that plumeria probably should not be defoliatedmuch before the end of August in order to permit initia-tion and development of the individual flowers in theinflorescence. The early inflorescences appear to havefewer flowers on them than do those produced at thenormal time, but flower size and shape were normal. Noother distortion of either flowers or foliage resulted fromtreatment with this concentration of ethephon.
The cultivars ‘Kimo’, ‘Celadine’, and ‘Donald An-gus’ continued to produce flowers from old inflores-cences through December 8. Nearly all plants were outof bloom by December 22.
New buds began to appear December 22 on the Sep-tember 13 treatment, with some of these reaching bloomJanuary 21 and flowering continuing to build well aheadof the other treatments and control. The late Septemberand October treatment yielded new buds on January 21and increased the bud count the rest of the observationperiod. Elongation was pronounced during March.Nontreated plants showed the same timing as the lateethephon treatments. Plants in this experiment were wellbehind the ‘Celadine’ plants in the previous experiment.
New buds were evident January 6 on the September 13treated plant, while the September 25 plant started push-ing buds on January 21 and the October 10 plant on Feb-ruary 8. The two plants from the September treatmentswere in good bloom the first week of March, while theinflorescences from the October treatment were elon-gating at this time and reached bloom by March 24. Anuntreated control plant paralleled the plant of the Sep-tember 24 treatment.
This selection is always an early bloomer, and newflower buds were apparent on all treated plants by No-vember 23. The nontreated control plant had many budsby December 22, slightly behind the treated plants. Buddevelopment continued on through the winter, with firstbloom on treated plants occurring December 22. Bothtreated and control plants were in good bloom from earlyJanuary on through the observation period.
This cultivar is usually a late bloomer. There was littlebud activity on the September-treated plants until Janu-ary 21, but the October-treated plant had prominent budsDecember 22. Development was slow, however, andwhile all plants had buds by February 8, there were noopen flowers until late March. Control plants had notproduced measurable buds even at that time.
The first new buds appeared in mid-January on the September 24 and October 10 treatments. Open flowersappeared between the March 10 and 21 observationdates. Control plants were slightly later in both bud andflower appearance.
This dark red cultivar is a sparse bloomer and usuallyquite late. Therefore it was a surprise to see developinginflorescence buds on December 22 on the plant treatedon October 10. A few buds appeared on the September13 plant by January 21. However, none of the early in-florescences matured into flowers. Control plants hadnot produced flower buds by March 24.
The extension of the ethephon treatment to otherplumeria cultivars to induce winter flowering appearspromising, although there are varietal differences in re-sponse, and the environment surely modifies the re-sponse. Cultivars that bloom heavily into the fall, suchas ‘Kimo’, ‘Donald Angus’, and ‘Celadine’, may be goodcandidates for the ethephon treatment because of thepresence of developed flower buds. The white hybrid‘18-41’ already blooms early and probalby does not gointo as deep a dormancy as do other cultivars. The earlyforcing of buds on ‘Lurline’ and ‘Scott Pratt’ was a sur-prise, which provided evidence that the treatments re-ally do work, even if early open flowers were notachieved.
The lack of water and fertilizer stressed the trees,making them perhaps more resistant to the defoliationtreatment, but as daylength decreased, they were morelikely to respond by uniform defoliation. Bud development was affected, as there were generally fewer budsthan on comparable trees in a watered and fertilized plot.A warm winter environment promotes rapid develop-ment of inflorescence buds, as occurred in the 1994–95season.
In conclusion, it appears that the ethephon treatmentto defoliate plumerias during September–October leadsto early inflorescence bud appearance and earlier flow-ering than on nontreated trees. In this trial, there wereflowers from new inflorescences before Christmas onlyfrom the ‘18-41’ hybrid, but healthier plants might beinduced to flower in time for the winter tourist trade.
This research demonstrated that plumeria trees withwell developed inflorescence structures can be forcedinto flower to meet the demands of the lei flower indus-try during a season when the availability of plumeriaflowers is normally low. A registration is being soughtfor this use with two companies that market ethephonfor various horticultural applications.
Criley, R.A. 1995. Enhanced winter flowering ofplumeria with ethephon. Acta Hortic. 394:325–330.
Hawaii Agricultural Statistical Service. 2000. Statisticsof Hawaii Agriculture 1998. Hawaii Dept. of Agri-culture, Honolulu, HI. p. 31.
Lawton, J.R.S., and E.E.J. Akpan. 1968. Periodicity inplumeria. Nature 218:384–386.
Murashige, T. 1966. The deciduous behavior of the tropi-cal plant, Plumeria accuminata. Physiol. Plant.19:348–355.
Sheehan, T.J., and T. Murashige. 1963. Growth responsesof plumeria to photoperiod and gibberellic acid. Proc.Fla. State Hort. Soc. 76:477–479.