Manipulating Dahlias By Garry Legnani and William B. Miller

Photoperiod scheduling can inhibit tuberous root growth in ‘Sunny Rose’ plugs and promote optimal flowering and height of ‘Sunny Yellow’pot plants

Tuberous root development and flower induction of dahlia arecontrolled by photoperiod; however, photoperiod manipulation is generally notused in their production. Seed-propagated plug production typically takes placeunder the short days of late autumn and early winter. The critical day lengthfor tuberous root formation is approximately 11-12 hours (Moser and Hess,1969). When the day length is shorter than this critical period, it signals theplant to store energy in tuberous roots at the expense of shoot growth. Manygrowers may have experienced difficulty when extracting plugs using mechanicaltransplanters because the tuberous roots have outgrown the plug tray.

Growers may have also experienced delays in flowering. Shortdays induce flowering in dahlia. This effect may be quantitative orqualitative, depending on the cultivar; however, longer day lengths arerequired for proper flower development. Konishi and Inaba (1964 and 1966) foundthe optimum day length for flower induction to be 10 hours or less; however,they determined that a day length of 12 hours Á or greater was requiredfor proper flower development. They observed that plants grown undercontinuous, 10-hour photoperiods had a high percentage of aborted flower buds.Halburton and Payne (1978) showed that while long days delayed bud set andflowering, the flowering percentage and overall flower and foliage quality ofDahlia ‘Redskin’ actually improved with long days. Plants grownunder short day lengths were shorter than those grown under longer day lengths,suggesting that photoperiod could be used as a method of height control. Dursoand De Hertogh (1977) observed that natural springtime photoperiods increasingfrom 10-14 hours were optimal for forcing of the tuberous rooted cultivars ‘Kolchelsee’and ‘Park Princess’. Brondum and Heins (1993) determined that theoptimum photoperiodic conditions for the production of ‘Royal DahliettaYellow’ were 12-14 hours at approximately 20° C.

In this article, we will share the results of two studies wehave conducted involving the use of photoperiod manipulation during plug andpot production of Dahlia ‘Sunny Rose’ and ‘SunnyYellow’. The first study demonstrates how night interruption lighting canbe used to inhibit tuberous root formation during plug production (Legnani andMiller, 2000). The second study investigates the use of photoperiodmanipulation for optimal flowering and height of plugs after transplanting into5-inch standard pots.

Materials and Methods — Experiment 1

Night interruption lighting inhibits tuberous root formationin Dahlia ‘Sunny Rose’ plugs.

Dahlia ‘Sunny Rose’ seeds (donated by Ball SeedCo., West Chicago, Ill.) were sown February 7 (two seeds per cell) in 288 plugtrays filled with Fafard Superfine Germinating Mix. The flats were covered withclear plastic wrap and placed in a growth chamber with 24-hour fluorescentlighting at 18° C. Germination occurred in approximately 4-5 days. Sevendays after sowing, flats were thinned to one seedling per cell and moved to aglass greenhouse for photoperiod treatments. Greenhouse night temperatures weremaintained at 17° C, with maximum day temperatures reaching 27° C.During the second week of production, flats were subirrigated and fertilized byimmersion in a tray containing 50 ppm nitrogen and potassium. Following thesecond week of production, as Á seedlings became established enough foroverhead watering, they were fertilized at each watering with 150 ppm nitrogenand potassium. Weekly sprays (to runoff) of 33 ppm A-Rest were appliedbeginning on February 28 for height control.

On day 7 in the greenhouse, five flats were placed under LD(nine hours of natural daylight + night interruption lighting with two 60-wattincandescent lamps between 10 p.m. and 2 a.m.) while the other five received SD(nine hours of natural daylight from 9:00 a.m. to 6:00 p.m.). Seedlings wererandomly harvested at two, four and six weeks following the start ofphotoperiod treatments and growth measurements were made.

Results Experiment 1

Although photoperiod treatments had no effect on total plantdry weight, SD increased tuberous root development at the expense of shootgrowth (See Table 1). These differences in dry weight of shoots and roots werefirst observed at week 4 (See Table 1). By week 6, LD plugs showed a 63-percentincrease in shoot dry weight over SD plugs, but the dry weight of theirtuberous root was less than half that of SD plugs.

No tuberous root formation was evident at week 2. By week 4there was noticeable swelling at the stem bases on both LD and SD plugs,although this swelling was greater on the SD plugs. Adventitious tuberous rootsoriginated from the swollen base of the stems independent of fibrous roots andwere greater in diameter than the latter. These tuberous roots were noticeablylarger on SD than on LD plugs. At week 6, SD plugs had developed large,rounded, tuberous roots while LD plugs had produced slender, elongatedstructures. Tuberous root dry weight was 2.4-fold as great in SD than in LDplugs at week 6. In contrast, LD plugs had a more extensive fibrous rootsystem; dry weight of the roots was twofold as great in LD as in SD plugs (SeeTable 1).

Week 4 plants had greater leaf area under LD than under SDwithout a substantial increase in the average number of leaf pairs. At week 6,LD plugs showed a 55-percent increase in leaf area over SD plugs and hadapproximately one more pair of leaves (See Table 2). Leaf number may affect thetime when the plug is capable of responding to photoperiod for flowerinduction, and thus the time to flower. Barrett and De Hertogh (1978) foundthat nonpinched, tuberous-rooted cultivars ‘Miramar’ and’Park Princess’ became reproductive following the unfolding of 4-6true-leaf pairs under inductive photoperiods.

Shoots were shorter under LD than under SD at week 2 (SeeTable 2). Long-day plugs continued to grow taller while growth in height of SDplugs ceased at week 4. By week 6, LD plugs were 50 percent taller than SDplugs. We suspect that this increase was attributable to both increasedpartitioning to the shoot and increased exposure to far-red light during thenight interruption.

In summary, LD promoted shoot growth, foliar development andfibrous root growth in plugs but reduced growth of tuberous roots. Long-dayplugs were of salable size and quality at week 5 (six weeks after sowing) butSD plugs were not salable until week 6 (seven weeks after sowing).

Materials and Methods — Experiment 2

Using photoperiod manipulation for optimal flowering andheight of Dahlia ‘Sunny Yellow’ plugs after transplanting into5-inch standard pots.

Dahlia ‘Sunny Yellow’ seeds were sown February 7in 288 plug trays and moved to a glass greenhouse in Ithaca, N.Y., on February14 for photoperiod treatment. Greenhouse temperatures were maintained at21° C both day and night. Both LD and SD plugs received nine hours ofnatural daylight from 8:00 a.m. to 5:00 p.m., with LD plugs receiving a nightinterruption between 10:00 p.m. and 2:00 a.m. Weekly sprays of 33 ppm A-Restwere made beginning on February 20. Fertilization was as previously described.On April 7, LD and SD plugs were transplanted into 5-inch standard pots(Metro-Mix 360) and moved to another greenhouse where temperatures weremaintained at 17° C both day and night with maximum daily temperaturesreaching 25° C. Fertilizer rates were increased to 200 ppm nitrogen andpotassium. Both LD and SD plugs were each subjected to six differentphotoperiod schedules after transplanting (10 plants per treatment) over aproduction period of 10 weeks:

1) Long days (as described above) for 10 weeks;

2) One week of short days (as described above) followed bynine weeks of long days;

3) Two weeks of short days followed by eight weeks of longdays;

4) Three weeks of short days followed by seven weeks of longdays;

5) Five weeks of short days followed by five weeks of longdays; or

6) Short days for 10 weeks.

Data collected included days to visible bud, days to firstflower, percent flowering diameter of first flower, height at flowering and dryweight of shoots after 10 weeks. Á

Results — Experiment 2

Following transplanting into 5-inch pots, LD plugs grewfaster than SD plugs. Long-day plugs reached visible bud and flowered earlierthan SD plugs, and providing just one week of SD (followed by LD) aftertransplanting accelerated flowering in LD plugs by 10 days (See Table 3). Twoweeks of SD following transplanting accelerated flowering in SD plugs; however,five or more weeks of SD after transplanting greatly decreased the floweringpercentage (See Table 3). Short days slightly decreased flower size andresulted in shorter, more compact plants when compared to plugs receiving onlyLD after transplanting. Plugs receiving five or more weeks of SD followingtransplanting were stunted, showing poor foliar and shoot development (SeeTable 3).

The data show that LD or SD plugs benefited from 1-2 weeksof short days (followed by LD) after transplanting. These benefits includefaster flowering (7-10 days), slightly larger flowers and shorter, more compactplants. In our study, the highest-quality plants were produced by growing plugsin LD, giving two weeks of SD immediately after transplanting, then finishingthe pots under LD.


Night interruption lighting during dahlia plug productioncan shorten crop time and inhibit tuberous root growth. It is important to keepin mind that the critical day length for tuberous root formation is 11-12hours. If the natural day length is greater than this critical period, the nightinterruption light will likely provide little benefit; however, crops sown inthe short days of late fall and early winter should benefit greatly. It shouldbe stressed that visible effects on inhibiting tuberous root growth were notobserved until four weeks after beginning photoperiod manipulation. This meansthat real benefits will be observed on plugs with a production time of fourweeks or longer.

Following plug transplanting, 1-2 weeks of SD will promoteflower induction. Growing on in LD will accelerate flower development andflowering. Short days also result in a shorter, more compact plant. When thenatural day length is shorter than the critical periods for flower induction(10 hours) or tuberous root formation and flower development (12 hours), wesuggest the following photoperiod schedule for producing 288 plugs and thengrowing on in 5-inch pots:

• Use night interruption lighting during plugproduction;

•After transplanting, provide two weeks of natural SDto induce flowering; and Á

•Provide a night interruption to promote shoot growthand flower development.

If the natural day length is longer than these critical daylengths, grow plugs under natural long days (less than 12 hours). Aftertransplanting into 5-inch pots, provide two weeks of short days using blackcloth and finish the crop under natural LD. In either situation, up to four SDweeks can be used after transplant for additional height control.

Editor’s Note: The use of specific trade names in thispublication does not constitute endorsement of these products in preference toothers containing the same active ingredients. The use of tradenames is solely for the purpose of providing specific information and does notsignify that they are approved to the exclusion of others. Mention of a productdoes not constitute a guarantee or warranty of the product by the author ormagazine.

Literature Cited

Barrett, J. E. and A. A. De Hertogh. 1978. Comparativeinflorescence development of two cultivars of forced tuberous-rooted dahlias.J. Amer. Soc. Hort. Sci. 103:767-772.

Brondum, J.J. and R.D. Heins. 1993. Modeling temperature andphotoperiod effects on growth and development of dahlia. J. Amer. Soc. Hort.Sci. 118:36-42.

Durso, M. and A. A. DeHertogh. 1977. The influence ofgreenhouse environmental factors on forcing Dahlia variabilis Willd. J Amer.Soc. Hort. Sci. 102(3):314-317.

Halburton, M.A. and R.N. Payne. 1978. Effects of photoperiodon growth and flowering of Dahlia pinnata ‘Redskin’ pot plants.Okl. Agric. Exp. Sta. Bull. B. 735:1-19.

Konishi, K. and K. Inaba. 1966. Studies on flowering controlof dahlia. 3. Effects of day-length on initiation and development of flowerbuds. J. Jpn. Soc. Hort. Sci. 35:73-79.

Konishi, K. and K. Inaba. 1964. Studies on flowering controlof dahlia. 1. On optimum day-length. J. Jpn. Soc. Hort. Sci. 33:171-180.

Legnani, G. and W.B. Miller. 2000. Night interruptionlighting is beneficial in the production of plugs of Dahlia ‘SunnyRose’. Hortscience. 35(7):1244-1246.

Moser, B.C. and C.E. Hess. 1968. The physiology of tuberousroot development in dahlia. Proc. Amer. Soc. Hort. Sci. 93:595-603.

Garry Legnani and William B. Miller

Garry Legnani is a Ph.D. student and William B. Miller is professor of flowerbulb and greenhouse crop physiology in the Department of Horticulture at Cornell University, Ithaca, N.Y. They may be reached via phone at (607) 255-4421 or via E-mail at

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