Manipulating Dahlias

December 28, 2001 - 15:12

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 are
controlled by photoperiod; however, photoperiod manipulation is generally not
used in their production. Seed-propagated plug production typically takes place
under the short days of late autumn and early winter. The critical day length
for tuberous root formation is approximately 11-12 hours (Moser and Hess,
1969). When the day length is shorter than this critical period, it signals the
plant to store energy in tuberous roots at the expense of shoot growth. Many
growers may have experienced difficulty when extracting plugs using mechanical
transplanters because the tuberous roots have outgrown the plug tray.

Growers may have also experienced delays in flowering. Short
days induce flowering in dahlia. This effect may be quantitative or
qualitative, depending on the cultivar; however, longer day lengths are
required for proper flower development. Konishi and Inaba (1964 and 1966) found
the 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 required
for proper flower development. They observed that plants grown under
continuous, 10-hour photoperiods had a high percentage of aborted flower buds.
Halburton and Payne (1978) showed that while long days delayed bud set and
flowering, the flowering percentage and overall flower and foliage quality of
Dahlia ‘Redskin’ actually improved with long days. Plants grown
under short day lengths were shorter than those grown under longer day lengths,
suggesting that photoperiod could be used as a method of height control. Durso
and De Hertogh (1977) observed that natural springtime photoperiods increasing
from 10-14 hours were optimal for forcing of the tuberous rooted cultivars ‘Kolchelsee’
and ‘Park Princess’. Brondum and Heins (1993) determined that the
optimum photoperiodic conditions for the production of ‘Royal Dahlietta
Yellow’ were 12-14 hours at approximately 20° C.

In this article, we will share the results of two studies we
have conducted involving the use of photoperiod manipulation during plug and
pot production of Dahlia ‘Sunny Rose’ and ‘Sunny
Yellow’. The first study demonstrates how night interruption lighting can
be used to inhibit tuberous root formation during plug production (Legnani and
Miller, 2000). The second study investigates the use of photoperiod
manipulation for optimal flowering and height of plugs after transplanting into
5-inch standard pots.

Materials and Methods — Experiment 1

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

Dahlia ‘Sunny Rose’ seeds (donated by Ball Seed
Co., West Chicago, Ill.) were sown February 7 (two seeds per cell) in 288 plug
trays filled with Fafard Superfine Germinating Mix. The flats were covered with
clear plastic wrap and placed in a growth chamber with 24-hour fluorescent
lighting at 18° C. Germination occurred in approximately 4-5 days. Seven
days after sowing, flats were thinned to one seedling per cell and moved to a
glass greenhouse for photoperiod treatments. Greenhouse night temperatures were
maintained at 17° C, with maximum day temperatures reaching 27° C.
During the second week of production, flats were subirrigated and fertilized by
immersion in a tray containing 50 ppm nitrogen and potassium. Following the
second week of production, as Á seedlings became established enough for
overhead watering, they were fertilized at each watering with 150 ppm nitrogen
and potassium. Weekly sprays (to runoff) of 33 ppm A-Rest were applied
beginning 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-watt
incandescent 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 were
randomly harvested at two, four and six weeks following the start of
photoperiod treatments and growth measurements were made.

Experiment 1

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

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

Week 4 plants had greater leaf area under LD than under SD
without 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 had
approximately one more pair of leaves (See Table 2). Leaf number may affect the
time when the plug is capable of responding to photoperiod for flower
induction, and thus the time to flower. Barrett and De Hertogh (1978) found
that nonpinched, tuberous-rooted cultivars ‘Miramar’ and
‘Park Princess’ became reproductive following the unfolding of 4-6
true-leaf pairs under inductive photoperiods.

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

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

Materials and Methods — Experiment 2

Using photoperiod manipulation for optimal flowering and
height of Dahlia ‘Sunny Yellow’ plugs after transplanting into
5-inch standard pots.

Dahlia ‘Sunny Yellow’ seeds were sown February 7
in 288 plug trays and moved to a glass greenhouse in Ithaca, N.Y., on February
14 for photoperiod treatment. Greenhouse temperatures were maintained at
21° C both day and night. Both LD and SD plugs received nine hours of
natural daylight from 8:00 a.m. to 5:00 p.m., with LD plugs receiving a night
interruption between 10:00 p.m. and 2:00 a.m. Weekly sprays of 33 ppm A-Rest
were 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 were
maintained at 17° C both day and night with maximum daily temperatures
reaching 25° C. Fertilizer rates were increased to 200 ppm nitrogen and
potassium. Both LD and SD plugs were each subjected to six different
photoperiod schedules after transplanting (10 plants per treatment) over a
production period of 10 weeks:

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

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

3) Two weeks of short days followed by eight weeks of long

4) Three weeks of short days followed by seven weeks of long

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

6) Short days for 10 weeks.

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

Results — Experiment 2

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

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


Night interruption lighting during dahlia plug production
can shorten crop time and inhibit tuberous root growth. It is important to keep
in mind that the critical day length for tuberous root formation is 11-12
hours. If the natural day length is greater than this critical period, the night
interruption light will likely provide little benefit; however, crops sown in
the short days of late fall and early winter should benefit greatly. It should
be stressed that visible effects on inhibiting tuberous root growth were not
observed until four weeks after beginning photoperiod manipulation. This means
that real benefits will be observed on plugs with a production time of four
weeks or longer.

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

• Use night interruption lighting during plug

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

•Provide a night interruption to promote shoot growth
and flower development.

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

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

Literature Cited

Barrett, J. E. and A. A. De Hertogh. 1978. Comparative
inflorescence 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 and
photoperiod effects on growth and development of dahlia. J. Amer. Soc. Hort.
Sci. 118:36-42.

Durso, M. and A. A. DeHertogh. 1977. The influence of
greenhouse 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 photoperiod
on 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 control
of dahlia. 3. Effects of day-length on initiation and development of flower
buds. J. Jpn. Soc. Hort. Sci. 35:73-79.

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

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

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

About The Author

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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