Producing Ravishing Ranunculus

January 28, 2003 - 13:36

University of Alaska-Fairbanks research on ranunculus varieties reveals the best production guidelines for this increasingly popular crop.

Ranunculus (Ranunculus asiaticus L.), with the common name Persian
buttercup, has primarily been produced as a field-grown cut flower, thanks to
extensive breeding work in England during the 19th century that resulted in
more than 500 varieties. These early, corm-propagated varieties grew at least
three feet tall, making them suitable as cut flowers. In the 1960s and 1970s,
efforts were intensified to develop seed-propagated ranunculus. Plants with
dwarf growth habits were also singled out during the breeding process to
identify suitable seed-propagated selections for bedding and potted plant
markets.

In 1983, Sakata Seed America, Morgan Hill, Calif.,
introduced the seed-propagated F1 hybrid series Bloomingdale. The growth habit
of the Bloomingdale series is particularly dwarf, with plant heights of 8-10
inches, and leaf size is reduced to complement the proportionally smaller
plant. Since the Bloomingdale series tolerates temperatures as low as 23°
F, winter plantings are suitable in regions with mild climates. Short and
sturdy stems efficiently support the up to 3-inch-large, double flowers. The
Bloomingdale series was originally available in Red, Rose, Pink, Yellow and
White Shades. Additional colors have continuously been developed, and today,
the assortment includes 14 shades, bicolors and mixtures. The most recently
introduced color choices are bicolors and shades in deep purple.

Current recommendations

The size of Bloomingdale seed is 37,000 per ounce. To
germinate the seed, a medium high in organic matter or a mixture of 30 percent
peat and 70 percent perlite is suitable. Cover the seed with a thin layer of
medium since light is not required for germination. Never allow the medium and
germinating seeds to dry out. Recommended temperature for germination is
50-60º F; avoid temperatures above 68º F at all times. Germination is
expected to take 10-14 days. Even under proper temperature and Á
moisture conditions, germination may be slow, sporadic and uneven. When 4-5
true leaves have developed, approximately eight weeks from seeding, transplant
one seedling per 4-inch pot or 2-3 seedlings into 5-inch pots. Fertilizer
applications are necessary early in development, although the medium should
remain low in soluble salts. Growth is often slow immediately following
transplant and regular fertigation with a complete fertilizer is necessary for
good plant establishment.

Although flowering has been observed at a range of day/night
combinations, 60-68º F has been recommended during the day. Dropping the
night temperature to between 45 and 50º F may improve flower bud development.
Excessive leaf and stem growth combined with few and small flowers are expected
at temperatures above 68º F. The irradiance can be relatively high as long
as temperatures are controlled. Instantaneous levels of up to 3,500
foot-candles (700 µmol·m-2s-1) are suitable prior to flowering and
2,500 foot-candles (500 µmol·m-2s-1) during flowering. Ranunculus
is commonly produced under natural short days. Days longer than 12 hours are
indicated to reduce the number of flowers and flowering plants. Earlier
selections of ranunculus respond to long days with corm formation, reduced
growth and dormancy.

Conditions of the study

Current recommendations and guidelines for production are
primarily based on corm-propagated ranunculus. The cultivar 'Bloomingdale Pure
Yellow' was, therefore, grown under various temperatures and daylengths to
determine suitable conditions for seed-propagated dwarf ranunculus. Germination
and early seedling development occurred at 60º F. Light was turned on
after two weeks at approximately 500 foot-candles (100
µmol·m-2s-1) for 16 hours each day. The seedlings were
transplanted six weeks after seeding into 4-inch pots that had been filled with
a peat-lite medium. Throughout the study, the plants were watered using a fertilizer
solution of 100 ppm nitrogen from a complete fertilizer amended with
micronutrients. The 12 growing environments were 46, 54, 60 or 68º F with
daylengths of eight, 12 or 16 hours. Plants were moved to the different
conditions eight weeks after seeding. To get similar daily amounts of light,
the instantaneous levels were adjusted to 2,000 foot-candles (400
µmol·m-2s-1) during the 8-hour day, 1,400 foot-candles (280
µmol·m-2s-1) during the 12-hour day and 1,000 foot-candles (200
µmol·m-2s-1) during the 16-hour day. These light levels correspond
to approximately 12 mol·day-1m-2. In a Midwestern United States glass
greenhouse, 12 mol·day-1m-2 is expected at bench level during a cloudy
summer day.

In ranunculus, the flower buds form close to the ground in
the rosette of leaves. The flower stems extend during bud development to
display the flowers above the foliage. Visible bud is used here to indicate the
first appearance of flower buds (approximately 1/5-inch-large bud) and
flowering as the initial opening of the flowers.

Findings and results

Temperature and daylength affected both the formation and the
development of flower buds. Visible bud and flowering were faster at longer
days or higher temperatures (See Figure 1, page 46). At 68º F and 16-hour
days, flower buds appeared 13 days from transplant. Two weeks later, these
plants flowered. Visible bud, at 46º F and 16-hour daylengths, was
observed after 29 days and flowering after 66 days. Increasing the day length
from 8 to 12 hours reduced the required time for plant development more than
four hours, from 12-16 hours. Flowering was 10-13 days earlier at days of 12
hours than eight hours. At 16 hours of daily light, flower time was only
reduced 6-8 days compared to 12-hour days.

In this study, plants had 9-16 mature leaves at flowering
(See Figure 2, page 46). The least number of leaves developed at 68º F and
the most at 54º F. Ranunculus grown at 46º F or 60º F had
between 12 and 14 leaves. Daylength had no effect on leaf number, and the
timing of flower formation could not be correlated to any specific number of
initiated leaves.

The tallest point, usually to the top of the flowers (plant
height), and the distance between the soil and the upper level of the foliage
(leaf height) increased with longer days and higher temperatures. The increase
in plant height was similar as temperature increased at the three daylengths.
Figure 3, page 46, illustrates leaf and plant heights under 12-hour days. The
height to the top of the leaves increased from 2.9 inches at 46º F to 5.9
inches at 68º F, and the plant height from 4.3 to seven inches.

Conclusions and recommendations

A few small flowers are expected to develop on tall plants
at high temperatures and long days. This study was terminated at the opening of
the first flower without opportunities to record total flowers per plant.
Observations suggest that you can expect more and larger flowers with improved
longevity at lower temperatures.

High temperatures promoted fast growth and flowering but
were less favorable for plant height and flower numbers. When temperature
cannot be maintained below 60º F, a growth regulator may be needed to
manage stem elongation. A B-Nine (daminozide) foliar spray at 2,500 ppm applied
at initial appearance of flower buds effectively controls height of ranunculus.

Another control measure for height is DIF, or the
relationship between day and night temperatures. Although earlier
recommendations suggest lowering the night temperature for a positive DIF,
constant temperatures satisfactorily supported flower bud development here.
Further studies are required to determine the impact of alternating
temperatures, especially higher night than day temperatures or negative DIF, on
the development of flower buds, leaf formation and plant height. A possible
schedule for producing short, high-quality and fast-growing ranunculus as
bedding plants is an average temperature of 60º F with a negative DIF.
Temperatures of 58º F day and 64º F night for a negative 6º F
DIF at daylengths of 12 hours can potentially be recommended as a suitable
temperature strategy for ranunculus production.

About The Author

Meriam Karlsson is professor of horticulture at the University of Alaska-Fairbanks. She may be reached by phone at (907) 474-7005 or E-mail at ffmgk@uaf.edu.

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