January 2003 Grower News

Producing Ravishing Ranunculus By Meriam Karlsson

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

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

Current recommendations

The size of Bloomingdale seed is 37,000 per ounce. Togerminate the seed, a medium high in organic matter or a mixture of 30 percentpeat and 70 percent perlite is suitable. Cover the seed with a thin layer ofmedium since light is not required for germination. Never allow the medium andgerminating seeds to dry out. Recommended temperature for germination is50-60º F; avoid temperatures above 68º F at all times. Germination isexpected to take 10-14 days. Even under proper temperature and Ámoisture conditions, germination may be slow, sporadic and uneven. When 4-5true leaves have developed, approximately eight weeks from seeding, transplantone seedling per 4-inch pot or 2-3 seedlings into 5-inch pots. Fertilizerapplications are necessary early in development, although the medium shouldremain low in soluble salts. Growth is often slow immediately followingtransplant and regular fertigation with a complete fertilizer is necessary forgood plant establishment.

Although flowering has been observed at a range of day/nightcombinations, 60-68º F has been recommended during the day. Dropping thenight temperature to between 45 and 50º F may improve flower bud development.Excessive leaf and stem growth combined with few and small flowers are expectedat temperatures above 68º F. The irradiance can be relatively high as longas temperatures are controlled. Instantaneous levels of up to 3,500foot-candles (700 µmol·m-2s-1) are suitable prior to flowering and2,500 foot-candles (500 µmol·m-2s-1) during flowering. Ranunculusis commonly produced under natural short days. Days longer than 12 hours areindicated to reduce the number of flowers and flowering plants. Earlierselections of ranunculus respond to long days with corm formation, reducedgrowth and dormancy.

Conditions of the study

Current recommendations and guidelines for production areprimarily based on corm-propagated ranunculus. The cultivar ‘Bloomingdale PureYellow’ was, therefore, grown under various temperatures and daylengths todetermine suitable conditions for seed-propagated dwarf ranunculus. Germinationand early seedling development occurred at 60º F. Light was turned onafter two weeks at approximately 500 foot-candles (100µmol·m-2s-1) for 16 hours each day. The seedlings weretransplanted six weeks after seeding into 4-inch pots that had been filled witha peat-lite medium. Throughout the study, the plants were watered using a fertilizersolution of 100 ppm nitrogen from a complete fertilizer amended withmicronutrients. The 12 growing environments were 46, 54, 60 or 68º F withdaylengths of eight, 12 or 16 hours. Plants were moved to the differentconditions eight weeks after seeding. To get similar daily amounts of light,the instantaneous levels were adjusted to 2,000 foot-candles (400µmol·m-2s-1) during the 8-hour day, 1,400 foot-candles (280µmol·m-2s-1) during the 12-hour day and 1,000 foot-candles (200µmol·m-2s-1) during the 16-hour day. These light levels correspondto approximately 12 mol·day-1m-2. In a Midwestern United States glassgreenhouse, 12 mol·day-1m-2 is expected at bench level during a cloudysummer day.

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

Findings and results

Temperature and daylength affected both the formation and thedevelopment of flower buds. Visible bud and flowering were faster at longerdays or higher temperatures (See Figure 1, page 46). At 68º F and 16-hourdays, flower buds appeared 13 days from transplant. Two weeks later, theseplants flowered. Visible bud, at 46º F and 16-hour daylengths, wasobserved after 29 days and flowering after 66 days. Increasing the day lengthfrom 8 to 12 hours reduced the required time for plant development more thanfour hours, from 12-16 hours. Flowering was 10-13 days earlier at days of 12hours than eight hours. At 16 hours of daily light, flower time was onlyreduced 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 andthe most at 54º F. Ranunculus grown at 46º F or 60º F hadbetween 12 and 14 leaves. Daylength had no effect on leaf number, and thetiming of flower formation could not be correlated to any specific number ofinitiated leaves.

The tallest point, usually to the top of the flowers (plantheight), and the distance between the soil and the upper level of the foliage(leaf height) increased with longer days and higher temperatures. The increasein plant height was similar as temperature increased at the three daylengths.Figure 3, page 46, illustrates leaf and plant heights under 12-hour days. Theheight to the top of the leaves increased from 2.9 inches at 46º F to 5.9inches 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 plantsat high temperatures and long days. This study was terminated at the opening ofthe first flower without opportunities to record total flowers per plant.Observations suggest that you can expect more and larger flowers with improvedlongevity at lower temperatures.

High temperatures promoted fast growth and flowering butwere less favorable for plant height and flower numbers. When temperaturecannot be maintained below 60º F, a growth regulator may be needed tomanage stem elongation. A B-Nine (daminozide) foliar spray at 2,500 ppm appliedat initial appearance of flower buds effectively controls height of ranunculus.

Another control measure for height is DIF, or therelationship between day and night temperatures. Although earlierrecommendations 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 alternatingtemperatures, especially higher night than day temperatures or negative DIF, onthe development of flower buds, leaf formation and plant height. A possibleschedule for producing short, high-quality and fast-growing ranunculus asbedding 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 FDIF at daylengths of 12 hours can potentially be recommended as a suitabletemperature strategy for ranunculus production.