November 2003

German Primrose: A Fresh, New Look By Meriam Karlsson and Jeff Werner

Primula obconica, sometimes referred to as German primrose has soft,pastel-colored flowers in shades of white, pink, orange, purple or lilac. Thisprimrose is attractive as a flowering potted plant or mixed with other plantsin color bowls or containers. German primrose cannot withstand frost but doeswell outdoors during periods of moderate temperatures. The leaves are up to 6inches long on 2- to 4-inch-long stems or petioles. Flowers appear in looseupright umbels or clusters on an extended main flower stalk or peduncle (6-7 incheslong).

Cultivar Development

The use and production of German primrose have been limitedbecause leaves of some cultivars produce an allergen called primin. After yearsof breeding work, primin-free selections are now available. The firstprimin-free cultivars were introduced in 1990 under the names Freedom andBeauty. Some individuals still responded with a skin rash to these cultivarsdespite the claims of no allergen content. In 1995, the Libre series (GoldsmithSeeds) was released as true primin-free selections of the German primrose. TheLibre series grows to heights of 8-10 inches, is suitable for 4- to 6-inchcontainers and comes in several flower colors including White, Pink, Salmon,Red and Blue. Additional primin-free cultivars in the Twilly Touch Me series(Schoneveld Twello) were released in 2000. Sensitive individuals when exposeddirectly to the earlier cultivars responded with a skin rash or in some casessevere dermatitis. The less attractive common name poison primrose probablystems from these problems with potential allergies.

Current Recommendations

As the risk for skin rash has dwindled with newer cultivars,the interest and production of German primrose have increased. Propagation iswith seeds, and the germination process requires 10 days to two weeks at65-68¡ F. Following germination, the temperature can continue at 65-68¡F with transplanting 6-8 weeks later. The expected time from production tomarketing varies from 4-6 months, depending on conditions and cultivar.

German primrose is considered day neutral, and no specificday length is recommended for promoting flowering. Contrary to the response ofEnglish primrose (Primula vulgaris) and polyanthus (Primula X polyantha),reducing the temperature to approximately 40¡ F does not promote Germanprimrose flower formation. Plant quality, flower color and size however,improve in the German primrose at temperatures somewhat lower than 65¡ Fduring the final stages of development.

Conditions for the Study

Information and recommendations for efficient production ofGerman primrose are rather limited. A study was therefore initiated todetermine flowering in response to day length at two temperatures for thecultivar selection ‘Libre Light Salmon’. The seed germinated at 68¡ F andapproximately 500 foot-candles during long days of 16 hours. Four weeks later,seedlings were transplanted into 4-inch pots filled with Premier Pro-Mix BX(Premier Horticulture), the temperature was maintained at 64-72¡ F andirradiance increased to approximately 875 foot-candles for 16 hours each day.

Three weeks after transplanting, when the plants had five orsix expanded leaves, the study was initiated. Plants were grown at 60 or64-72¡ F and days with eight or 16 hours of light. To properly maintain theshort eight-hour days, black cloth was pulled over the plants manually at 4p.m. and retracted at 8 a.m. to avoid interference of adjacent lightedgreenhouse areas and outside lights. Irradiance was 10 moláday-1m-2independent of day length with approximately 875 foot-candles for 16 hours or1,750 foot-candles for eight hours from high-pressure sodium lamps.

This study was conducted during the winter. The influence ofnatural light was therefore limited due to the short days and low sun angles.Horizontal airflow fans were supplemented with fans underneath benches toensure proper air circulation and consistent temperatures around the plantswithout compromising desired day length conditions. As the plants developed,flower bud appearance (0.08-inch large flower buds) and first open flowers werenoted and recorded. The study was terminated 146 days from seeding or 97 daysfrom initiating the experimental conditions.

Results and findings

Time for developing flower buds and reaching floweringvaried for the Libre Light Salmon plants grown at the eight-hour short days andç those grown at long days of 16 hours (see Figure 1, page 70).Temperature also affected floral and overall rate of development. Flower budsappeared 10 days earlier at short days and 60¡ F than at 68¡ F. At longdays, on the other hand, flower buds appeared at a similar time independent oftemperature. The average number of days to flower buds was 38 under long days.

The development of buds into flowers responded strongly tothe temperature and day length conditions. Flowering under long days was morethan a week faster at the higher temperature than 60¡ F. At short days, thedevelopment of buds into flowers slowed or was even halted. Flowering occurredon average after 82 short days at 60¡ F and was delayed compared to longdays with nine or 10 days. No open flowers were observed within the 14 weeks ofshort days and 68¡ F in this study.

Conclusions

Production conditions with longdays were most favorable for the development of German primrose in this study.At the recommended production temperatures of 65-68¡ F, long days appear arequirement for efficient crop development. For rate of development, there wasno advantage of maintaining 68¡ F compared to 60¡ F during early developmentprior to flower bud appearance. These results suggest lower temperaturesfollowed by 65-68¡ F will result in the quickest flowering. Independent oftemperature conditions, long days are likely to produce a faster and more uniform, high-quality crop of German primrose than short-day conditions.