August 2002

Primula: an all-time favorite By Meriam Karlsson

The most important primula species in production todayinclude English primrose or acaulis primula (Primula vulgaris, synonym P.acaulis), polyanthus, polyantha primrose or hybrid primrose (P. xpolyantha),fairy primrose or baby primrose (P. malacoides), German primrose or poisonprimrose (P. obconica) and cowslip (P. veris). Chinese primrose (P. sinensis)is also produced in limited numbers and some interest exists in producingdrumstick primrose (P. denticulata) as a container plant.

Primula is widely produced in Europe as a flowering pottedplant for the late winter or early spring market. In the United States, growingconditions in the Pacific Northwest are particularly well-suited for producingprimula. For garden and bedding uses, English primrose and polyanthus areconsidered hardy to Zone 5, while fairy primrose, German primrose and Chineseprimrose cannot withstand frost. Species that are more appropriate for garden plantingsinclude garden auricula (P. xpubescens, a cross between P. auricula and P.hirsuta), cowslip, Julian primrose (P. juliae), siebold primrose (P. sieboldii)and red-leaf primrose (P. rosea).

Cultivars

English Primrose.The English primrose has 2- to 10-inch-long leaves in a compact rosette, assuggested by the word acaulis, meaning “stemless.” The flowersdevelop on individual, 2- to 8-inch-long pedicels from the center of the plant.The native English primrose has pale yellow flowers. Through breeding effortsand the development of F1 hybrids, cultivars of English primrose are nowavailable with flowers in many colors from white to purple. The flowers mayhave a yellow or white eye and are sometimes fragrant.

English primrose has been extensively bred, and a largenumber of cultivars are available. The Danova series was introduced in 1989 byDæhnfeldt Inc. (Odense, Denmark) and may be the most widely producedprimula cultivar today. The Danova series is intended for early-seasonmarketing in November and December in the Northern hemisphere from a Juneseeding. The Danova series comes in more than 20 colors and includes severalcultivars with 2-toned flowers. Other early-season cultivars include the Lovelyand Pageant series, bred by the Sakata Seed Corporation (Yokohama, Japan), andthe Quantum series from Goldsmith Seeds Inc. (Gilroy, Calif.). The Lovelyseries has smaller flowers than the Pageant series, although they both have acompact growth habit suitable for 4-inch or smaller pots. Several red and pinkbicolors are included in the Pageant series. The Quantum series has uniformgermination rates and good postharvest quality.

The Dania (Dæhnfeldt Inc.), Finesse (Ernst Benary SeedGrowers Ltd., Hann. Muenden, Germany) and Gemini (Goldsmith Seeds Inc.) seriesare suitable for mid-season marketing (January and February). Compared to theDanova series, the Dania series has somewhat larger flowers but a more limitedcolor range. A unique feature of the Finesse series is the narrow silver orgold border of the flower petals. The limited leaf growth of the Gemini seriesmakes it suitable for 4-inch pots or in combination with other plants in patioplanters or color bowls.

Cultivars for late-season marketing (February and March)include the Daniella (Dæhnfeldt Inc.), Joker (Ernst Benary Seed GrowersLtd.) and Paloma (Royal Sluis Ornamentals, Leyland, UK) series. The Daniellaseries was introduced Á

in 1995 as a complement to the Danova series to extend themarketing period. The Joker series consists of both bicolored and clearflowers, and the Paloma series is a leading cultivar in Europe.

Polyanthus.Polyanthus is a hybrid, primarily between cowslip, oxlip (P. elatior), Englishprimrose and Julian primrose. The flowers of polyanthus develop in a cluster onan extended main flower stalk or peduncle (4-6 inches). Sometimes thisinflorescence is referred to as the polyanthus-type in contrast to thesingle-pedicel acaulis type. Production guidelines, flowering requirements andplant development are similar for polyanthus and English primrose.

The flower colors of polyanthus are dominated by yellows andreds but cultivars with white, purple, bronze or gold flowers, with or withouta yellow or white eye, are available. For years, the most important polyanthuscultivar has been the Pacific Giant series. The Sakata Seed Corporation hasbeen maintaining and developing the Pacific Giant series since 1968. Thisvigorous-growing series with long peduncles has large clusters of flowers. Thenewer Concorde (Dæhnfeldt Inc.), Hercules (Royal Sluis Ornamentals) andRumba (Goldsmith Seeds Inc.) series have more compact but stronger pedunclesthan Pacific Giant. The Hercules series is also known for its postharvestability to ship well.

Fairy Primrose. Theflowers of the fairy primrose are arranged in a loose cluster with 2-6superimposed whorls of 4-6 flowers. The flowers come in white, pink, red, mauveor lavender shades. The leaves are 1-2 inches in length with 2- to 3-inch-longpetioles. The primary use of the fairy primrose is as a flowering potted plant,since sensitivity to frost limits bedding and landscape applications.

The Prima series (Dæhnfeldt Inc.) is the most commonlygrown fairy primrose. This series comes as a seed mix of pastel flower colors witha large proportion of bicolored pink and rose flowers or segregated intospecific flower colors. The expected production time is 5-6 months, and thegrowth habit is uniform and compact.

German Primrose. TheGerman primrose has leaves up to six inches long with petioles of 2-4 inches.Flowers appear in loose, upright clusters on 6- to 7-inch-long peduncles.Cultivars are available with flowers in many pleasing, soft pastel shades fromwhite to lilac, purple, pink and orange. Expected production time for Germanprimrose varies from 4-6 months. The leaves of some German primrose cultivarsproduce the allergen primin, which may cause skin dermatitis. The risk forallergic reactions has limited its use and production. It is advisable toobserve caution when producing, handling and marketing the German primrose andlimit exposure by wearing gloves and long sleeves.

The German primrose series Juno (S&G Flowers, DownersGrove, Ill.) has been grown extensively in the United States. The Juno serieshas a height of 12 inches, relatively small leaves and abundant flowering. Therecent development and introduction of cultivars that do not produce primin hasreduced the skin rash problem and renewed the interest in German primrose. Thefirst cultivars stated to lack primin were introduced in 1990 under the names‘Freedom’ and ‘Beauty’. Some individuals, however,developed a skin rash following direct contact with these cultivars. In 1995,the Libre series (Goldsmith Seeds Inc.) was released as the first trueprimin-free selection of the German primrose. The Libre series grows shorterthan the Juno series at 8-10 inches, is suitable for 4- to 6-inch pots andcomes in several flower colors, including white, pink, salmon, red and blue.Schoneveld Twello b.v. (Twello, The Netherlands) recently released theprimin-free Twilly series Touch Me.

Chinese Primrose.The leaves of the Chinese primrose have a rounder shape than the other speciesdiscussed here, and the leaf margins are scalloped. Cold hardiness is limitedto Zones 8-10. The flowers of the Chinese primrose form clusters on 4- to6-inch-long peduncles. Various flower colors are available, from white andpurple to pink. Pigments often color the roots and the lower sides of theleaves red. The Fanfare series (Dæhnfeldt Inc.) is produced in 5-6 monthsand has exceptionally good shelf life, with large flowers. DrumstickPrimrose. Selections of drumstick primroseare suggested to survive temperatures down to 50° F and have potential formarketing as a flowering potted perennial for Northern landscapes. Flowersappear in a dense, globular cluster on a peduncle. The flower color is white orvarious shades of purple.

Cowslip. The cowsliphas fragrant, bright yellow flowers on an extended peduncle and is used inrock, alpine or other types of gardens. Flower colors other than yellow(orange, apricot, crimson, light purple and white) are now offered. The abilityto withstand low temperatures varies for cowslip from Zone 3 (-40 to -30°F) to Zone 8 (10-20° F).

Propagation

Primula is exclusively propagated by seed. Germination isoften erratic and low, but improved seed quality and close attention tomoisture and temperature conditions have improved germination uniformity andrates. A well-drained peatlite medium works well for germination and earlyseedling development. A medium with 60 percent fine peat, 25 percent perliteand 15 percent vermiculite may be recommended for primula plug production. Thesowing medium should have a low nutrient content and a pH between 5.5 and 6.0.A medium electrical conductivity (EC) less than 0.75 dS·m-1 is essentialas high soluble salt levels may interfere with germination.

Recommended germination temperature is 60° F for Englishprimrose, polyanthus and Chinese primrose; 60-65° F for the fairy primrose;and 65-68° F for German primrose. For good germination, maintaining thetemperature below 70° F for English primrose and polyanthus is important.Chilling the English primrose seed to 40-50° F for one or two weeks toimprove germination at 65° F has been suggested. The response to chillinghas been variable and may depend on the cultivar and seed quality. One breedinggoal has been to improve germination. The currently available cultivars havefaster and more uniform germination rates and do not appear to require orbenefit from a low-temperature seed treatment.

Several reports indicate that light is required for maximumgermination. In other studies, light was not necessary for germination butuseful for controlling the height of seedlings immediately after germination.In addition to providing light, sufficient moisture appears crucial for goodgermination and early seedling development. Covering the seeds with a layer ofvermiculite, perlite or any well-aerated medium can be used to improve the humidityaround the seeds. The layer must remain thin due to the potential lightrequirement. The small seeds of fairy and German primrose especially benefitfrom a thin cover to avoid desiccation. For English primrose and polyanthus, aprotective layer can be added later when the radicle emerges 7-10 days afterseeding.

Early plant development

The germination process requires 10-14 days. Followinggermination, the temperature can continue at 60-65° F for English primrose,polyanthus and fairy primrose, and 65-68° F for German primrose. After 6-8weeks, seedlings should have two or three true leaves and are suitable fortransplanting.

A single seedling is planted in a 3- or 4-inch pot. Forlarger pots, two or more seedlings are used. Similar to the germination medium,the growing medium should be high in organic matter and well-drained at a pH of5.5-6.0. The planting depth should be the same as in the seedling flat to avoidcrown rot and other diseases. Plants are spaced when leaves reach the edge ofthe pot. Suitable final spacing for 4-inch pots is four pots per sq ft.

Flowering requirements

English Primrose and Polyanthus. Production guidelines for flower initiation inEnglish primrose and polyanthus include several weeks at 40-50° F. Thetemperature is dropped when plants have developed a good root system and 6-10leaves. To promote bud set, some growers increase the fertilizer rate from60-200 ppm nitrogen and double the potassium rate in relation to nitrogen twoweeks prior to temperature drop. The use of calcium nitrate and potassiumnitrate with low-proportion ammonium nitrogen has worked well. When flower budsare visible, the temperature can increase to between 50 and 55° F or remainbelow 50° F.

Cooling plants for up to 10 weeks increases the quality ofEnglish primrose and polyanthus by increasing flower number, maintainingsmaller leaf size and reducing the pedicel length. The lower temperature,however, slows overall plant development. Flower initiation has been observedin plants grown continuously at 60-68°F, and the newer cultivars do notrequire a cold treatment.

Slower flower initiation at lower temperatures has beenconfirmed in more recent studies with rapidly flowering . Flower initiation at46° F was especially slow when combined with low light (2mol·d-1m-2) and short days (eight hours). Estimated optimum temperaturefor flower initiation was 55° F. At 75° F, the English primrose Daniafailed to initiate flower buds.

A higher night than day temperature (negative DIF) has beensuggested to hasten flower initiation in English primrose. Flower formation anddevelopment was faster at the negative (54/70° F, day/night) than thepositive DIF (75/54° F) at the same average daily temperature. Especiallyunder short days (eight hours), negative DIF appears to promote faster budformation and flowering than constant or positive DIF.

Even though primula is considered a low-light crop, dailyirradiance affected flower initiation efficiency. From seeding to floralinitiation at 54° F and long days (more than 12 hours), time to initiationdecreased from 72-57 days for English primrose at 10 mol·d-1m-2 comparedto 2 mol·d-1m-2. The optimum daily irradiance for flower initiation wasestimated to be 11 mol·d-1m-2 or approximately 1,250 foot-candles (250 µmol·m-2s-1)for 12 hours. High natural light conditions may warrant shading to preventsunscald and improve temperature control since plant quality of Englishprimrose decreases above 68° F. Maximum peak irradiance in primulaproduction should not exceed 3,000 foot-candles (600 µmol·m-2s-1).For most rapid flower initiation, the optimum daily light integral appears toincrease from 11-13 mol·d-1m-2 with either an increasing or decreasingtemperature from 55° F.

In contrast to earlier studies and recommendations, longdays have been found more beneficial than short days (less than 12 hours) forrapid flower initiation and development. Cultivars that initiate flowerswithout a drop in temperature appear to also have altered response to daylength. Long days are more beneficial for flowering of cultivars that do notrequire a low temperature exposure. When natural day lengths are less than 12hours, extending the day to 16 hours or utilizing 4-hour night interruptions ata minimum of 10 foot-candles (2 µmol·m-2s-1) is now recommended.

Unsightly long and large leaves have been correlated to longdays or night break treatments and therefore avoided in primula production.However, temperature now appears to be more critical than day length for plantmorphology. In polyanthus, more and smaller leaves were recorded at 68° Fthan at 50° F, although plant leaf areas remained similar at the twotemperatures. High fertilizer levels and nitrogen in ammonium form are alsofactors likely to result in large plants and leaves. Á

Primula flower initiation probably does not relate to plantleaf area, leaf number or plant maturity. In Dania, flower initiation wasidentified in plants with 6-26 leaves. At 46° F, 10 mol·d-1m-2 and11-hour day length, plants with six leaves initiated flowers in 72 days fromseeding. At 68° F, 2 mol·d-1m-2 and 8 hours day length, plantsinitiated flowers in 138 days but had 26 leaves.

Fairy Primrose. Adrop in temperature for six weeks is recommended for flower bud initiation offairy primrose. The temperature is reduced to between 45 and 50° F whenplants have reached the desired marketable size. Following six weeks of reducedtemperatures, 57-65° F is recommended. Fairy primrose will initiate flowerswithout a reduction in temperature, although the final plant and flower qualitymay not be as high.

Flower bud formation in the fairy primrose‘Prima’ was independent of day length when grown at 60 or 68°F.The appearance of flower buds was 20 days earlier at 60° F than at thehigher temperature. The development of buds into flowers on the other hand, wasaffected by day length and was more rapid with long (16 hours) than short (8hours) days at either temperature.

German Primrose.German primrose is commonly grown commercially at 65-68° F. Reducing the temperatureto induce flowers is not recommended; however, the temperature can be droppedto lower than 65° F during the final stages of plant development to improveplant quality, flower color and size.

Guidelines for producing German primrose usually do notinclude specific recommendations for day length. A high light integral (minimum10 mol·d-1m-2) may shorten the production time, although during thesummer, shading is required to control temperature and avoid burning of thefoliage. The response to day length is temperature-dependent. The Germanprimrose Libre had 0.08-inch-large flower buds 90 days from seeding when grownat 60° F and 8- or 16-hour day length or under 16 hours at 68° F. Shortdays (eight hours) at 68° F delayed bud appearance by two weeks and flowersfailed to develop within 145 days from seeding. In contrast, long days at68° F resulted in flowering after 111 days. At 60° F, the primin-freeLibre flowered faster under long (122 days) rather than short day length (133days).

Nutrition

Fertilizing should start as soon as the cotyledons begin todevelop, about two weeks from seeding. The initial fertilizer rate should below at levels of 60 ppm nitrogen and potassium. The rate can increase up to 200ppm nitrogen immediately prior to transplanting. During the production phase,fertilizer rates of 90-100 ppm nitrogen and potassium from a completefertilizer with micronutrients are suitable. Excessive nitrogen and fertilizereasily result in plants with too much leaf growth. Primula is sensitive to highsoluble salt levels that may result in necrotic leaf margin burns. Avoidnitrogen in ammonium form under growing conditions with low temperature andirradiance. High ammonium nitrogen levels may result in plants withdisproportionately long leaves. Micronutrient deficiencies or toxicities arecommon in primula production and a medium pH between 5.5 and 6.2 is essentialfor micronutrient availability. Regular monitoring of pH, soluble salts andnutrient balance, through soil tests, is highly recommended.

Nitrogen deficiency appears as chlorosis in newly developingleaves and as a chlorosis and necrosis of older plant tissues. Prematureflowering may also occur under nitrogen deficiency. The initial symptoms of aphosphorous deficiency are bronzing of older leaves, inward curling of youngerleaves and leaf tip necrosis. Similar to nitrogen deficiency, plants mayprematurely flower. Signs of low potassium are chlorotic lower leaves slowlyturning necrotic and curling of leaf margins. Some plants may also diefollowing unexpected and sudden wilting with potassium deficiency. Polyanthusgrown with low or no calcium had poor root growth and pale green foliage.Magnesium deficiency appeared as interveinal chlorosis with tip and marginalnecrosis of older leaves.

Primula is highly sensitive to reduced iron availability,and deficiency symptoms are common. Early symptoms of iron deficiency arechlorotic new growth followed by completely bleached white tissue due to a lackof chlorophyll. Iron uptake and availability are greatly restricted above pH 6.A boron deficiency initially appears as a light green color later turning intochlorosis of recently matured leaves. The leaves may get cupped or crinkledwith leaf edges turning downward. The veins become excessively prominent,especially on the lower side of the leaf. If a boron deficiency is notcorrected, the apical growing point dies and the stem becomes hollow.

German primrose requires higher fertilizer levels than theother primula species. Nitrogen rates of 250 ppm with equal or greaterpotassium levels are recommended for German primrose. On the other hand, Germanand fairy primrose are highly sensitive to elevated soluble salts, whichmanifests as leaf edge necrosis. Leaching at regular intervals is recommendedto avoid salt buildup.

Irrigation

During germination and early seedling development, themedium should never be allowed to dry out. Seedlings are, however, sensitive toover-watering. When seedlings of English primrose and polyanthus have establishedfollowing transplant, they can be allowed to dry slightly before being wateredthoroughly again. Fairy and German primrose are more sensitive to moisturestress, and the media should be kept continuously moist. Plants allowed to dryor grow at uneven moisture readily develop brown, dried leaf edges. High saltlevels in the medium result in similar symptoms as water stress in fairy andGerman primrose.

Diseases and insects

Primula is relatively pest-free. Aphids, thrips, whitefliesand caterpillars are the most commonly encountered pests. The primin-free Libreis reported to be more susceptible to thrips than primin-containing cultivars.During germination and early seedling development, fungus gnats and shorefliesmay become a problem. Control of algae growth and the use of insecticides maybe required to restrict the fungus gnat populations during early stages ofplant growth.

Tomato spotted wilt virus (TSWV) and impatiens necrotic spotvirus (INSV) have been identified in primula. The symptoms include browningalong the veins of the leaves and yellow mottling. No chemical controls existfor viral diseases. The best control of TSWV and INSV is to rogue infestedplants and eliminate thrips, the insect vector that spreads the virus.

Postharvest

Primula is marketed when the first 5-7 flowers have opened.Proper temperatures for shipping and holding are 36-43° F, and maintainingwell-watered plants is vital for longevity. Primula is highly sensitive toethylene. A silver thiosulfate spray application at 65-165 ppm has successfullyimproved the keeping quality of English primrose.

The home environment is often at low relative humidity andhigher temperatures than the preferred 60-65° F. Although the keepingquality is expected to be limited under these conditions, a high-qualityprimula should flower and remain attractive for 10-12 days. With proper care,the German primrose is expected to continue flowering for 2-4 weeks in anappropriate postharvest environment.