an old bulb forcing faq

FORCING BULBS FOR WINTER BLOOM 1st draft, 2 October 1992 "Do you think 200 paperwhites are enough? I'm giving a few pots for Christmas presents, and I haven't got enough refrigerator space to do that many tulips..." --3rd year bulb-forcer, who just discovered wholesale catalogs A. Why force bulbs? To (in)temperate zone gardeners, winter can be those 8 months before there's something blooming outside again. About mid-January, seed catalogs pall, and snow piles up, and the gardeners get a bit testy. There are several ways of coping with the winter-garden blues. Pruning the apple orchard is productive, but a bit chilly. Revamping the entire garden on paper, adding drystone walls, subterranean irrigation, a new pond and small arboretum is exciting, but it's a bit expensive to actually do the work and guilt-inducing to not do the work... Or, with a bit of planning, you can beat the winter doldrums and have spring flowers all winter, with modest space and equipment requirements. Because flower buds are already formed in dormant bulbs, if you buy bulbs from a reputable supplier, you are almost guaranteed success. This FAQ was prepared by Kay Lancaster, and reflects her experiences and prejudices. B. Bulbs that need no prechilling The simplest bulbs to "force" (bring into out-of-season bloom) are species that require no prechilling. These include _Hippeastrum_ (amaryllis), _Narcissus tazetta_ (paperwhite narcissus), and _Colchicum autumnale_ which will bloom in the fall just sitting on a shelf. Cultural instructions are in the list below. C. How to force bulbs 1. Equipment and supplies Equipment needs are fairly minimal: a sunny, cool window for bringing the bulbs to flower, pots and ordinary potting soil for growing, and a refrigerator, coldframe, or a box stuffed with leaves-- something to maintain the pots at about 40oF. Choice of bulbs is critical: some species and cultivars force better than others, and larger bulbs give a better show than smaller bulbs of the same cultivar. 2. Planting the bulbs Some authors advocate special "bulb fiber" for forcing, or specially shaped glasses to allow bulbs to be rooted in water. Others specify a mixture of pebbles and charcoal, glass marbles, or similar non-absorbent rooting media. My experience with bulb fiber is that it is good, but expensive. The other media are prone to a number of ills, including tipping, rot, and malodors. However, most professional growers use a simple, well-drained, standard potting soil in an ordinary clay or plastic pot with drain holes. This offers a heavy enough base that top growth won't flip the pot. If available, "bulb pans" or "azalea pans" (pots that are very squat or sort-of-squat in comparison to the standard flower pot shape) can be ^ Prepare the pot as if you were potting a house plant. New clay pots should be soaked in plain water for 24 hours before use*. Old clay or plastic pots should be scrubbed, disinfected and dried** before re-use. Crock the pot (put broken overlapping pot shards into the pot) to prevent soil running out the drain holes. Add enough soil so that the tips of the bulbs set on this base are about 2" (5 cm) below the rim. Place the bulbs on this base, one cultivar per pot, with the bulbs just barely separated. Put as many bulbs in as will fit. Continue filling the pot with soil to about 3/4" (2 cm) below the rim. Tag with cultivar and date. Place filled pot in water and allow the soil to soak up from the bottom; drain for 12 hours, then placed into the chilling chamber. Don't forget to write some "check on it dates" on the calendar! * New clay pots can "steal" water from roots as the clay molecules rehydrate from firing. ** Scrub mineral deposits and old soil and algal crusts from pots with a stiff-bristled brush. Soak clean pots in 10% sodium or calcium hypochlorite solution (chlorine bleach) diluted to 10% of commercial strength for an hour. Drain, rinse in clean water, and air dry until no trace of chlorine smell remains. It is possible to use undrained containers for bulb forcing, but I do not recommend this until you've mastered forcing at least a few species in ordinary pots-- watering is infinitely trickier in undrained pots. Instead choose a growing pot that will slip into the undrained display container. 3. Chilling period Place the pots in a cool chamber: something with a temperature that can be maintained at 35-40oF. This can be a refrigerator (beware, though, ethylene gas from ripening fruit, esp. apples, can cause flowers to abort), a cold basement or root cellar, a cold frame, or an outdoor trench filled with insulating but non-freezing materials such as straw over a gravel base. A cardboard box stuffed with straw and placed on insulating materials like a couple of inches of styrofoam will provide adequate temperature control in an unheated garage in USDA zones 4 or 5. You will want to be able to check on the pots about once a month. Arrange the pots in the chilling chamber, and remember to give them a drink every month or so. Actual length of the chilling period will depend on planting date and cultivar. In general, pots are ready to be brought on (grown) when the root system is well developed (go ahead, turn out a pot and peek!) and the new shoots are an inch or so high. 4. "Bringing them on" Abnormally high temperatures (for spring bulbs) will lead to loose, floppy growth, and perhaps flower abortion (esp in Iris). 50-55oF is an good growing temperature for most hardy spring bulbs, often obtainable in a bright windowsill, or in a basement under fluorescent light. Put new pots in half-bright light for a couple of days until the shoots green up, then move to a strongly lit cool area to keep growth compact. In general, do not fertilize (if you must, no N-- it makes floppy leaves!). You may also need to do a certain amount of discreet staking: I use chopsticks and a variety of shades of green embroidery floss.
5. Aftercare After bloom has finished, the hardy bulbs are badly exhausted. If you feel morally obliged to save the bulbs, move the pot back to a cool, very bright area and water with a balanced fertilizer solution. As soon as possible, remove the soil and bulb mass and transplant to a discreet location in the service garden where the foliage can be allowed to die off naturally. Transplant to a better location in fall, but do not expect good bloom for 2-3 years. D. Species and cultivars suitable for forcing Species marked as "Green thumb" require some experimentation and at least a coldframe or cool greenhouse; other species are suitable for growing entirely in the house. A rough pronunciation is given after the genus name; these are "American Botanical Latin", based on Church Latin. European botanists and gardeners tend to use Classical Latin pronunciation, with some slightly different vowel and consonant sounds. For instance, Americans would pronounce "Julius Caesar" as JEW-lee-us SIEZE-er; Classical Latin would be approximately YOU-lee-us KI-sar. ======================================================== BABIANA [Baboon root] rating: Green thumb [bah-bee-ANN-nah] Sunny; Growing temp: 40-50oF 6-10" tall with dark green, pubescent foliage and blue, violet, red or white flowers. 5 bulbs per 4.5" pot, 2.5-3.5" deep, sandy soil mix, strong sun. B. disticha: blue [DIS-tick-ah] B. plicata: violet and blue [pleh-CAH-tah] B. ringens: scarlet [RIN-jens] B. stricta: white with lilac [STRICT-tah] ________________________ COLCHICUM rating: Brown thumb [COAL-chee-come or COAL-key-come] Any light, including none; temps above freezing Technically, a corm. Will bloom sitting on a bookshelf, producing 8- 20 "giant crocus" flowers. Plant outdoors in sunny, well-drained soil, 2" deep. Leaves produced spring and summer. Hardy to southern Canada. C. autumnale var. album: white single [aw-tum-NAH-leh] C. autumnale var. minor: rose-lilac, star-shaped, shorter than C. autumnale. C. autumnale: pink-lavender single C. byzantinum rose-lilac single [biz-zan-TEEN-num] ---Hybrids and cultivars--- Autumn Queen (+ Princess Astrid): purple checkered over white Lilac Wonder: late-flowering lilac The Giant: late-flowering lilac-mauve with white base Waterlily: double lavender ________________________ CONVALLARIA MAJUS [Lily of the Valley] rating: Green thumb [con-val-AIR-ee-ah MAY-jus] Shaded window; Growing temp 55-65oF You can buy Lily of the Valley pips prepared for forcing, and even arrange for them to be shipped to you throughout the winter. The prepared pips are easy to grow, and will flower 3 weeks after planting. Maintain a 60-65oF temperature at first, rising to 70oF and full light. It is also possible to grow Lily of the Valley from your own plants: This is much less certain, but fun to try. Treat your Lily of the Valley well the previous growing system, applying a balanced fertilizer several times in the growing season. Before hard frost, select a healthy clump and put it in a dark cool place, watering once a week. About 1 February, bring into light. As the flowering stalks appear, you can separate out the flowering pips from those that are too small to bloom, and put the flowering pips in a container of pebbles and water. Or just let the clump bloom. In the spring, you can set the clump back in the bed for bloom some other year. ________________________ CROCUS rating: Easy [CROW-cuss] Sunny window; Growing temp: 45-55oF Both fall and spring blooming species are available. Fall crocus (Crocus sativus, C. speciosus, C. byzantinus) is potted upon receipt in August or September. Pots will bloom almost immediately in an east-facing window. Remove from the pot after flowering and set out in the garden. Spring flowering crocus includes both species and hybrid crocus. For reasons I do not understand, I've never had luck forcing yellow crocus, though the lavender and white cultivars flower easily. Plant several corms just below the surface of the soil in a 4" pot; hold in the cold until shoots reach about 1.5" (3-4 cm); bring into a dim 50oF room, then to a sunny 55oF window. Higher temperatures can prevent flowering. Children may enjoy "crocus balls"; crocus corms are embedded in sphagnum moss, shaped into a ball, and enclosed in coarse netting or a network of string. The sphagnum ball is moistened, then chilled. Following the pre-chill period, the ball is suspended on a string until bloom. Water by dipping in a bucket, being sure to place a pan on the floor after to catch the drips! ________________________ ERANTHUS HIEMALIS (winter aconite) rating: Experienced [air-AN-thus hi-em-MAY-lis] Shaded window; Growing temp 45-55oF Eranthus hiemalis, winter aconite, forces about 70% of the time for me; I plant the tubers just below the soil surface, and treat as for tulips. They will flower in late January if brought in to a sunny cool window in late December. ________________________ ERYTHRONIUM (trout lily; dogtooth violet) rating: Experienced [air-reh-THRONE-knee-um] Shaded window; Growing temp 45-55oF Most species and hybrids are handled just as for Eranthus; but planted 3" deep, several per pot. ________________________ GALANTHUS (snowdrop) rating: Experienced [ga-LAN-thus] Sunny window; Growing temp 40-50oF Galanthus nivalis [niv-VAL-us] has smaller flowers than G. elwesii [el- WES-ee-eye], but both are small charmers with green markings on the perianth that look a bit like rabbits. Plant several to a pot, and force slowly as for tulips. These prefer sun and 45-50o temperatures. Blossoms open in warmth, close in cooler temperatures. This genus will survive and grow if planted in the garden. ________________________ HIPPEASTRUM [Amaryllis] rating: Easy [hippie-AST-rum] Shaded window before flowering, sunny after; Growing temp 65-75oF Standard size: 20-25", blooms 8-10 weeks after planting, 3-4 flowers 5-6" across, per stem. Usually 2 stems per bulb. Plant in potting soil in 4 or 5" clay pots, or several in a larger pot (1/2" between bulbs); 1/3 of bulb above soil. Houseplant fertilizer (10-10-10 or 10-20-20) once monthly from planting to Sept 1. Withhold water gradually, no water from 1 Oct to early or mid-December. Remove yellowed foliage. Temps: 60-70 oF. Sun. Some cvs. will bloom at Christmas if planted mid-Nov. These are marked with an asterisk. Colors in parentheses are the overall impression of flower color from 10 ft. SINGLE STANDARD HIPPEASTRUM Apple blossom: pink stripes over white (light pink) Cantate: fuchsia * Carnival: red with white edge Christmas Gift: white * Cocktail: red with white star Dazzler: white * Desert Dawn: salmon Donau: rosy-red Liberty: red Masai: white with a few scarlet stripes (peppermint stripe) * Milady: pink Minerva: scarlet with white central star; green throat Orange Sovereign: orange Oscar: blue-red Picotee: white with red margin Rilona: peach * Safari: red * Springtime: light pink Star of Holland: red with white star Susan: pink Valentine: med. pink with white central star Vlammenspel: scarlet with white central star * Wedding Dance: white DOUBLE STANDARD HIPPEASTRUM Double Picotee: white with red edge, also red blush on petals Lady Jane: dark coral with a small amount of white striping Pasadena: red with white star "MINIATURE AMARYLLIS" Culture as for standard size Hippeastrum. 15-20" tall, usually 4 flowers per umbel, 2-3 umbels per bulb. Nicest grown in groups of 3-5 bulbs. Charm: orange with yellow-green throat Germa: yellow Pamela: red Scarlet Baby =(+ Gracilis Dulcinea): scarlet Spotty: terra-cotta red with narrow white stripes (light terra-cotta)) ________________________ HYACINTHUS [hyacinth] rating: Easy [hi-ah-SIN-thus] Shaded window while in flower: sunny after; Growing temp 60oF; FRAGRANT You can purchase hyacinths in a number of forms. The quickest to bloom will be "prepared" bulbs of Dutch Hyacinths-- L'Innocence will be in bloom by Christmas if planted by early October. These will bloom about 2-3 weeks before "unprepared" bulbs. If planting bulbs in soil, figure 3 bulbs per 6" pot of the largest size bulbs, 4-6 medium bulbs for a 6" pot. Plant with about 3/4" (1.5 cm) exposed bulb tip. Allow 8-10 weeks of chilling; a bit of leaf and a small tip of flowerbud should be visible before they are removed from the cold. Gradually bring into a 60oF dimly-lit room for growing. When flower stalks are about 4", bring them into full sunlight, but avoid temperatures over about 65oF. Allow 12-16 weeks start to finish. Dutch hyacinths can also be grown in special "hyacinth glasses" in water, and treated much like the soil-grown plants. I have not had as much success with this as with soil culture. Some cultivars suitable for forcing: (Dutch hyacinths that can take water culture are marked with an asterisk*) Ann Marie: single pink; early Bismark; large single sky blue; midseason City of Harlem: soft yellow; late * Grand Maitre: single lavender; midseason Jan Bos: single red; early * King of Blues: single dark blue; late King of Lilacs: light mauve; late * L'Innocence: white single; early if prepared; midseason unprepared * La Victoire: deep red; midseason * Lady Derby: light salmon pink; midseason * Myosotis: single light blue; midseason Ostara: single blue-violet; very early * Pink Pearl: pink single; early Queen of Blues: mid blue; late Queen of Pinks: largest of pinks, late Queen of Whites: late Rosalie: small, bright pink; very early Yellow Hammer: single soft yellow, midseason French-Roman Hyacinths are offered only in color strains of white, pink or blue. These plants should be rooted at 55-60oF, and produce short flower stalks. For continuous bloom from January on, plant every 2-3 weeks. __________________________ IRIS rating: Experienced [EYE-riss] Sunny window; Growing temp: 50-55oF Never allow iris to dry out or the temperature to rise above 55oF, or you will never have flowers. If you can provide these conditions, however, you can easily grow the Dutch Iris cultivars Wedgewood, White Excelsior or Yellow Queen or the tiny Iris reticulata (reh-tick-you-LAH-tah]. These are grown in a sunny window, not prechilled. Feed every two weeks with a balanced fertilizer and salvage for the garden. ________________________ MUSCARI rating: Experienced [muss-CARE-ee] Sunny window; Growing temp: 60oF
Two of the best for forcing are 'Early Giant', a large blue,, and Muscari botroides var. album [bot-ree-OY-dees ALB-bum]. Plant several in a 6" pot, just under the soil surface, and root in the cold until the pot is full of roots. Gradually bring into a sunny window of 55-65oF. This species will continue to grow and increase if planted outdoors after flowering. ________________________ NARCISSUS rating: Easy [nar-SIS-us] Sunny window; Growing temp: 55-65oF Force most "non-paperwhites" as you would tulips, choosing varieties noted in the catalog marked as suitable for forcing. There are so many cultivars, I can't begin to list them, but some are: Early: Cragford: white with orange corona; fragrant Forerunner: yellow trumpet Magnificence: golden yellow trumpet Rembrandt: large yellow Midseason: Allard Pierson: yellow corona, white perianth Cheerfulness; double-white; fragrant Early Beauty: yellow corona, white perianth Early Perfection: yellow corona, white perianth Fortune: lemon yellow with orange corona Geranium: white with red corona Golden Harvest: golden yellow Helios: perianth yellow, corona orange Innocence: white with yellow corona John Evelyn: white, frilled corona with orange edge King Alfred: giant yellow La Fiancee: perianth white, corona light orange Laurens Koster: white, red eye; fragrant Mount Hood: creamy white Music Hall: creamy white with yellow corona Scarlet Elegance: yellow with scarlet-bordered corona Von Sion: double yellow Miniature Narcissus species suitable for forcing include N. bulbocodium, the hoop-petticoat narcissus, N. cyclamineus ('February Gold', 'February Silver', 'Beryl' and 'Peeping Tom') and N. triandrus ('Thalia' and 'Moonshine'). ________________________ PAPERWHITE NARCISSUS rating: Novice Sunny window; Growing temp 55-65oF Several umbels of strongly-scented flowers are produced 3-5 weeks after planting. Soil or pebbles or bulb fiber. Room temperature, no prechilling. Sweet odor may be cloying in a small room. No fertilizer. Toss after flowering. If bulbs are not planted immediately, store cool and dry, but does not require refrigeration for storage. (45o-50oF ideal) Erlicheer: cream double Galilee: large, pure white flowers; improved version of old paperwhites. Omri: white flowers with yellow trumpets Soleil d'Or: old favorite yellow; orange trumpet Ziva: white ________________________ PUSCHKINIA SCILLOIDES rating: Experienced [push-KIN-ih-ah skil-LOY-dees] (yep, named for the Russian poet Pushkin) Culture as for Muscari. ________________________ SCILLA rating: Easy [SKILL-ah] or [SILL-ah] Sunny window; Growing temp: 55-65oF Culture as for Muscari. Scilla siberica and S. campanulata (=S. hispanica) are both easy species. ________________________ TULIPA [tulip] rating: novice to experienced [too-LEAP-ah] Sunny window; Growing temp 55-65oF
Tulips are classified according to blooming season, doubleness, and species -- most of our garden tulips are of hybrid origin. The garden tulips in the list below are slightly more difficult than paperwhites, but not much... Plant several bulbs per pot in soil, bulbs not quite touching. (usually 6-7 bulbs in a 6" pot, 8-9 in a 7", 9-12 bulbs in an 8" pan) Conventionally, the flat side of the bulb is oriented toward the outside of the pot: the first leaf will appear on this side. Store at 35-50 oF for 12 weeks; you may need to water if the pots dry out in this period. Bring indoors when top growth has reached about 2", gradually bringing them into full sun and 55-65 oF growing temperature. Taller cultivars may require staking. After bloom, toss the bulbs. If you must try to grow them next year, fertilize with houseplant fertilizer, and keep in as cool and sunny a spot as you have until you can transplant (in clumps) to a pre-dug trench in the garden. Be prepared for very disappointing flowering for the next 2-3 years. Cultivars marked "late" in the list below may require longer prechilling, and will bloom about March. As you gain more experience, you can branch out into other classifications of tulips. Some experiments will be rewarding, others, well -- live and learn! ---Singles--- (mostly easy to force; # marks cultivars used for commercial forcing: details available in Ball Red Book. Dates in parentheses denote year of origin. # Albury: red; late All Bright: deep red with white base and ring of pure blue Ambassador: red Anna Jose: pink with white edge, 16-18"; late # Apricot Beauty: apricot, late # Attila: dark lavender; 18-20"; late Attraction: orange scarlet; early # Atom: red; late # Bellona: yellow, 15"; fragrant (outstanding yellow!); early Bestseller: coppery-apricot pink; fragrant, 16-20"; late # Bing Crosby: red # Blenda: rose # Blizzard: white; late Boccherini: lilac blue, 20"; late Brilliant Star: scarlet, 12" Broadway: red with white edging: 16-18"; late # Carl M. Bellman: bicolor, late # Cassini: red # Charles: red Cheerleader: red; 16-20; late # Christmas Gold: yellow # Christmas Marvel: cherry pink, 12 # Comet: bicolor: bicolor Cordell Hull: white, splashed deep cherry red (Rembrandt) # Couleur Cardinal: dark red, 12", fragrant (1845); late Crater: dark red # Danton: red Demeter: violet; early # Denbola: bicolor; late # Diplomate: red; late Don Quichotte: medium fuchsia; 16-18; late Douglas Bader: pale pink; 16-18"; late Dreaming Maid: rosy violet, white edge; 16-18"; late Duc de Berlin: red, edged yellow; early Duc van Tol Scarlet: scarlet; early Duc van Tol White: white, early Early Queen: deep rose; early Easter Fire: red, late, 14-16" # Edith Eddy: bicolor; late Elmus: cherry red, edged pure white Ganders Rhapsody: pink shaded to red edge; 16-18" General de Wet: orange, 13"; fragrant (outstanding); early # Golden Eddy: bicolor; late Golden Glory: yellow, early Golden Harvest: deep lemon Golden Melody: soft yellow, 20", late Ibis: dark rose, 13" # Hibernia: white # Invasion: bicolor Jo-Ann: soft pink, 20"; late # Karel Doorman: bicolor # Kareol: yellow # Kees Nelis: bicolor Keizerskroon: dark rose, edged yellow, 15"; fragrant (1750) King of the Yellows; yellow; early Leen van der Mark: bright red with white edge; late; 22" # Madame Spoor: bicolor # Makassar: yellow; late # Merry Widow: bicolor: red with white margin # Mirjoran: bicolor # Monte Carlo: yellow Mon Tresor: yellow, 12"; early Negrita: deep purple; 18-20"; late New Design: pale yellow, white midpetals, pink edges; 16-20"; late Niphetos: soft lemon; late # Olaf: red Olga: violet red, edged white; early. # Orange Sun: orange, late Orange Wonder: orange, fragrant, 18", late # Ornament: deep yellow; late Page Polka: white with broad pink border; 18"; late # Palestrina: rose; late # Paris: bicolor; late # Paul Richter: scarlet; mid season # Pax: white # Peerless Pink: pink, large flowers, 20"; late Praestens Fusilier: bunch flowering scarlet; February, grow 55-60oF # Preludium: rose Prince Carnival: red/yellow, 15" # Prince Charles: lavender # Princess Irene: orange, fragrant, late; 10-12" Prince of Austria: scarlet, 15" # Prominance: red Proserpine: deep rose; early Queen of Bartigons: salmon pink with yellow stamens # Red Giant: red; late # Robinea: red; late # Stockholm: red Tambour Maitre: red, slight fragrance, 18"; late # Thule: bicolor # Topscore: red # Trance: red Van Der Neer: plum, 12"; fragrant (1860) Vermilion Brilliant: scarlet Victor H. Ries: purple, 16-20"; late # Virtuoso: rose; late White Dream: pure white, 16-18"; late White Dream: white, 20"; late White Hawk: white, 12" Yellow Present: pale yellow, 16-18"; late --Early Doubles Dante: red Mr van Der Hoef: pure yellow Peach Blossom: rosy pink Scarlet Cardinal; scarlet double; early Willemsoord: bright carmine, edged white ________________________ VELTHEIMIA rating: Green thumb [velt-HI-me-ah] Growing temp 50-55oF Culture as for Hippeastrum; plant in a pot just an inch or so larger than the bulb in November or December. Feed with a balanced fertilizer every couple of weeks until growth starts. A tall stalk with 20-30 long lasting, pinkish flowers will appear. When flowers fade, rest the plant dry in the pot until August or September. ________________________ Kay Lancaster

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