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
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
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
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
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
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.
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,
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
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
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
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
ERYTHRONIUM (trout lily; dogtooth violet) rating: Experienced
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)
* Carnival: red with white edge
Christmas Gift: white
Cocktail: red with white star
* Desert Dawn: salmon
Masai: white with a few scarlet stripes (peppermint stripe)
Minerva: scarlet with white central star; green throat
Orange Sovereign: orange
Picotee: white with red margin
* Safari: red
Springtime: light pink
Star of Holland: red with white star
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
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
Charm: orange with yellow-green throat
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
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
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:
Cragford: white with orange corona; fragrant
Forerunner: yellow trumpet
Magnificence: golden yellow trumpet
Rembrandt: large yellow
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
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
Erlicheer: cream double
Galilee: large, pure white flowers; improved version of old
Omri: white flowers with yellow trumpets
Soleil d'Or: old favorite yellow; orange trumpet
PUSCHKINIA SCILLOIDES rating: Experienced
[push-KIN-ih-ah skil-LOY-dees] (yep, named for the Russian
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!
(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
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
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 email@example.com