Urban Gardening

From November 2007 E-newsletter





Forcing Bulbs for Winter Blooms

The gardening season is ending, and bleak, dreary winter is on the way. But take heart; by forcing bulbs you can enjoy a dose of spring many weeks before it officially arrives. Your windowsill can be a miniature garden of vivid colors and intoxicating perfumes even when the weather outside is frightful.

Bulbs are uniquely adapted to forcing. The flowers and leaves are already formed inside the dormant bulb, and with the right temperature and moisture, they begin growing. Both hardy bulbs and tender bulbs can be forced indoors.

Hardy bulbs from temperate areas require cold winters and plenty of water (rainfall or snow) to grow. The earliest bloomers in nature are usually the best performers when it comes to forcing. Early-blooming choices include crocuses, anemones, glory-of-the-snow (Chionodoxa), hyacinths, some tulips, some daffodils, striped squill (Puschkinia), and netted iris.

Hardy bulbs are available at garden centers and big box stores, and through mail-order catalogs. Be sure to choose bulbs that are firm, fresh, and healthy, like good produce. Reject all bulbs that are soft, moldy, or diseased. If bad bulbs arrive by mail, send them back to the company for replacement or refund.

Some sources, such as the NGA Garden Shop, Brent & Becky's Bulbs, and Van Bourgondien, offer pre-chilled hardy bulbs just for forcing. These bulbs will bloom much earlier than non-chilled bulbs. They cost more, but you may be able to have blossoms by the holidays. The selection of pre-chilled bulbs will be less extensive than the regular ones.

Pitfalls of Forcing Bulbs

    Bulbs rot during the chilling phase. Either the bulbs were already diseased or wet soil caused them to rot. The soil should be slightly moist, not wet.

    • Bulbs don't grow. The chilling period was too short. Make sure the bulbs have lots of roots and growing shoots before they are brought into warm temperatures to bloom.

    • Blooms are sparse or nonexistent. The temperature was too warm or the sunlight insufficient for blooming. A room that's too warm mimics summer, not spring. Bulbs need cool temperatures and bright light to bloom their best.

Tips for Success

    • Select the largest, firmest, healthiest bulbs.

    • For a more varied display, mix pots of different bulbs in baskets or large containers. Don't mix different bulb species in a single pot.

    • For continual blooms, plant many pots and bring them out of the chilling period every couple of weeks.

    • Don't get discouraged. It may take a couple of tries before everything works according to plan.

Tender or tropical bulbs are easier to force because they do not require a chilling period. There are literally scores of suitable tender bulbs for indoor bloom, such as the popular amaryllis (Hippeastrum), paper whites (Narcissus), calla lilies (Zantedeschia), cyclamen, and voodoo lilies (Amorphophallus).

Some tender bulbs are available at garden centers. Gloxinias (Sinningia) and calla lilies are common but usually sold as blooming plants. Check the Internet and catalogues for freesias, ever-blooming gladiolus (Gladiolus tristis), sun star (Ornithogalum dubium), velthemia, cape cowslip (Lachenalia), voodoo lily, devil's dung (Sauromatum venosum), and others.

The Potting Process

A 6- or 8-inch-diameter pot will be large enough for a nice display but small enough to move around easily. Make sure there are drainage holes. Plastic pots have the advantage of being lightweight and inexpensive. A sterile, loose, and well-draining potting medium will help prevent the bulbs from rotting during the chilling phase. Either make your own or buy a professional potting mix.

Add 2 inches of soil mix to the bottom of the pot. Pack in the bulbs, nestling them close together but not touching. Then cover the bulbs with the soil mix. The soil line should be 1/2 inch below the rim of the pot. It's okay if the noses (tips) of the bulbs are visible above the soil.

Give the pot a good soaking. Water until the soil is saturated and water leaks out of the drainage holes. If you have the time and space, plant several pots over a period of several weeks for a succession of blooms.

Labels are absolutely necessary when forcing bulbs. After a few months, you can't expect to remember what you planted or when. It's helpful to write the name of the bulbs in the pot and the approximate date you need to remove the pot from storage. Garden centers have many different kinds of labels available. I often use the plastic knives leftover from eating take-out. They are durable, plentiful, and easy to write on, plus this keeps them out of my local landfill. Buy permanent markers and write neatly (a major problem for me) for sure identification.

Giving Hardy Bulbs a Cold Shoulder

The chilling (faux winter). At this point hardy and tender bulbs are treated differently. Tulips, crocuses, daffodils, hyacinths, and other temperate bulbs normally develop their roots in winter for quick shoot growth in spring. The chilling process mimics winter and allows time for the roots of the potted bulbs to grow.

Place pots in a dark, chilly area (35 to 45 degrees F) such as a cold frame, refrigerator, unheated basement, garage, or in-ground trench. The required chilling period can last 10 to 16 weeks, depending on the type of bulb. Snowdrops, species tulips, and crocuses need less time than daffodils, Darwin tulips, and camass.


At first, check the pots monthly and water if necessary to keep the soil moist. After a couple of months, begin to monitor them weekly. By then the roots should be growing out of the bottom holes. The pot is ready for the next stage when both the shoots and roots are visible. (Your outdoor bulbs typically reach this level of readiness in December and then wait for spring to sprout from the ground.)

Growth and bloom (pseudo spring). You are ready to begin forcing the flowers when the yellowish white shoots are 1 to 2 inches tall. Remove pots from storage and place them in a rather cool (55 to 65 degrees F), bright place for two to four weeks. This is the trickiest period. The bulbs must have some warmth but they cannot go directly from winter to summer. If temperatures are too warm, the foliage may grow but the flower buds will die. Having weeks of your efforts blasted away because the pot was set too close to the heating vent is disheartening (I've been there). Keep the plants cool and in good light, and the soil moist. Think of spring; it gets warm gradually. Your job is to fool the bulbs into thinking it's April.

When flower buds show color, move the pots to your living areas so you can enjoy your hard-won blooms. Dining tables, bedroom windowsills, and bathroom counters are good spots. Combine pots of forced bulbs in decorative containers or baskets. Mix in houseplants and annuals for dazzling displays. At night move the pots to a cooler location to prolong blooms. As flowers fade, replace them with pots at the bud stage to continue the show.

Here's a list of some of the most common hardy bulbs, the number of weeks of cold treatment required, and the number of weeks to flowering:

    Amaryllis: no cold treatment needed; weeks to bloom varies by species

    Chionodoxa luciliae: 15 weeks; 2-3 weeks

    Crocus: 15 weeks; 2-3 weeks

    Daffodil: 15-17 weeks; 2-3 weeks

    Eranthis hyemalis: 15 weeks; 2 weeks

    Fritillaria meleagris: 15 weeks; 3 weeks

    Galanthus nivalis: 15 weeks; 2 weeks

    Hyacinth, prepared: 10-12 weeks; unprepared: 11-14 weeks; 2-3 weeks

    Iris reticulata: 15 weeks; 2-3 weeks

    Muscari armeniacum: 13-15 weeks; 2-3 weeks

    Narcissus tazetta (paper whites): none; 3-5 weeks

    Scilla siberica: 15 weeks; 2-3 weeks

    Tulip: 14-20 weeks; 2-3 weeks

(Source: Flower Bulb Information Center)

Post-bloom care. Many gardeners view forced hardy bulbs as annuals and discard or compost them after they flower. The process of forcing saps much of the bulb's energy and it cannot be forced again. However, I can never intentionally kill a bulb. To give them a chance of surviving, treat them as bright-light houseplants through the rest of winter. Fertilize them occasionally. After your last frost date, they can safely be planted in the garden. It will take a few years before they can build up the reserves to bloom again.

Quick-Blooming Tropicals

Tropical and tender bulbs do not need a chilling period. They prepare for blooming by going through a dry dormancy, which is usually completed prior to shipping. When you get the bulb, it's ready to go. With some of the larger bulbs, only one will fit in a pot with its neck and nose sticking out, like potted amaryllis. Be sure to leave a half-inch of space above the soil line. Water thoroughly and place in a sunny area.

Do not water again until growth begins. Once there is visible aboveground growth (leaves or flower shoots), treat the potted bulb like a normal houseplant and water and fertilize regularly. After they flower, care for tender bulbs like other houseplants. Then, in late summer, allow the pots to dry out (which simulates drought) so the bulbs enter dormancy. Begin the flowering process again by watering in late autumn.

Get Creative With Containers

Many gardeners choose more artistic approaches to forcing bulbs. Instead of using plastic pots and combining them in a decorative container, they force the bulbs directly in bowls, vases, and baskets. You can substitute pea gravel, stones, and marbles for nutrient-rich soil. Hyacinths and crocuses can be forced in specially made glasses with water only, no soil. This gives a clean, Zen-like look, but completely exhausts the bulb. Discard them after flowering. (Hyacinth glasses are a neat way for kids to observe root growth and flower development.)

Forcing bulbs is part science, part art, and part luck. Start with easier bulbs like hyacinths (pre-chilled if possible) and amaryllis. Once you've had success and honed your skills, then expand your palette. With practice you can synchronize your forced bulbs with Christmas, Martin Luther King Day, and Valentine's Day. You can have colorful, fragrant flowers all throughout winter to chase away cabin fever and recharge your spirits

Related Articles:

Forcing Tulips Indoors

Keeping Paper White Narcissus From Flopping

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