How to Force Bulbs for Indoor Bloom
Almost any container can be used for forcing bulbs. Some do particularly well forced only in water, and specially designed vases for forcing individual hyacinth bulbs are available. Paperwhite narcissus can be easily forced in a shallow container of water using pebbles for support. Hyacinths can be forced in special hyacinth vases. More extensive forcing projects are best done in clay or plastic pots that have adequate drainage holes. Plastic retains soil moisture longer, and can always be set inside a decorative outer planter.
The best soil mix for forcing bulbs contains equal parts of soil, sphagnum moss, and perlite or vermiculite. Commercial "soil-less" potting mixes can also be used. Bulbs for forcing should not be planted in ordinary garden soil or in potting mixes that are labeled "potting soil." Potting soil mixes are often no more than a fine form of peat moss and holds too much moisture which can cause water-related disease problems.
Fill three-quarters of the container with potting mix. Plant bulbs closely together. Spacing considerations that apply to planting bulbs in the garden do not apply when the bulbs are to be forced. Place tulip bulbs with the "flat" side facing the edge of the container. After you arrange the bulbs, place additional media around them. Do not fill the container to the surface with the potting mix. The tops of tulip and narcissus bulbs do not need to be covered. The bulbs should then be watered in. Bulbs needing pre-chilling should be planted in inexpensive plastic pots that can be set inside decorative planters.
All of the spring-blooming bulbs, with the exception of paperwhite narcissus, amaryllis and iris reticulata, must have a cold period of at least three months to initiate bloom. You do this pre-chilling a variety of ways. Potted bulbs can be stored in a refrigerator or in an unheated garage or cellar. Pots in a refrigerator tend to dry out rapidly, so check periodically to ensure that the soil is moist.
Bulbs can be chilled in a cold frame as well. If you use this method, make sure you open the cold frame on sunny winter days. Even when the outside temperature is under 40 degrees F, the inside of the cold frame can rapidly heat up, which can initiate early flowering.
A simple method involves chilling the pots under natural cold conditions outdoors. Dig a trench or pit in the vegetable or flower garden approximately as deep as the containers. Place the planted pots in the trench or pit and mulch thickly with loose, dried leaves, straw or sphagnum moss mounded up a few inches higher than the ground surface. Cover the mound with plastic (anchor it with soil, bricks or rocks to keep in place). The mulch acts as a buffer zone. Bulbs will receive the cold temperatures they need but will not freeze. The plastic cover makes it much easier to remove the pots after the cold period has been completed. The length of the cold period needed depends on the specific bulb and, in some cases, the cultivar. The following table gives cold treatment guidelines for bulbs that are easily forced.
After bulbs have been chilled, bring the pots inside for blooming. Check the pots to see if the bulbs have produced an adequate root system (look to see if any roots are visible through the drainage holes). The number of weeks it takes before the plants actually bloom depends on the environmental factors in the home, but the average is two to three weeks.
Water the pots thoroughly when bringing them inside. Place pots in a cool area of the home (high light intensity is not important at this point) and leave pots in a cool location until active growth is visible. Take care not to over-water.
When the shoots are 4-6 inches tall, you can move the pots to a warmer location that receives more light. Forcing bulbs slowly is more desirable than placing them directly in a bright, warm location. The quick transition from chilling to warm temperatures can sometimes "blast" the buds, which means everything moves too fast and the bulbs do not bloom. Because of the warmer indoor temperatures, flowers from bulbs that are forced indoors do not last as long as outdoor flowers. Forcing several containers of bulbs on a staggered schedule extends the indoor display.
If blossoms begin to develop too quickly, you may be able to retard blooming a bit by moving the pots out of direct sunlight and into a cooler location. Re-acclimate them to sunlight and warmer temperatures when you want them to resume growing.
After blooming, hardy bulbs such as hyacinths and tulips cannot be forced again and should be discarded. Or they can be planted outdoors where they may rebloom within a year or two.
Forcing is hard on most bulbs. The easiest after-bloom care is pitching the bulbs on the compost pile. If you wish to recycle these bulbs for the garden, after-bloom care is critical. The key to success is keeping the foliage actively growing as long as possible. Bulbs will need to be fertilized with a water-soluble fertilizer. Follow label directions. After the foliage has died back naturally, the bulbs can be planted directly in the garden or stored for later planting. If they do not perform well in the garden, do not be disappointed. Forced bulbs are most useful for indoor enjoyment. By all means, do not try to force the same bulbs the next season. It is difficult to recreate the natural bulb cycle indoors. Most homes simply do not have the necessary light conditions to be successful.
Unlike most other bulbs, amaryllis bulbs will bloom again and again. See Holiday Houseplant Care for information on amaryllis after-bloom care.
Paperwhites (narcissus tazetta), 'Soleil d'Or', 'Chinese sacred lily' and colchicum are among the most popular forcing flowers that don't require the 12-week rooting period. They are easy to start and can give you indoor blooms from Thanksgiving until late March. If you plant them successively, batch after batch in late fall to early winter, they will provide many containers of blooms.
Paperwhites are most often (and most easily) potted in shallow containers of gravel. Place bulbs on a layer of gravel and carefully fill in enough gravel to hold bulbs but not cover them. A crowded grouping will be the most attractive.
Add water to the container. It should go just to the base of the bulbs, but not touching the bulbs. Place container in a sunny spot, step back and watch 'em grow! You'll see roots in a day or so and in three to five weeks you'll have gorgeous flowers. Keep the water level at the same point.
Other Bulbs for Forcing
Forced tulips do not do quite as well as garden planted tulips because they require a fairly long rooting period at even (non-fluctuating) temperatures to be successful. Allow at least 15 weeks.
Experiment with a few pots of different varieties. One to try is the distinctive 'Princess Irene'. This single early tulip is orange with purple flame markings. The bright yellow 'Monte Carlo' and pink Angelique are double early tulips, a cultivar that has twice as many petals as most tulips.
Tulip Tip: Plant bulbs with flat side facing the rim, this will position the larger outer leaves toward the pot rim, where they will drape gracefully over the edge of the pot.
Daffodils require very bright light, such as that found in a greenhouse, to flower well. Too little sun results in leggy growth and no blossoms. Only the miniature varieties (hybrid) daffodils are recommended for home forcing. Daffodils usually require a 12-14 week rooting period.
Once removed from the rooting area, daffodils must be placed in a location that receives lots of sun, say an enclosed porch or sun room or under a skylight.
Lily-of-the-Valley are often pre-cooled when you buy them, so they will bloom three to four weeks after planting. Ask your supplier.
Freesias don't require a cooling period, however they usually require a lot of sunshine and about three months of growth time before they bloom. These fragrant flowers do best in a very bright room with daytime temperatures of about 70°F and nighttime temperatures of about 50°F.