Hey there, time traveller!
This article was published 1/12/2008 (2829 days ago), so information in it may no longer be current.
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For those who like to fill the house with flowering plants during our dreary winter months, 'tis the season for forcing an assortment of bulbs to bloom indoors.
Forcing is the process of fooling bulbs to bloom out of season. The big commercial greenhouses do it all the time, to provide gift plants at certain times of the year for the market. (Think lilies at Easter and oxalis for St. Patrick's Day.) By simulating the conditions required in nature to stimulate growth and flower production, you can persuade those bulbs to bloom for you, too.
Purchase your bulbs before they've sat on the shelf too long. Bulbs are perishable items and should feel firm to the touch and not be sprouting in their packaging. Before potting them up, store them in the crisper section of the fridge, but be sure to keep them away from fruits and vegetables that emit ethylene gas, which can cause the bulbs to abort their blooms.
Species to force include the big bulbs -- tulip, daffodil and Dutch hyacinth. The best varieties for forcing are the ones that bloom early and are short in stature. Minor bulbs -- grape hyacinth, crocus, scilla, chionodoxa, snowdrop, winter aconite and iris reticulata -- are also fun to try. Because they are hardy bulbs, they need a period of cold and dark that simulates winter to break their dormancy. Amaryllis and paperwhite narcissus are the easiest of all bulbs to coax into bloom as they don't require any chilling period.
You can use any pot that has drainage holes, or plain, clean plastic pots that can be slipped into a decorative container later on for display purposes. Special bulb pans or bowls are also available in squat sizes that make them ideal for bulbs, which can get top-heavy. Clay pots should be soaked for a few hours before planting. Using a sterile, well-draining potting medium, fill the bottom third to half of the pot loosely with soil and pack in the bulbs with the tapered or pointy side up. The more, the merrier, as the bulbs can touch each other, and the idea is to have as many blooms as possible per pot. Cover the bulbs with soil and very gently firm them in. Water thoroughly and let the pot drain completely. If you are using a variety of different bulbs, label each pot with the names of the plants and make note of the date. Fertilizer isn't necessary as the bulbs contain all the food they need for one season's growth.
The potted bulbs now need to go back into the fridge or a cold storage area that maintains a temperature of about 40 F for six to 15 weeks, depending on the bulb. (See the chart below). Again, don't store them with produce such as apples, avocados, bananas, melons, peaches, pears and tomatoes that produce ethylene gas. I like to wrap the pots loosely in newspaper to help conserve moisture and prevent soil spills or damage to the bulbs . During this time, the bulbs will begin producing roots and, over time, shoots will begin to emerge from the soil. Check the pots periodically to see if the soil is dry -- you want the soil to hold some moisture but not remain soggy.
When the appropriate chilling time has elapsed, bring the pots into a cool, bright room but away from direct sunlight. The cooler temperature will ensure the foliage grows in a compact manner and not become leggy and floppy, which makes the plants top-heavy and messy-looking. The bright light will encourage bigger, brighter blooms. As flower buds form, you can move the plants to a brighter, warmer location.
Because amaryllis and paperwhite narcissus don't require chilling, they are a snap to force indoors.
Paperwhites can be displayed in any water-tight container anchored in pea gravel, pebbles or decorative stones in just enough water to cover the bottom quarter of the bulbs. Amaryllis should be planted in potting soil in a sturdy pot with the top third of the bulb exposed. Don't forget the drainage holes, as amaryllis won't tolerate saturated soil. The cool, bright room rule applies to both. Paperwhites produce delicate-looking and highly scented clusters of small white flowers, though not necessarily pleasant to everyone's taste. (For my nose, hyacinths produce the most pleasant fragrance).
Forced paperwhites are a one-shot deal and should be discarded after forcing. Amaryllis are stunning and dramatic, producing three to six large lily-like blooms on each tall stalk. Once they are finished blooming -- and they may produce more than one flower stalk -- cut back the stem and keep the foliage healthy and green. The plants can go out into a sheltered part of the garden for the summer, and after a couple months of rest by withholding water the following fall, they should repeat the flower show.
Bulbs need to be fully rooted to bloom properly. If you are chilling your potted bulbs in the fridge, you can deduct three weeks from the maximum times listed.
Tulips require the longest cold period: 14-16 weeks.
Daffodils: 12-13 weeks.
Hyacinths and crocus: 10-12 weeks.
Scilla and muscari: 8-9 weeks.
Chionodoxa and iris reticulata: 6-8 weeks.