Bulbs That Require a Chilling Period
Bulbs that are traditionally planted in the fall need a period of cold temperatures to stimulate growth and flower production. These include: non-tropical narcissus, hyacinth, tulips and crocus. You can buy bulbs prechilled and ready for forcing, but they are expensive and doing it yourself takes planning, but not a lot of work.
Potting Up: Shallow pots are traditionally used for forcing and work well, but you can use most any container you choose, if it has holes for drainage. Fill the pot about 3/4 full with a peat based potting mix, for moisture retention. Squeeze in as many bulbs as can fit. You can use all one type or mix and match. You can even plant smaller bulbs on top of larger bulbs, but try and pick varieties with a similar bloom time. Just be sure to plant the bulbs flat side down. Cover the bulbs with about one inch of potting mix. If you are planting tulips, leave the shoot tips poking out above the soil line. Water until you see it coming out of the drainage hole.
Temperature: Throughout the chilling period, the temperature needs to be around 35 - 45 degrees F.
Duration: The period of chilling required will vary with the type of bulb, but most require at least 16 to 18 weeks. A little extra chilling wont hurt the bulbs. However, if they arent allowed enough chilling time, the flower may not fully form. This means the bulbs should be kept where the temperature will not fluctuate greatly.
Location: If you live in an area where winters are cold, but rarely dip below 25 degrees F., (maybe zone 8), you can keep your potted bulbs outdoors. Place them in a convenient location and cover them with some straw mulch for protection.
Where winter temperatures are commonly below 25 degrees F., you can still chill your bulbs outside, but it will be more work. The bulbs will need to be in a hole or trench below ground level. A popular technique is to dig a trench about 2 feet deep and place your loose or potted bulbs in and cover them with a couple of layers of floating row cover or even old blankets. Then fill in the trench with a thick layer of straw or leaves. You will need to keep tabs on the temperature in your trench, to insure the bulbs do not freeze. If you have vole or squirrel problems, store the bulbs in wire mesh.
An easier method is to chill your bulbs in an unheated basement, crawlspace or attic, a partially heated garage or a cold frame.
You can also chill the bulbs in a refrigerator. This is the default method for those living in zones 9 and above. The catch here is that the bulbs cannot be stored where there is produce. Many ripening vegetables and fruits, especially apples, release ethylene gas, which can kill or damage the flowers.
Timing: Check this Bulb Chart for the average chilling periods of commonly forced spring bulbs.
Post-Chill: When the required chilling time is up, the bulbs should exhibit some root growth. Move your pots to a warm spot in your house, about 60 degrees F., with indirect sunlight. Shoots should emerge within a couple of weeks.
When the shots are 4-5 inches high, the pots can be moved to direct sunlight and the temperature can be increased to 68-70 degrees F., to encourage budding.
When you begin to see color in the buds, move your pots back to indirect sunlight. Remember, these are spring flowers and they arent happy in harsh light.
After the Bloom Fades: Forcing bulbs knocks them out of their regular routine and saps their energy. Most people simply discard the bulbs, once theyve finished blooming. However except for the tropical narcissus, they can be saved and planted outdoors. Treat them like an outdoor bulb. Keep watering the plants and give them a little bulb food as the blooms fade. You should deadhead, but allow the foliage to yellow on its own. Then find a spot for them in your garden. The plants should come back the following year, but it may take a few more years before they have the strength to rebloom. For a quicker payoff, try forcing tropical bulbs, like paperwhites and amaryllis. These bulbs need no prechilling and very little fuss.