Plants have internal time clocks that determine when they bloom -- daffodils in spring, roses in summer and mums in fall. But plants can be convinced to flower sooner or later than normal by using a technique called forcing. If you have ever attended a late winter flower show, you have seen thousands of expertly forced trees, shrubs, perennials and bulbs that have all been coaxed into bloom for a particular weekend.
Flowering bulbs are some of the easiest plants to force. There are two types: bulbs that need to go through a period of cold temperatures before they will flower, and bulbs that don't require a chilling period.
NO CHILLING NEEDED: AMARYLLIS AND PAPERWHITES
Amaryllis and paperwhites are winter-blooming bulbs that don't require a cooling period to trigger flowering. This is because they are native to warm climates where temperatures never drop below freezing. When these bulbs become available in late fall, they can be planted indoors immediately and will bloom 4 to 12 weeks later.
CHILLING NEEDED: TULIPS, DAFFODILS, HYACINTHS AND OTHERS
Spring-flowering bulbs such as tulips, daffodils, hyacinths, and crocus, require a chilling period before they will bloom. Cold temperatures (35-45°F) stimulate a biochemical response that "turns on" the bulb and tells the embryonic flower to start developing. If these bulbs don’t go through an extended period of cold temperatures, they either won’t bloom or their flowers will be poorly formed.
WHEN TO PLANT THE BULBS
Bulbs that you are planning to force can be planted in pots at the same time you are planting bulbs outdoors -- anytime during October or November. The bulbs should be fresh, firm and not dried out.
WHAT TO PLANT THEM IN
Forced bulbs look best in shallow pots that are 4 to 6” deep. The pots should have a drainage hole in the bottom. Using a standard potting mix, put several inches of soil in the bottom of the container and then set the bulbs on top, pointy-end up. Forced bulbs look best when they’re planted very closely together with the bulbs almost touching each other. Cover the bulbs with more soil, until the tips are about an inch below the soil surface and then label each pot so you know what’s been planted. Water thoroughly to settle the bulbs in place.
WHERE TO STORE THE BULBS
Finding the right place to chill your bulbs is important. If you live where winters are moderate (zone 7) it’s relatively easy. The potted bulbs can simply be left outdoors as long as the soil doesn’t freeze or get waterlogged. In climates where winter temperatures typically drop well below freezing (zones 3-6), the bulbs need to be stored in a protected place where the soil will not freeze. An unheated basement, attached garage, ventilated crawlspace or cold framework can work well. A spare refrigerator is ideal as long as it doesn’t contain any fruit. Ripening fruit emits ethylene gas, which will damage the bulbs.
HOW TO CARE FOR THE BULBS
Keep the soil moist, but not wet for the entire chilling period. And keep the bulbs in the dark or they may start growing before they're fully chilled. The temperature needs to maintained at 35 to 45°F throughout the entire chilling period. Recommended minimum chilling times are as follows (extra chilling time is fine):
- Glory-of-the-Snow (Chionodoxa): 15 weeks
- Crocus: 15 weeks
- Hyacinths: 13-16 weeks
- Grape Hyacinth (Muscari): 12-15 weeks
- Daffodils: 16 weeks
- Siberian squill (Scilla siberica): 12-15 weeks
- Tulips: 14-20 weeks
- Snowdrops (Galanthus): 12-15 weeks
- Iris reticulata: 13-15 weeks
To learn about planting spring-blooming bulbs in containers for outdoor display, read: How to Grow Spring Bulbs in Containers