Most spring-blooming bulbs, including tulips, daffodils and crocus, come from climates with cold winters. So if you live where winters are mild and frosts are rare (zones 8-11), you will need to "pre-chill" these bulbs so they bloom properly. Instructions are below.
Bulbs that are native to warmer climates do not require a chilling period. These include Dutch iris, Anemone DeCaen, Anemone St. Brigid, Freesia, Ranunculus and Scilla peruviana. Where these bulbs are winter hardy (zone 8-10) they can simply be planted in fall for spring flowers.
The ideal chilling period for most spring-blooming bulbs is 12-14 weeks at 35 to 45°F. Exposing bulbs to these cold temperatures stimulates a bio-chemical response that “turns on” flower formation and initiates root growth. Without a chilling period, the bulbs will try to bloom, but the flowers may be small and malformed. Bulbs can be pre-chilled for longer than the recommended amount of time, but not less.
For best results, begin chilling the bulbs in mid to late October and plant them outdoors as soon as the chilling period is complete. This gives the bulbs some time to develop roots before they focus on flowering. If you start the chilling period in late fall, the bulbs will be ready to plant in January or February.
Chilling your bulbs in the refrigerator is an easy option as long as you don't keep any fruit in there while the bulbs are chilling. Ripening fruit gives off ethylene gas that will damage the embryonic flowers inside the bulbs. If you do store the bulbs in the refrigerator, put them in ventilated plastic bags so they don't dry out during the chilling period.
When the bulbs have chilled for the appropriate amount of time (see below), remove them from the refrigerator and plant immediately. To extend the bloom time, choose a planting location that will be protected from afternoon sun. After planting the bulbs, you can spread a few inches of mulch on top of the soil to help retain moisture and keep the soil cool.
Most bulbs that have been pre-chilled will bloom 4 to 6 weeks after planting. Pre-chilled bulbs should always be treated as annuals.
Spring-blooming bulbs that should be pre-chilled at 35-45°F
Bulbs that should be pre-chilled in zone 8:
Bulbs that should be pre-chilled in zones 9-10:
All tulips, plus others including hyacinths, muscari, crocus and alliums
Bulbs that will bloom in zones 8-9 without pre-chilling:
Daffodil varieties well-suited to warm climates, including Carlton, Erlicheer, Thalia, Avalanche
Others: Dutch iris, Anemone DeCaen, Anemone St. Brigid, Freesia, Ranunculus, Scilla peruviana