Forcing is a growing technique that puts bulbs through an artificial winter cooling period. The bulbs are planted in pots and then chilled (outdoors or in a refrigerator) for a specific number of weeks. After being chilled, the bulbs will bloom either indoors or out.
Precooling or pre-chilling bulbs exposes them to cold temperatures that mimic a natural winter. Most spring-blooming bulbs, including tulips and hyacinths, will not bloom properly unless they are chilled at 40-45°F for 10-14 weeks. This means that in warm climates (growing zones 8-10), these bulbs need to be chilled artificially before being planted into the garden.
The easiest way to pre-cool bulbs is to put them in the fruit/vegetable drawer of your refrigerator. Make sure not to store them with ripening fruit, which can give off ethylene gas that will damage the bulbs and prevent them from blooming. As soon as the chilling period has ended, plant the bulbs in the garden or in containers.
In regions with mild winters, you can avoid precooling by choosing bulbs that don’t require a winter chilling period. These include iris, freesia, scilla, most alliums and some varieties of narcissus.
The USDA plant hardiness zone map divides North America into 11 different growing zones based on the lowest average winter temperature. Each growing zone is approximately 10°F warmer (or colder) during an average winter than the adjacent zone. The lower the zone number, the colder the winters.
Some bulbs and plants are naturally more cold tolerant than others. Hardiness ratings help gardeners select plants that are likely to survive the winter where they live.
Annuals are plants that live only one year. They go from seed to flower to seed in one growing season. These plants either reseed themselves or need to be replanted each spring.
Perennials are plants that live for more than one year. In most cases, these plants will die back to the ground in the fall and regrow in the spring.