Exploit the natural cycle
The bulbs that are suited to forcing have a topsy-turvy cycle of growth in nature and in the garden. Most of them flower early in spring, while trees are still bare. Their leaves and roots persist just long enough—through mid- to late spring—for the bulbs to store a new supply of energy. Then the leaves and roots die and the bulbs go dormant for the summer and part of the fall.
In fall, when the soil cools, the bulbs awaken and send out new roots, the first step in their preparation for the next spring. The roots continue to grow until the soil cools to 40F. In most of the United States, the bulbs produce a dense mat of roots before winter sets in. In cold climates, freezing temperatures halt the roots’ growth but won’t kill them; in warmer climates, the bulbs may continue to make roots throughout the winter.
When spring temperatures rise to between 40F and 60F, bulbs reawaken, send up leaves, and then make flowers. This growth is rapid, thanks to the full complement of roots. Before the bulbs go dormant again by fall, the leaves have plenty of time to replenish the energy of the bulbs.
Forcing exploits this cycle. In fall, you buy dormant bulbs, pot them, and keep them cold for several months to stimulate them to make roots. Once plants have roots, you bring the pots to a windowsill, where the warmth tricks the bulbs into thinking that spring has arrived. In short order, they flower, and you have an early spring.
A parade of bulbs: Bulbs for forcing vary in shape and size. Shoots grow from the top; roots from the bottom. Plant bottom-end down.
Photo/Illustration: Susan Kahn