Anxious nail-biting isn’t my only association with chilling. Once a plant’s chilling “bank account” has been filled, the readiness of its stems to flower can be put to more convivial use. All that’s needed is warmth, so, brought indoors, flowering stems are urged to unfold their buds for a colorful show of flowers—an early spring, in a vase, on my dining table or window sill.

Starting with branches of flowering trees...even though leafless in winter, some trees and shrubs do notice when days are getting longer, their sleeping buds jerking awake as soon as they feel day length has become sufficiently long. Trees most affected by photoperiod—that is, day length—include birches, some species of maple, sycamore and London planetree, black locust, elm, and American beech. Least affected trees include ashes, gingko, pines, pin oak, and, in contrast to the American species, European beech. White oak, red oak, linden, and ironwood are among trees intermediate in response. Plant species, as well as duration and amount of cold, all interact with photoperiod in its influence on awakening plants in spring. (Photoperiod can also be a stimulus for plants to enter dormancy in autumn, a response that can affect winter hardiness of trees exposed to artificial light, such as street lights.)

Apples, plums, and most other fruit trees—all of whose branches bear beautiful, sometimes fragrant, blossoms that can be forced indoors—wait only for temperature changes to tell them when spring has arrived.

Branches snipped from trees and shrubs and brought indoors for forcing can be exposed to more steadily warm temperatures than the fluctuating warmth of late winter and early spring outdoors. I just have to wait long enough to begin forcing blossoms from these branches.

Even then, to force any branch well demands patience. Rush the process, and blossoms open sporadically along the branch, then dry up and fall off.

To stay alive and flower, a cut stem needs water, which is no longer afforded by roots. To prevent drying, I first plump up all the buds by completely immersing the cut branches in tepid water for a few hours, then recut their bases to expose fresh xylem cells (through which water and nutrients flow), and put them in a vase of water in a cool room. The vase gets moved out for display just as the buds are about to burst into bloom. The time for this will be shorter the closer to the natural bloom time that I begin forcing.

Am I occasionally too impatient to wait for the chilling “deposits” to be fulfilled to even begin forcing? Banking rules for the chilling bank are not all that strict, and a dormant branch might be awakened from its sleepy state with a high temperature shock. Immersing branches for a few hours in 90 degree water usually does the trick. On the living plant, chilling needs can also be reduced by exposing a plant to long days, but this is not as convenient as the warm water treatment.

Hardy, spring-flowering bulbs, such as tulips, daffodils, crocuses, and hyacinths, can similarly bring spring indoors early. Bulbs are not really out of place when considering forcing stems to flower. Most bulbs are, after all, some kind of modified stem.

So forcing flowers out of season from a spring-flowering bulb is not very different from forcing a forsythia stem, the main difference being that a bulb, at least when purchased, lacks roots. But bulbs are fleshier and moister than branches, so can pump some moisture up into a developing flower, and roots can even be coaxed from bulbs in preparation for their show. The first step in forcing a bulb is to plant it, usually in a pot of soil or stones, or to suspend the bulb above water with only its base dipping in. A bulb’s roots, like the roots of other plants, grow whenever soil temperatures are above 40°F., so they can be in place and ready to support leaves and flowers when spring comes.

A bulb’s flower buds are initiated the previous growing season, just like the flower buds of spring-flowering trees and shrubs. And, like the flower buds of spring-flowering trees, a bulb’s flower bud has a chilling requirement and will remain dormant until that requirement is met. So after planting a bulb for forcing, it needs to be kept cool, in that magical 30° to 45° Fahrenheit range.

In time, typically 6 to 8 weeks, which varies with the type and variety of bulb, roots will grow and eventually the flower’s winter chilling requirement will be satisfied. Once the chilling “bank” has been filled, bud growth simply awaits temperatures warm enough to grow. A well-grown, spring-flowering bulb comes packed with a flowerbud-in-waiting...waiting, that is, for a chilling period to break its dormancy and then sufficient warmth to allow growth. Keeping a potted bulb cool at this point is useful for staggering flowering for multiple pots of bulbs or delaying flowering for a specific date, such as someone’s birthday.

When I’m ready to enjoy the flowers, I don’t just bring the pot into a hot room. The flowers would blast open and collapse. The plants, at this point, need gradually increasing warmth, and enough light to draw out a sturdy flower stalk. Forcing bulbs to blossom out of season demands a certain amount of artistry in addition to science.

The whole forcing process can be bypassed when forcing Paperwhite narcissi. These bulbs hail from perennially warm climes and will bloom without any prior chilling. All that’s needed is to pot them up and wait as long as it takes for the fragrant, white blossoms to unfold. To stagger their blooms, pot them up sequentially; lack of water keeps them dormant.