The technique of bringing plants into flower outside their natural season is known as forcing. It is routinely practiced by growers to produce plants in flower over a longer period, and involves manipulating the plants’ days. Flowering is induced by the amount of light in hours (day length) rather than by its intensity, so if you use black plastic to simulate nighttime and electric light to simulate day, the plants can be fooled into triggering the formation of buds.
Bulbs that naturally flower in the spring, such as hyacinths, can be brought into flower during the winter by planting them in the fall and keeping them cool and dark (and the soil moist) for 6—10 weeks. When the shoots are 1—2 in. (2.5—5 cm) high, the bulbs can be brought indoors into a cool, well-lit spot; the leaves will extend and turn darker green, and the flower buds appear. They can then be moved to a warmer, brightly lit place to flower.
Choose bulbs specified for indoor use, and keep the soil moist throughout. As the flowers die down, it is important to feed and water the plants in order to replenish the food within the bulb ready for next year. After the fear of frost has passed, they can be planted outdoors to be enjoyed for years to come. Or you can lift them and store them over the summer in a cool, dry, dark place.
Plants that flower indoors fall into two categories: those which flower annually without help, and those which are discarded after flowering. Of the latter, some, such as poinsettias and azaleas, can be coaxed to flower again. Poinsettias can be brought back into color, but will be taller than before, because commercial growers use growth regulators to produce a compact, bushy plant.
After the bracts and leaves fall, cut the stems down to 4 in. (10 cm) stumps. Keep the plant almost dry until early summer, then repot (in the same size pot), and increase watering and feeding, selecting 4—5 strong stems. In fall, cover nightly with black plastic for 14 hours for 8 weeks, then treat normally to produce bracts for winter.
If you have the time, space, and patience to persevere with them, there are many plants that can be brought back into flower a second time. They may not be as spectacular second time around, but this is much more satisfying than discarding them.
Plants for forcing
- Bulbs: Narcissus, hyacinth, crocus, ins, tulip, amaryllis, lily, and freesia can all be brought into flower outside their natural season by controlling their growing conditions.
- Flowering plants: Azaleas and poinsettias can be brought back into flower by manipulating day length and temperature. Other flowering plants can be brought forward or held back when they have set buds by moving them to a warmer or cooler position.