Most years I go to the Philadelphia Flower Show. The show is held in March and is filled with incredible numbers of flowers. Last year, in the entry hall alone there were more than four thousand flowers, all in bloom and all on display. Considering that Philadelphia is in the northern part of the United States where it can be snowing in March, if ever there was a testament to how to use a greenhouse, this is it. In the northern half of the United States, growing this many flowers takes a heated greenhouse, or the flowers need to trucked or flown from southern climes to the show. However, driving vast numbers of flowers from Florida can be delayed or marooned by snowstorms, so most of the flowers are grown locally.
This is especially true for the prizewinning blooms entered by Pennsylvania Horticultural Society members. Almost all of which are locally grown and the profusion of blooms on display suggests that a large number of gardeners in the Philadelphia area are greenhouse owners who know a lot about forcing bulbs and plants to flower ate exactly the right time.
So how do you force a plant to flower exactly at the right time? The technique depends on the plant, the light period and the temperature. For example, when forcing bulbs for an event, timing is critical. Daffodils and tulips can be started eight to ten weeks before the flowers bloom. Amaryllis will take ten to twelve weeks. All you need do, is put the bulb in a pot and keep them at a temperature of about 65 to 70 degrees., Indoors is usually fine. The bulbs will require additional light, ideally 12 -14 hours of light, which is about what you use indoors. In your greenhouse, you might use a timer and a grow light to give the plants exactly the right amount of light.
Other plants are a little more difficult. Perennials need six to eight weeks of low temperatures (roughly equivalent to the winter months) and more darkness than daylight, before bringing them into the warmth to get them grow. You can simulate this effect in the middle of summer by putting your plants in a cool spot (a working old refrigerator, for example) and keeping the door closed for up to 14 hours per day. To get the plants to bloom, shorten the periods of darkness to less than 12 hours per day and pace the plants in an area where the temperature is at least 65 degrees.
You can do this with many plants to force them out of season, for example chrysanthemums normally bloom in the fall. If you want them to bloom in spring, you need to reverse the seasons. Keep the plants warm and well-lit for the winter months and reduce light levels in spring to trigger the blooming period.. By varying light and heat levels, greenhouse owners can produce flowers at exactly the right time for the show or can force flowers all winter long for home use.