Have you wondered how florists get flowers to bloom at different times of the year? It is done by forcing them, a simple process if you know how and when to do it.

For example, a poinsettia from last Christmas can be made to bloom again this Christmas by controlling the light to which the plant is exposed in October and November. It needs 14 uninterrupted hours of darkness per day, starting about Oct. 1 and continuing until early December.

The garden lily, properly cooled and stored in the fall, can be made to bloom at Easter.If you love cut flowers and would like to have them fresh in the home the year around without having to buy them from a florist, you can grow and force them yourself.

In "Florwers When You Want Them" (Hawthorne Books, 266 pp., illustrated, $10.95), John James, a former greenhouse operator, suggest that forcing flowers extends your gardening horizons.

"The first requisite for forcing is knowledge of what makes a flower flower. Most plants have amazing botanical time clocks," says James. "The deeper we delve into it, the more we discover about this aspect of nature's processes, and the more astonishing it becomes."

In addition to explaining what makes plants flower and how to regulate the lighting and temperature for each flower variety, there are instructions for forcing branches of flowering shrubs and trees, seeds, bulbs, corms and tubers. There is also a special chapter on how to cut flowers and keep them fresh.

Sometimes it can be difficult to get a seed to produce a plant, the author says. Suppose all seeds germinated immediately upon contact with moisture. Since most seeds ripen in the fall, they would sprout about the time winter was at its worst, and the tender seedlings would be killed. Before long the species would be extinct.

Nature must prevent seeds from sprouting until conditions are right for subsequent growth. Some of the more common blocks to germination are requirements of moisture, time, temperature, aeration or light. In order to survive, most seeds have more than one block to germination. Some seeds need times as well as moisture; they need a rest period known as after-ripening, which may last several weeks or months.

Most seeds will not germinate in low temperatures. Other seeds will not start to germinate at high temperatures. And some seeds, even though they germinate under relatively high temperature conditions, first require exposure to near freezing before germination will begin.

Aeration is another germination block. Some seeds will germinate under water, but most will not. Then there is light. Most seeds are not influenced by light but many like begonia, columbine, cineraria, feverfew and kalanchoe, need light to germinate. Such seeds should be planted on top of the soil, with little or no covering.