John Sylvester's back yard in Northwest Washington is shaded by three huge, leafy trees: an oak, a tulip poplar and a maple. Grass grows only reluctantly; planting a vegetable patch is not worth a try.

But Sylvester, a retired admiral, is an avid gardener. He likes a constant bloom with a lot of colors and enjoys all phases of gardening, from planting to propagation. He first caught the fever in the early 1950s when he was asked to take care of some gardenias in a house he rented for a few months. The gardenias thrived under his management. In the 1960s he acquired plants of his own when he had at his disposal the conservatory of the Navy Yard in Southeast Washington, and was hooked for the rest of his life.

After his retirement in 1964, he began thinking of ways he could do some serious gardening. With the help of his son, he set up a greenhouse: a classic aluminium-and-glass structure based on cinder blocks, 16 feet long and 10 feet wide. And because in the winter those huge trees all around are leafless and his greenhouse gets plenty of sunshine, he decided to concentrate on winter gardening.

The first thing I noticed on a visit to his greenhouse is the overpowering scent: a sharp fragrance between jasmine and carnation, coming from two orchids (zygopetalum). Sylvester has had them for years, and they bloom every winter, for a full month. They are spectacular, with their purple and white striped lips and mottled green-and-brown petals. He also has some cymbidiums that bloom later.

The temperature variation Sylvester has decided on is ideal for orchids: 65 degrees Fahrenheit during the day and 50 at night. If the temperature rises above 65 degrees, vents open automatically.

Sylvester's pride is his cyclamen collection. He raises the plants from seeds, divides them regularly, and has too many of them. "I have to make a decision soon," he says. "I have to give some away." His cyclamens are luxuriantly healthy, full of buds. "They bloom all winter," Sylvester says. "They don't stop blooming."

His other favorites are bulbous plants of South African origin. He has a collection of up to a dozen species, and they bloom one after the other, all winter long. The one currently in bloom is Lachenalia pendula, with a profusion of delicate, hyacinth-like flowers. The purple variety is always the first to bloom, Sylvester says. "It is always ready by Christmas, and it blooms until the end of January." Next to bloom is the yellow variety, then a tricolor--red on top, yellow and green at the bottom.

"I have bulbous South African plants blooming up to April," he says. "But when it starts getting hot, they quit."

When summer comes, Sylvester puts his plants outside and keeps watering them. After the foliage turns brown, he cuts off everything above the bulb. He stores the pots under the greenhouse bench, and lets them stay there without watering until late August or early September. Then he starts watering them, and they come back to life.

He also grows some annuals, such as a coleus with blue flowers, mostly from the Pacific northwest. In addition, he has herbs, geraniums, lantanas and one orange tree, which, though not one of his prides, gives an orange or two every year. His is a crowded greenhouse, he concedes, but the only problem he has is with whiteflies.

He keeps his greenhouse shipshape and caulks it regularly. He labels each plant meticulously--"it's confusing otherwise," he says. "You've got to keep order."

But he doesn't know how many plants he has there--"I have no idea," he says. He uses a gas heater, but has never tried to figure out how much it costs--"perhaps $15 on a cold month" is his reluctant, "surely imprecise" estimate. The only chore he complains about is his having to wash pots when repotting. "I use a lot of pots," he says.

"The only problem with a greenhouse is that you have to be here," he says. "Or have someone very obliging to look after your greenhouse. It's a very confining hobby. But fascinating. In the winter, when everything else is drab, you go into the greenhouse, and there is color everywhere."