Deep snow, 40 degrees below, and time to harvest the tomatoes

A couple of years back John Pierce was being interviewed on a radio talk show in a part of Canada where temperatures of 40 below zero F. are commonplace in winter.

Just about anything can be grown in the passive-solar greenhouse that is built right and sited correctly, he told listeners.

''The snow may be deep and the bitter wind howling, but the orchids are in bloom and the tomatoes are delicious,'' was the thrust of his message.

But the interviewer wasn't buying any of it. ''Come on, Mr. Pierce,'' he said , ''this is Saskatchewan; you're misleading the public.''

Pierce insisted he was right, but soft-pedaled just a little. Then came the phone calls from listeners. ''Get off Mr. Pierce's back,'' the very first caller demanded. ''I've picked watermelons in my greenhouse at Christmas!'' They were grown on ''sun power'' only, the caller added. That sort of production is within reach of all of us, Pierce maintains.

John H. Pierce, botanist, teacher, and solar expert, built his first greenhouse specifically for the production of off-season vegetables when he was a high school student in Somerville, Mass.

He has built scores more and advised on the construction of several hundred, both in the United States and Canada, where he now lives.

In the process he's learned a lot, written books and scores of articles on the subject, and been an invited speaker at seminars around both countries. It was at one such conference, here at Longwood Gardens, that I caught up with Mr. Pierce.

Pierce believes that the solar growing device could and should ''become the most common household appliance there is.'' These ''appliances'' range from a heat-storing cold frame, through window greenhouse, to two-story affairs that cover the entire south-facing side of a house. Imagine home-grown oranges in Maine or Minnesota. It's not impossible, according to Pierce.

But why bother? How important are these devices beyond giving us that ''green feeling'' 12 months of the year? As Pierce sees it, such solar devices:

1. Save energy. A Toronto engineer, on Pierce's advice, built an eave-to-ground greenhouse. The next heating season he saw his home fuel consumption drop from 900 gallons of oil a year to 250.

A solar grower also means ''less use of the refrigerator or freezer, because you grow your own fresh vegetables year-round.''

Consider, too, how much energy is consumed to harvest a California-grown lettuce mechanically, pack it, store it, ship it, and finally display it on your nearest supermarket shelf. Compare that with the energy used in walking to your solar grower, picking a fresh head of lettuce, and washing it for the table. Looked at this way, owning a solar grower can even be a patriotic thing to do.

2. Save money. Food savings can be substantial if you grow vegetables even in relatively small greenhouses. Beginners frequently grow vegetables valued at $2 for every square foot of greenhouse space. The experienced grower might readily boost that figure to $5 a square foot and even higher.

Pierce has filled his yard overlooking Lake Ontario with flowers that would have cost a small fortune if all of them were bought as bedding plants from a garden center. Because of his greenhouse, they cost him ''the price of a few packets of seed.''

3. Provide good taste and fragrance year round. The previously mentioned Toronto engineer may have built his greenhouse primarily to heat his home, but it also provides an off-season taste treat from Alpine strawberry plants growing in hanging baskets.

During the bleakest weeks of winter, a majority of growers find harvesting in a passive-solar greenhouse limited to frost-tolerant crops, such as kohlrabi, turnip, or Chinese cabbage. But the engineer harvests tomatoes and melons months before their outdoor counterparts because the ''summer season'' arrives so early in a greenhouse. It also ''holds'' these more tender plants during the more bitter spells, even if doesn't provide ideal growing conditions.

This is why the Saskatchewan listener was able to harvest melons at Christmas. They had done most of their growing during the longer-light periods of early fall.

The best-tasting and highest-quality foods are eaten on the day of harvest. That becomes a relatively simple practice when you own a greenhouse.

4. Bring the tropics to your doorstep. Let no one deceive you. Owning a greenhouse is not necessarily the equivalent of a winter trip to the Bahamas. But there is ''great satisfaction in being among green growing things when the snow lies thickly outdoors.''

Some people who take breakfast in the warmth of their greenhouse describe the experience as a ''delight all its own.''

Pierce's most recent book, ''Home Solar Gardening'' (Van Nostrand Reinhold, New York and Key-Porter Books, Toronto) deals with the construction and operation of solar greenhouses for house, backyard, or apartment. Particularly valuable is the nine-page resource section at the back of the book which lists companies and outlets in the United States, Canada, and Britain which make greenhouse components, sell them, or both.

Next week: Which type of greenhouse is right for you and why passive-solar structures work the way they do