Cauliflower under glass; The virtues of a greenhouse: saving on the greengrocer's bill, for one

When an ordinary can of vegetables edges up to half a dollar or more, and fruit is completely out of sight, it's time to start looking into all the ways you can save on the food bill.

Greenhouses were once a rich man's tropical paradise where exotic flowers were raised, but now they are growing a new image. Instead of flowers you're apt to find cauliflower. Instead of orchids you might find onions.

Just think of the advantages: no cutworms to mow down seedlings, no wind to thrash plants about, and no drought to wither them away.

Garden insects are at a minimum, but if some do manage to invade, a chameleon , some praying mantises, or even a toad can bring the enemy under control. These warriors can also provide an interesting science lesson for the children, not to mention what the youngsters will learn from watching plants sprout and grow close at hand.

Greenhouse enthusiasts claim a modest greenhouse can pay for itself in just a few years, and there are many intangible advantages as well.

What vegetables can you grow that would help pay for a greenhouse?

Don't limit your thinking to lettuce and radishes.Beans, beets, peppers, peas , squash -- all do well under glass. Most vegetables can be grown in a greenhouse and you can enjoy one crop after another all year long.

Seed houses have developed miniatures that are ideal for greenhouses: cabbage , cantaloupe, carrots, short-vine cucumbers, short sweet corn, and watermelon, for example. You can also go vertical with pole beans, peas, and tomatoes on trellises.

Special strains of tomatoes developed for the lower light conditions in winter greenhouses don't taste like those plastic supermarket monstrosities, because they're ripened on the vine.It is wise to keep your space-yield ratio in mind, however, and mainly grow vegetables that will produce a lot for the space they take.

A greenhouse can save you money in other ways.

You can raise all of your long-season seedlings for outside: cabbage, cauliflower, broccoli, Brussels sprouts, and tomatoes. You can start muskmelon, cucumbers, squash, and watermelon inside if you plant them in peat pots so as not to disturb the root systems when you plant them outside.

Herbs are another garden treat suited to greenhouse growing. Basil, dill, parsley, chives, marjoram, mint, and thyme are a few that are easily grown.

Raising flats of bedding plants for your flower garden can save many dollars and give you a better garden, because you can get seeds of many more varieties than you can buy in plants from nurseries.

Another way to save money in your greenhouse is to raise houseplants from seed or cuttings. These make welcome and inexpensive gifts, and you can easily sell them as well. Many people have started doing this in a small way and soon found themselves in big business.

You can even raise fruit. You can have delicious free strawberries in midwinter from plants you potted up in summer and brought in about November. Fig trees adapt well to container growing, and you can even have a grape trellis along the ceiling.

Heating greenhouses used to be prohibitive, but the developments in double walls and insulating materials have cut heating costs considerably.Keeping a cool greenhouse saves another hefty chunk of heating post. Pennsylvania State University greenhouse engineers found a saving of 30 percent on the heat bill by lowering the thermostat from 65 degrees to 55. Most vegetables will even thrive with a nightime temperature of 50 degrees.

Another heat cost saver is passive solar heating, using something like plastic bottles or large cans painted black, filled with water, and put under a bench where the sun will warm them all day.

At night the water releases its heat upward to the bench where it's needed.

Another tactic is a plastic tent drawn over the plants at dusk to keep in daytime heat.On the coldest nights, soil- heating cables can be used to keep the bench soil warm without heating the air.

These have all been financial considerations, but you also have to consider the pleasure of eating food grown in a greenhouse. Besides the beautiful flowers and greenery that make you feel good every time you see them, the actual work in the greenhouse is calming and revitalizing at the same time. Who can be restless when they're transplanting eager little seedlings or propagating a tired houseplant?

Just being able to sit, relax, and soak up some of the sunshine is worth the cost for many people.

If you have decided that there is a greenhouse in your future, you have a fascinating search ahead of you. But don'tm start by writing for brochures from 25 greenhouse manufacturers. Instead, go to your local public library or bookstore and get some books, such as "The Greenhouse Gardener," By Elvin McDonald, or "Organic Gardening Under Glass," by George and Katy Abraham. These will tell you about the products on the greenhouse market and help you make decisions, such as the type of greenhouse to buy (window sill, free-standing, or lean-to), materials (glass, fiber glass, or plastic, clear or frosted, redwood, other wood, or aluminum), glass-to-ground or sidewalled, foundation or not, size , shape, single or double walled, type of ventilation, and location.

After you've made some of these decisions, then look at the greenhouse ads in garden magazines. You now can find those few that might satisfy your requirements and write for their brochures and prices.

Don't overlook the possibility of building your own greenhouse.

You can probably save about one-third of the cost of a comparable kit greenhouse this way, and if you have some of the materials already on hand (lumber, windows, a storm door), you can save even more.

Another advantage in building it yourself is that you can design it to your own particular needs.

Books that will help you plan and build your own greenhouse include "Build Your Own Greenhouse," by Charles Neal, and "Your Homemade Greenhouse and How to Build It," by Jack Kramer. A plan for a simple free-standing greenhouse can be obtained by writing for Cooperative Extension Service Circular No. 880, "A Simple Rigid Frame Greenhouse for Home Gardeners," from the Department of Agricultural Engineering, University of Illinois, Urbana, IL 61801 (if you live out of state send 10 cents). Or you can write to your own state extension service for similar publications.

If commercial greenhouse kits are too expensive and building plans too complicated, or if there is no good place to build one on your property, there is one more alternative for you to consider: a basement greenhouse.

At first the idea appears ridiculous because you don't have the sun available. But you do have something that is of great importance these days and that is heat without additional cost.

You can enclose a corner to keep the humidity in, put some boards or an old flush door on some concrete blocks for shelves, and hang some fluorescent light fixtures on chains from the rafters with a timer to turn them on and off automatically for light.

Most basements have the needed source of water. One family built a 6-by-8 foot basement greenhouse with windows in it for less than $300 (see Plants Alive magazine, February 1978).

Owning a greenhouse can be a wonderful, money-saving hobby that provides not only vegetables and flowers, but pleasant relaxation and recreation. The only problem is in getting yourself to actually pluck a nd eat those picture-perfect vegetables.