Mother's Mini-Manual: Greenhouse Gardening

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A rule of thumb for estimating the safety margin for most greenhouses is a heat loss of 1.2 BTUs per hour for each degree of temperature differential (difference between inside and outside temperature) per square foot of exposed surface.

When you arrive at the greenhouse BTU requirements, select a heating system which is equal to at least 10 percent more than required. There is far less wear and tear if the capacity is a little larger than actually required. Then, too, if you encounter a record-breaking year with low temperatures, the heating system will carry you through. Even though greenhouse plants are not directly exposed to outside temperatures, they can be lost if the weather is unusually cold and your heating system is adequate only for seasonal temperatures.

SAVING MONEY AND FUEL IN YOUR GREENHOUSE³

A greenhouse is a pretty big consumer of fuel, so you should be aware of the many things that you can do to keep your heating needs as low as possible.

The biggest mistake most hobbyists make (and many commercial growers, too) is to run their greenhouses too high, day or night. You can save a lot of money and fuel by growing "cool" rather than "warm" plants. Whatever plants you do grow, it's smarter to run the temperature a little on the low side. Although growth may be slowed, many plants can get along fine with a few degrees less than optimum.

In addition to just plain lowering the greenhouse thermostat, there are lots of other things that you can do to conserve heating fuel:

1. See that your greenhouse is as airtight as possible. Replace broken and slipped glass and torn plastic. Make sure that all doors and vent sashes close tightly, especially in winter. A film of plastic sheet or sheets of clear fiberglass, installed inside the greenhouse, will create an air space between the outside and inside layers. Air, as you might know, is the best insulation there is. A good hermetically sealed inside layer of plastic or fiberglass could reduce conducted heat loss by as much as 40 percent, although 20 percent is more realistic because of the difficulty of getting the inside layer fastened tightly all around the greenhouse. Don't worry about the reduction in light due to the extra layer of plastic or fiberglass. Today's greenhouse plastics cause a minimum of light reduction.

2. To cut out drafts, have two outside doors, so that one can act as a storm door. Better yet, build onto the outside door a small foyer. This foyer will create an air pocket, and have an insulating effect as well as helping to cut out drafts.

3. If your benches are open on the bottom, use drop cloth "skirts" around the base of the benches to help confine heat above them, where it's really needed.

4. Mulching the soil in raised benches can help insulate the soil and prevent heat from the soil from being lost to the air.

5. During the cooler seasons, consider watering tropical foliage plants and other warm-season plants with water warmed to 65° F.

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