Mother's Mini-Manual: Greenhouse Gardening

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If there is a humidity problem, it's usually because there is too much humidity, not too little. The temperatures in greenhouses are higher than the temperatures outside, and the warmer the air the more water vapor it can hold. If ventilation is poor, greenhouses will be like "steaming jungles," with the high moisture content encouraging vegetative or leaf growth at the sacrifice of fruit and flower production. It also invites plant diseases like damping-off and botrytis, which thrive in excessively damp and humid areas.

Of course, some growers do have problems with too low a relative humidity level. This generally occurs in dry parts of the country where greenhouses are over-ventilated. Too little humidity hastens the development of roots, flowers and fruit. If your problem is overall too low a humidity, hose down your gravel, sand or cement aisles frequently, reduce ventilation and mist those plants that need a lot of moisture.

HEATING THE GREENHOUSE¹

Since fuel oil, electricity, natural gas, bottle gas or coal are all satisfactory fuels for use in greenhouse heating, choose the one most likely to be available during the heating season. Low operating cost is another factor to consider in selecting a fuel.

EXTENSION OF THE HOME HEATING SYSTEM¹

If the BTU capacity of your home heating system is large enough to heat only your home, do not burden it by also trying to heat the greenhouse with it. On the other hand, if the system is oversized, there is no reason why you cannot extend it to the greenhouse. This is particularly true if the greenhouse is small or is attached to the home.

Unless you're a heating expert, secure advice from a heating contractor, mail-order house or greenhouse supplier about installing an additional zone for heating the greenhouse. Generally, this is not an expensive job to do with hot water or steam systems. It's a simple matter of extending the heating pipes to the greenhouse. For hot air systems, cold and warm air ducts can be extended into the greenhouse, providing it's not located too far from the home. With either of the three systems, a thermostat located in the greenhouse can control temperatures without interfering with the home heating requirements. Locate the thermometer at plant growing height and not for your own convenience. It is best to use alcohol-filled thermometers. Thermometers filled with mercury will damage certain plants, such as roses, with vapors given off if broken.

SIZE OF HEATING SYSTEM¹

Since most heating systems state their output rating in British thermal units (BTUs), you should know the BTU requirements for your particular greenhouse. These ten guidelines indicate why greenhouses of the same size may require vastly different heating systems:

1. The minimum inside temperature desired.
2. The lowest anticipated outside temperature.
3. Amount of exposure to winds.
4. Nature of the greenhouse surface area.
5. Double or single layering of side walls.
6. Double or single glazed greenhouse.
7. Amount of straw or other material used to cover the roof (not to the point of blocking out too much sunlight) during extremely cold weather.
8. Desirability of maintaining maximum temperatures during extreme periods of cold weather.
9. Attachment of greenhouse to heated building (as with a lean-to greenhouse).
10. Excellent, good or poor operating and maintenance schedule.

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