Solar greenhouses: Low maintenance, low-cost gardening season extenders

By The Old House Web

Solar greenhouses: Low maintenance, low-cost gardening season extenders

A passive solar greenhouse system is one in which the greenhouse acts as the collector of solar heat. This heat is then stored in an internal water or rock storage. In contrast, an active solar heating system has external collectors and fans or pumps that circulate the warmed air or water into the greenhouse or into a storage.

A passive solar greenhouse has several advantages to the home grower. Its construction is relatively simple, it is low maintenance and there is a low initial cost in building such a greenhouse. The greenhouse is very useful as a season extender, providing the grower with a greater longevity in the growing season and a greater variety of crops for growth.

Vegetables can be started in the greenhouse earlier in the spring which makes them mature sooner. In the fall, tomatoes, peppers, and similar crops can be transplanted and moved into the greenhouse before a frost. By doing this, harvesting can be extended.

Mid- winter vegetable crops are marginal in the Northeast because of shorter days, low angle of the sun, and cold and cloudy weather. Some vegetables will grow under these conditions but at a much slower rate. The use of a greenhouse will serve to improve the quality of these vegetables and an enhanced rate of growth.

Some suitable vegetables and herbs for production in a winter solar greenhouse include:

  • Excellent: Basil, celery, dill, fennel, kale, leaf lettuce, marjoram, mustard greens, oregano, parsley, spinach, Swiss chard, turnip.
  • Good: Cabbage, collards, garlic, green onions, leek. Cherry tomatoes and cucumbers(European type) are considered good, but require warm temperatures.
  • Fair: Broccoli, edible pod peas. Eggplant, large tomatoes, and peppers are considered fair, but require warm temperatures.
  • Poor: Beans, carrots, corn, radishes, turnips. Melons and squash are considered poor because they are naturally pollinated by bees. If hand pollinated, they can be listed as fair.

Misconceptions about greenhouses

Many home growers do indeed opt for the creation of a home greenhouse. One misconception in planning a greenhouse is the assumption that the structure will aid the grower to save on high energy bills. However, there is a contradiction between the use of a greenhouse to grow plants and the use of it as a solar collector for heating the house. While it will cost little money to heat the greenhouse itself, the addition of plants increases the size of the heating bills.

It must be considered that the greenhouse itself is not a reliable heat source. Heat builds up on sunny days, occasionally building up to a temperature high enough to cook the plants if the house is not vented. If the heat is vented into the main building, it will warm up the adjacent rooms. Also, if the hot air is forced to circulate over a bed of stones (incorporated in some greenhouses), the stones will retain the heat and radiate warmth during the night. When the sun is not out, the greenhouse is not trapping much solar heat and will need supplemental heating, as most houseplants will not tolerate temperatures below 50 degrees F. Additional heat may be supplied by a greenhouse heater or piped in from the main building; either of which cost additional money.

Consequently, heating the greenhouse is often a problem at night. During the coldest winter months, even after a day of sun the greenhouse will require additional heat at night as well as in the early morning before the heat levels build up. The average daily heat loss is about 310,000 BTU's (in an average 8 by 12' greenhouse) while the daily heat gain is only 26,000 BTU. Even a solar greenhouse will lose about 21,500 BTU's, an amount slightly less than one-half of what it takes to heat it. Only on sunny days will it collect enough heat to maintain itself for twenty-four hours and provide a surplus heat to the main house.

If the greenhouse is intended to function as a solar collector, it will serve to counteract the amount of money saved. However, because it houses plants, it cannot be closed off from the main house and not heated when not conserving heat. Even if the plants could stand the temperature drop, they would be faced with a number of circumstances trying to their survival. In a tightly sealed greenhouse, the carbon dioxide within would be used within two hours of sunrise. Excessive humidity may also build up, causing disease and rot to set in. Watering problems also become common at colder temperatures, leading to root rot and death.

Minimizing costs

In order to minimize heating costs, many growers are forced to close their greenhouses down during the coldest part of the winter. This practice may be feasible if the greenhouse is intended for growing transplants; however if may be difficult if the greenhouse contains more plants than there is room for in the main house. Under these conditions, the plants could be kept on wheeled carts and moved into the house during the night or on a cloudy day.

The location of the greenhouse also aids in minimizing its heating costs. The greenhouse should be located on the south side of house; if the greenhouse is oriented 45 degrees off from solar south it will only be 72% as efficient. Western and eastern exposures will provide some heat savings but are even less efficient, and northern exposures should be avoided altogether, as the greenhouse will receive little or no direct sunlight in this location.

Another source of heat loss is the glass of the greenhouse, which often allows the escape of heat. The glass area may be reduced by sinking the greenhouse partially into the ground at construction time or by the use of concrete blocks for the construction of the lower portion of the wall. Polethylene plastic and fiberglass are also alternatives to glass. The greenhouse will receive maximum light and solar heat if the roof angle is at a 50 degree F slope; this plan should be implemented upon construction.

Other strategies for heat conservation include: the covering of the glass with clear plastic, with air forced between the layers, the use of insulated shades which can be pulled down at night (but may be cumbersome if plants are in the way), the caulking of all glass panes with a transparent greenhouse material and making sure that all doors and vents leading to the outside fit tightly to their frames.


"A Home Greenhouse--Dream or Nightmare?" by Nancy J. Butler Weed 'Em and Reap; Feb.-March, 1985 MSU Cooperative Extension Service

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