Michigan State University has an extensive plant and idea
data base at:
http://www.msue.msu.edu/imp/mod03
Here is a piece on solar greenhouses
----------------------------------------------------------------------
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 underthese 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.
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.
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.
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.
Polyethylene 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.
