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Larry Sagers
Good construction techniques helps conserve the heat in a passive solar pit green house.

As NASA and other agencies probe deep into space, one of the things they are looking for is planets that might support life. While there are many different necessities for life as we know it to exist, one that is absolutely critical is the sun, or star as it is called in another solar system

The sun is the basis for life. If the sun is too close, the planet gets too hot and burns up. If it is too far away, it would be too cold to support life as the planet would be a constant deep freeze.

In addition to providing heat, the sun also provides light. Light energy is critical for plants to grow. If plants did not grow, there would be nothing for animals to eat and we would die.

While ancient civilizations did not understand the scientific principles of photosynthesis, they knew the sun was critical to their life. Many worshipped the sun as a deity and adopted many rituals in hopes of influencing the power of the sun in their life.

While our reliance on the sun to grow our plants in northern Utah is automatic in the summertime, in the winter it takes special attention to capture and utilize the sun to grow our plants. It might be as simple as a few interior plants on the windowsill, as inexpensive as a cold frame or as extensive as a large solar greenhouse, but all depend on the sun to get plants to grow.

Solar devices take many forms, and while all greenhouses are solar, the difference between a solar greenhouse and conventional ones is that solar structures collect and store the heat that the sun radiates during the day and then release that heat on cloudy days or at night.

Solar designs have three basics. They must maximize South-facing glazing, they must have an efficient way to store the heat and they must minimize heat lost with good construction techniques.

To make a solar greenhouse function efficiently, it is important to size the collection system to the greenhouse size. In our climate, a greenhouse needs between 0.65 and 1.5 square feet of South-facing double glass greenhouse collecting surface for each square foot of floor area. This varies according to the climate and the storage media.

Storing solar heat is somewhat difficult because the heat is almost always collected inside the growing structure. That being the case, the temperature cannot rise above the level that is best for growing plants. While solar heat collectors could operate efficiently at temperatures between 150 and 170 degrees, it would be far too warm to grow plants.

In addition to collecting heat, storing heat is critical. Storing heat requires storage media and the two most common are water and masonry. Each has advantages when trying to grow plants.

Temperatures inside greenhouses fluctuate as much as 60 degrees Fahrenheit on a sunny day. To level out fluctuations, masonry storage or containers filled with water reduce temperature fluctuations. One cubic foot of water for each square foot of South-facing glass reduces temperature fluctuations about 25 degrees.

Water storage is most efficient as it absorbs the heat more slowly and releases it more slowly than masonry. This allows the greenhouse to stay above critical temperatures for a longer period of time.

The disadvantages of water are that it has to be contained in an efficient, affordable and durable container. This usually means a container for less than five gallons. At one time, 55 gallon drums were widely promoted as good heat storage devices, but they take too long to heat up and do not collect the heat efficiently because of their surface-to-volume ratio.

Affordability is another issue. Outfitting your greenhouse with enough containers to store the needed amount of solar heat may be cost-prohibitive. While several dollars each is affordable if you are only buying one or two, it might make buying 500 or 1,000 containers cost- prohibitive.

You can reduce costs by recycling certain containers, but durability becomes another issue. While milk jugs are abundant, they are manufactured so that they break down quickly in the ultraviolet light from the sun, which makes them a poor choice to use to store heat in water.

Metal cans rust, and glass bottles are expensive and break easily. That leaves the most logical choice as the pop bottles made from polyethylene terephthalate (PET) plastic. This strong, lightweight plastic is readily available and resistant to breakdown in the sun when filled with water.

Other people use masonry to store heat. It is not as efficient at dampening the temperature fluctuations, but it often is easier in terms of construction techniques and maintenance. Bricks, rocks, concrete and other materials often are used to store heat.

Construction techniques are also critical. Double glazing, tight construction to prevent heat loss and good insulation and caulking are all included in this category. Paying attention to these are often the least costly but very important in making solar devices work.

Solar energy harvest is a great way to extend the gardening season. This might be the year to add a hotbed, a cold frame or even your first greenhouse. It can help grow your plants out of season and dispel the winter blues.

Garden tips

Utah State University Extension is offering two classes in January. To register, call 801-768-7443 or go online at www.thanksgiving point.com.

A solar greenhouse construction class will be held Jan. 17, 24 and 31 at 2-4:30 p.m. or 6-8:30 p.m. Take advantage of the sun to heat your greenhouse. Extend your outdoor garden or become more self-reliant. Course topics include choosing a site, energy conservation, low-cost construction techniques, heating, cooling, glazing, hot beds and cold frames. This is the only time of the year that this class is held.

A basic landscape design class will be held Jan. 17, 24 and 31 at 10 a.m.-12:30 p.m. Each class is $40 per person.

Larry A. Sagers is a horticulture specialist for the Utah State University Extension Service at Thanksgiving Point.