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The Skinny On Solar Greenhouses

The Skinny On Solar Greenhouses

By Larry A. Sagers | Posted - Dec. 31, 2011 at 8:04 a.m.

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Solar greenhouses have always been the dream, but they take on many forms depending on how, when and what you plan to grow in them.

All greenhouses are solar — meaning they depend on the sun to provide light and at least some of the heat during the day.

What makes a greenhouse solar is the fact that it is designed so you add little or no extra heat. You collect the heat when the sun is shining and try and store it so it is released when the sun is not shining.

This week's focus is on a type of greenhouse called a high tunnel. Brent Black and Dan Drost, professors at Utah State University, are doing extensive research on using the high-tunnel system to help gardeners extend their growing season so their produce will command a higher price and be available over a longer period of time.

A high-tunnel greenhouse is an inexpensive structure you can stand up in. Low-tunnel greenhouses, by contrast, are also used to extend the growing season, but they're much smaller, usually about 18 inches high.

The advantage of a high-tunnel system is the cost. Cost for the basic construction starts at less than $1 per square foot and can go up from there. Small greenhouses will often run 10-20 times as much because you put so much into them for additional equipment.

High tunnels are essentially passive solar greenhouses. By definition they lack any electrical components and any source of external heat. There are no fans to cool or distribute heat, and temperatures are regulated by raising or lowering the sidewalls of the greenhouse.

The crops grow directly in the soil, which serves as the solar storage for the heat. In other words, the soil gets warm during the day and that helps keep it warm at night. This helps protect plants from temperature fluctuations and light frosts and protects them from extreme moisture fluctuations and wind.

While much of the work on high tunnels is focused on commercial growers, the system is adaptable to home gardeners.

The first choice is to look at the crops you plan to grow. High tunnels can extend the season on either end by several weeks using warm-season crops including tomatoes, peppers, squash and cucumbers.

They are even better adapted to cool-season crops. Some local growers are using them to produce vegetables during severe winter weather. Imagine picking all the ingredients for a fresh garden salad in January from your backyard greenhouse.

Lettuce — particularly the leafy types — and all of the cabbage crops including kale, broccoli, kohlrabi and various root crops such as radishes and green onions are good choices for high-tunnel production. For additional treats, try some snap peas for a real touch of spring. Black is also doing research on high tunnels to produce blackberries, raspberries and strawberries out of season. These high-value crops are more work but are even more of a treat out of season.

When using these devices, plan carefully to maximize their benefit. Many commercial growers try to maximize their return by producing two or three crops each season in the same house.

They might start growing a short season radish crop and then get melons off to an early start. After they finish, they might plant a late-season crop of summer squash or salad greens.

If the idea of using one of these appeals to you, see if you have a good site. The structure will work best on good, well-drained fertile soil. Avoid areas with serious perennial weed problems, including field bindweed and quackgrass because weeds grow well in the high tunnels.

Look at the light. Now is a good time to check things out because all greenhouses need the maximum possible light in the wintertime.

Make certain the greenhouse will get full sunlight during the months you plan to use it. You also need a water source that is accessible year-round and that will not freeze, because the growing plants will require regular irrigation.

If you only want the greenhouse for extending crops, put it to other uses during the winter. Fill it with you winter firewood supply or use it to store unused garden equipment.

Some people plant a winter cover crop of grain and till that in before planting in the spring.

At least one family also lets the weeds and volunteer vegetables grow and then lets their chickens graze them off during the winter. The birds clean up all the weeds and all of the insects.

If you are looking to extend your gardening season, consider a high-tunnel greenhouse. It might be a lot cheaper than a trip to Hawaii this winter.

Garden tips and events:

For free plans and directions on building your own high-tunnel greenhouse, go to Utah State University Extension is offering two classes in January.

A solar greenhouse construction class will be held Jan. 17, 24 and 31st 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 31st at 10 a.m.-12;30 PM. Each class is $40 per person. To register, call 801-768-7443 or go online at

Written by: Larry A. Sagers Horticulture Specialist Utah State University Extension Service Thanksgiving Point Office

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Larry A. Sagers


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