Greenhouses can be cheap yet effective

Greenhouses come in all shapes and sizes. Last week's article discussed a superb solar-heated greenhouse in South Jordan that was built by Steve Petersen.

But there are more modest alternatives to extending the plant-growing season in Utah.

The earliest climate-modifying devices were bough or branch coverings that protected plants from frost. A cloth or blanket on a frosty night can also modify climatic conditions. While these methods may keep plants from dying, they do nothing to help them grow better because they block the light.

One early Roman emperor loved cucumbers and wanted them year 'round. His slaves used sheets of translucent mica to cover cucumbers and other vegetables to help them grow out of season.

Later, when glass bell jars became common, they provided a way to help tender plants survive out of season. But the jars were expensive, and when the sun came out, they had to be removed to prevent the plants from burning. Some gardeners tried using clay pots with lids, but they were labor-intensive and didn't let in enough light.

Credit the Dutch with building the first greenhouses, uncomplicated devices known as hotbeds or cold frames, more than 250 years ago. These small — yet effective — structures let gardeners get a jump on the growing season by starting plants earlier using the natural heat of the sun. After temperatures rise and the plants are established, they can be moved in the garden.

Replicas of these devices can be found in Colonial Williamsburg, and many still use these devices to start plants for their gardens today. While the first cold-frame greenhouses took advantage of extra solar heat on the south sides of buildings, growers have learned to add extra heat to accelerate growth.

While hotbed and cold-frame greenhouses aren't big enough for you to move around in, they're plenty warm for your plants. Besides, a greenhouse big enough for you to move around in would be more expensive to build and heat.

By definition, a cold frame is a plant bed that protects plants. It is called a cold frame because no artificial heat is added while growing the plants. The temperature difference inside of the frame is generally five to 10 degrees warmer than the outside, just enough to get plants off to an earlier start or to protect them from fall frosts.

Cold frames are true solar-heat collecting devices, and a well-designed structure maximizes heat collection and takes full advantage of the heat conservation and heat storage.