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Larry A. Sagers Regional Horticultural Specialist Utah State University Extension Service Thanksgiving Point Office © All Rights Reserved
The recent snows made it very evident we live in an area with four definite seasons. Growing plants out of season require special protection. The earliest climate modifying devices were boughs or branches covering that protected plants from frost. A cloth or blanket on a frosty night also helped modify climatic conditions. While these methods kept the plants from dying, they did nothing to help them grow better because they blocked the essential light.
Credit the ingenious Dutch with building the first greenhouses. They first constructed less complicated devices better known as hotbed or coldframes. These structures let gardener get a jump on the season. Later, plants were moved in the garden to let them grow and mature.
Replicas of these devices that operated more than 250 years ago are in Colonial Williamsburg. “Colonists” still use these to start plants for their gardens today. Cold frames use the natural heat from the sun. The first ones took advantage of extra solar heat on the south sides of building. Later, growers added extra heat to accelerate their plant growth.
While I have no way of proving my hypothesis, I suspect that an ingenious gardener who was tending plants inside a cold frame on a winter day invented the large greenhouse. While reaching inside to care for the plants, he realized inside the cold was warm, outside was cold so why not make the structure for the plants big enough so I can move inside. That way I can be warm and protected from the weather like the plants.”
The upside seemed obvious to the gardener, but they downside is the same complaint as today. Making the structure big enough for you to fit inside makes it much larger so it is much more expensive to build. It is also much more expensive to heat and to cool simply because the larger the volume of a structure, the more heat as needed.
For those who want to save money, hotbeds and coldframes are another alternative. No, you cannot crawl inside to stay warm with the plants but if your primary interest in having is to grow plants inside to get a jump on the gardening season outside, these structures are for you.
By definition, a coldframe is plant bed that protects plants but that has no artificial heat added while growing the plants. Although cold frames have no artificial heat provided, they are not without heat. They are true solar collecting devices and a well-designed structure maximizes heat collection and takes full advantage of the heat conservation and heat storage.
A hotbed is a heated coldframe. It is essentially a miniature greenhouse and provides a heated environment for growing plants at minimal expense. The first hotbeds got their heat from decomposing manure and that is how I heated my first hotbed.
The recipe was simple but smelly. Dig a pit three feet deep under the coldframe. Fill the pit with fresh horse manure. Add several inches of sand or soil on top of the manure. Plant the seeds in the soil or pots and let them grow. The smell of shoveling manure in and out of the hole and the lack of heat control prompted gardeners to select less smelly ways of heating the hotbed. These include buried electric heat cables, light bulbs or other devices.
The design is very simple. Build a rectangular box with the back higher than the front and covered with a transparent roof. Whatever the size, make the slope from back to front 1 inch per foot. Other variation can increase the size or the complexity but the basic premise is the same.
Covers include fiberglass, acrylic plastics and double walled greenhouse plastic or clear polyethylene stretched on wooden frames. Glass is heavy but also works well. Because of the danger when it breaks only use tempered glass.
Make the sides of the structure of wood, brick or concrete. Concrete or brick are more permanent but are more costly than wood. Wood is the easiest to fabricate and will last longer when treated with a preservative that is not toxic to plants. Paint the structure with white paint for added light reflection. Lining the frame with water-resistant foam insulation will greatly increase the heat efficiency of the device.
Poor man’s greenhouse does not mean poor quality plants. Learn to build and use one of these devices and grow wonderful transplants for your garden. If you own a greenhouse or have ever thought of building one, consider attending our hobby greenhouse short course. The class will cover whether you want or need a greenhouse in your gardening situation. We will also cover constructing a greenhouse with attention to greenhouse structures and greenhouse coverings. Selection of construction materials is important to keep your greenhouse safe, practical, enjoyable and efficient.
The class is Tuesdays from 2:00-4:00 or 6:00-8:00 P.M. at the Thanksgiving Gardens Visitors Center in classroom. Classes begin on January 6 and run for four weeks. The class fee is $40.00 and includes the greenhouse design booklets and other written materials. Please register by calling 801-768-7443 or toll-free 1-888-672-6040 (ext. 7443) to register to guarantee your place in the course. You can also register online at www.thanksgivingpoint.com and then click on the education icon.