In summer, sunlight is plentiful and readily available to plants. Flowers and plants receive light and heat from the sun and convert this energy into plant mass. Available sunlight is reduced in spring and fall, though—and in winter, light levels drop to their lowest annual point. To extend your growing season into these less well-lit months, things like cloches, hoop houses, cold frames, hotboxes, greenhouses or hothouses can all be employed.
Sunlight is comprised of a wide range of electromagnetic radiation that travels from the sun to your plants. Plants make use of both visible light (for photosynthesis) and infrared radiation (heat). Full summer sunlight has approximately 400 to 450 watts of visible light and over 500 watts of infrared energy per 10 square feet.
In comparison, a 400 watt HPS light is about 20 per cent efficient for visible light, so it would take five to six of them per 10 square feet to get the equivalent amount of visible light—and there would be a huge excess of infrared radiation. Even using mighty 1,000 watt HIDs, you would need two or three per 10 square feet to bring the same amount of light indoors. Outdoor gardens enjoy free light, which can greatly reduce the expense of bringing a plant to harvest.
While the visible portion of sunlight might still be powerful enough in cooler months to fuel plant growth, the amount of available infrared radiation is greatly reduced. By capturing and storing available heat, however, growing seasons can be extended even when outdoor temperatures drop below your plants’ comfort zone.
One simple way to trap the heat from the sun is to paint an object black—a black object will absorb more infrared radiation than a white one, which will reflect much of the heat and thus remain cooler. Sunlight striking a black surface will release its infrared radiation into that surface—thereby warming it.
Heat from the surface of an object will eventually warm its interior. Soil solarization uses this principle to raise soil temperatures high enough to kill most weed seeds and other pathogens. Black sheets of plastic are used to cover soil under summer sunlight—the sunlight heats the plastic and the air trapped under it, bringing temperatures high enough to sterilize the soil by killing most seeds and many harmful disease spores.
While black is an ideal color for collecting and storing solar heat, it is opaque to visible light. Covering a plant with a sheet of black plastic would certainly help keep it warm, but the plant would die from the lack of visible light. Since plants need both visible light and infrared radiation to survive, the material covering your plants should not only be at least semi-opaque to infrared light but transparent to visible light.
If an elevated transparent material is used the amount of heat that is collected and trapped will be reduced, but the visible light that plants need to grow can pass though. This is why glass, plastic sheeting and other clear materials are used when trapping solar radiation for plants—the material should be as clear as possible to allow for visible light transmission, but still retain some opacity to infrared radiation for retaining heat energy.
As long as a structure absorbs heat faster than it loses it, it will enjoy a net gain and the internal temperature of the structure will rise. In this way a structure can absorb heat during the day and release it later, during the cold hours of night. The smaller the area covered, the less buffered temperature fluctuations are—a small object heated by the sun will cool faster than a large object due to the difference in mass, so a cloche will not store heat as well overnight as a full greenhouse would.
Cloches and cold frames are often useful for growing cool-weather plants in spring and fall, but they generally don’t supply enough heat during the winter months, especially at night.
One way to add heat retention to an existing transparent structure is to include large black objects—often filled with water—to the interior. If heat loss during the night is still too great, additional heat might need to be added to maintain minimum plant temperature requirements.
During a cold snap, covering your transparent material with something insulating—such as burlap sacks or black plastic—in the afternoon can help the stored heat last until morning. During summer months when heat is abundant, strips of opaque material can be used to help shade the plants inside.
Humidity will also tend to become trapped inside—which might be an advantage in dry environments—but this might create the kind of high-moisture environment that is attractive to pathogens if things are kept too wet. Humidity levels can be manipulated with proper venting.
A cloche can help protect small spring seedlings. Originally bell-shaped glass domes that were placed on seedlings to protect them, most cloches today are made of plastic. For an inexpensive cloche for a single seedling, cut the bottom off of a two quart clear plastic soda bottle and use it to cover the seedling—this will help protect the tender sprout from the elements, keeping the seedling warm and humid inside the cloche.
A larger cloche can be made for a section of your garden by building a simple frame and covering it with transparent poly sheeting. Hoop houses are a common style of large cloche and can be easily made at home—wooden boards or PVC tubes are bent into ‘D’ shapes, then raised into place, attached to each other and used as a frame to hold the outer transparent layer of sheeting.
Southern or southeastern exposures are both well suited for taking advantage of solar heat and light collection—if insulation is going to be applied, it should be added to the north face and sides. When larger cloches cover entire rows of plants, they are known as row covers or high tunnels.
A semi-permanent or permanent small, rigid-framed structure that uses this same principle is called a cold frame—a sturdier and more permanent version of a cloche. Usually these are wooden-sided boxes with transparent tops. Light passes through to the plants, but some of the heat is trapped inside. Cold frames can be used to start seedlings before transplanting outside.
A simple cold frame can extend the growing season further into fall for cool weather vegetables and allow early spring starts to be prepared for summer growth. Cold frames are also often used as a staging area for plants started indoors, to help them harden off and transition from a protected environment to the rigors of outdoors. If a change in environment is too extreme—such as when seeds started indoors are moved outdoors—plants could die from shock. By placing indoor-grown plants into a protected cold frame, however, this change can be introduced in steps and your plants can adjust to their new conditions more gradually.
To increase the internal temperature of a cold frame, composting material can be added. The heat from the composting material is retained in much the same way solar heat is kept in. A cold frame that employs this method of retaining heat is called a hotbox. To add composting material, dig down one or two feet inside the hotbox and fill the hole with a straw-manure mixture, then cover it well with soil. As the mixture turns into compost, heat from the process will rise from the material and add to the warmth inside the frame. Electrical heating cables or solar-heated water can also be used to increase available heat.
A glass room connected to a residence is a conservatory, while a stand-alone structure of glass or transparent plastic is a greenhouse. A greenhouse is basically an upscale version of a cold frame. A greenhouse that functions well enough to maintain growing temperatures in the winter months—either through efficiency or by utilizing additional heat sources—is known as a hothouse.
One advantage to greenhouses is that not only can they be used to warm plants in cooler months, they can be cooled to reduce their internal temperatures in summer. Greenhouses often have windows that can be opened to release excess heat in the summer and sometimes fans are added for greater control.
Greenhouses can range in size from a handful of square feet to huge structures the size of commercial warehouses. Although greenhouses often involve a more substantial financial investment than smaller structures, they also tend to be more durable and afford growers a greater level of control.
Cool-weather plants such as lettuce and spinach are particularly well suited for off-season growing, but in some circumstances tomatoes and other tender plants can be grown as well—obviously the harsher the winters, the more difficult the grower’s task becomes.
There are many variations on the same principles of light availability, moisture supply and heat retention. Construction can entail anything from simple structures made with reclaimed household items to very expensive large-scale building projects.
For the beginner, though, I recommend starting with a homemade cold frame and working up to more ambitious structures in a series of steps, as your knowledge and experience develop and your confidence grows.