Question: What is the difference between a cold frame and hotbed for the garden. My husband says we need a coldframe and I say hotbed. I’m not sure if they are the same thing or not. All I know is that they are suppose to extend the growing season. Lil, Rochester, NY
Answer: The place of the coldframe and hotbed in the culture of plants is second in importance only to the garden itself. Managed correctly, no piece of garden equipment will yield more per dollar invested or do more to extend the growing season.
A coldframe and hotbed have many purposes. Although the primary function of these garden aids is to supply plants for early crops, long before it is possible to sow seeds outdoors, the coldframe can be kept in operation throughout most of the year as an auxiliary to the garden.
Beginning in late winter or very early spring, when seeds are started, through late spring, when plants are being prepared to withstand early outdoor temperatures, and then successively through the following months, the cold frame can be devoted to raising perennials, increasing plants from cuttings, protecting tender plants, or prolonging the growing season by sowing late vegetables which mature around Thanksgiving, or for growing such plants as violets and pansies for out-of-season flowers.
Cold Frame Of Hotbed – A Marked Distinction
There is a marked distinction between a cold frame and a hotbed. Simple cold frame design is a frame like structure, usually made of wood, set on top of, or enclosing a small area, of prepared soil. The roof, or top, is of glass sash through which the sun supplies light and heat. When this same structure is supplied with artificial heat, in addition to the sun’s heat, it becomes a hotbed. In the past, this heat was usually supplied by fermenting horse manure placed in the bottom, under the growing soil. But, now an electric soil heating cable which is buried under the soil, has served as a substitute for the manure. This gives a cleaner appearance and the heat, controlled by a thermostat, can he kept at any desired temperature which is not possible when manure is used for heating. Titus the same structure can function as a hotbed early in the season and a coldframe later on when the heating element is turned off.
The over-all size of cold frames are determined by the size and number of glass sash used (some people use a plastic sheet), until recently, the 3 X 6-foot sash was the standard unit; commercially, it still is. The width is 6 feet but the length depends upon the number of sash units used. For example, a two-sash frame would be 6-feet square.
Since sash of this size is difficult for one person to handle and since it is impossible to reach across the 6-foot space without stepping inside the frame, a smaller and lighter 2 X 4-foot frame has recently been introduced. This is much easier to handle and well suited to home gardening use. The introduction of plastic sheeting substitutes for glass also makes frames lighter and easier to handle. Some of these are quite practical and are gaining favor for coldframe use. The future will see a greater development along this line.
Cold Frame Design and Construction
Frames and sash can be purchased ready to be assembled. Or, the sash itself can be purchased and the frame made from any suitable material. It is possible, however, to use any kind of glass sash available whether it is window or storm sash, or any other kind that a handy person can put together. Sometimes a temporary frame will serve, just to start early seeds. This could be made by digging a 12-inch pit and covering it with glass sash of any size, placed on a level with the soil surface.
Materials for construction will vary with the purpose for which the frame is to be used. Light shelving lumber is suitable but a more permanent structure is possible from 1-1/4 to 2-inch lumber. Pecky cypress and redwood are the most durable but pine. when treated with a wood preservative and then painted, will last a long time. Preservatives such as Cuprinol are the safest to use. Materials such as creosote are injurious to plants and should be avoided. Tight construction and good fitting are essential. especially if the frame is to he used as a hotbed. More permanent structures may be made of brick or poured concrete.
When planning a frame in which several sash are to be used, crosspieces, made from 2 by 4-inch lumber are needed to facilitate sliding the sash on and off and to cover the slit between each of two sash. In the small frame, however, this crosspiece can be eliminated by fastening a 2-inch wide strip of wood on one sash so that it will overlap the other sash when in place. This could be used on the heavier sash, too, but would make handling them more difficult. Crosspieces should be removable so that digging, transplanting and other operations can go on unimpeded.
In a frame composed of several sash, stakes or pegs made from 2 by 4-inch lumber may be driven into the ground a foot deep and the boards used for the sides attached to these. One stake is used inside the frame on each corner, others are spaced at intervals along the structure on the outside to hold the boards in place.
Getting The Best Sunlight
In order that the most sunlight possible may enter the frame, it should be built with the rear 6 inches higher than the front so that the sash is at an incline toward the sun. The best direction for the frame to face is south or southeast and protection is desirable on the north from a wall, building or hedge. If none of these situations is available a mound of soil or manure should be placed outside the frame for protection.
The depth inside the frame (or the air space between the soil surface and the glass) influences the temperature within the frame. The deeper it is, the greater the protection from cold. The depth may vary from 8 to 12 inches for seedlings to over 2 feet where larger plants are to be set out from flats or pots. A convenient depth is from 8 to 12 inches at the front and 14 to 18 inches at the rear. Any depth above this would require a greater degree of slope to the glass sash and since a high sun is low on the horizon in early spring, the front board would throw a shadow halfway across the frame to the detriment of the plants.
The site for the frames must be well drained. Water lying at or near tile soil surface, especially in spring, is fatal to seeds and seedlings and will play havoc with whatever method of heating is used for a hotbed. If the soil is heavy and holds water, excavate to a depth of 18 inches and put several inches of cinders or gravel on the bottom. If the bottom is hard clay, slope it in one direction to throw the water away from the frame. The drainage area should extend several inches beyond the structure.
The best soil for a coldframe is composed of 3/4 good garden loam mixed with 1/4 peatmoss, leafmold, or well decayed compost. If the loam is heavy, mix it with enough sand to give good porosity. The mixture should be passed through a 4-inch screen before spreading it in the frame to a depth of 6 inches. Here it should he firmed flown slightly and raked level.
