A hotbed basically is a heated coldframe. In many ways it is a miniature greenhouse, providing the same benefits with limited space at minimal expense. It is a means for extending the growing season. It is most often used to give an early start to warm-season vegetables such as tomato, pepper or melon. It may also be used to root cuttings of some woody plants. The temperature difference between the inside and outside of the frame is generally not more than 5 to 10 degrees. A mat or blanket may be placed over the frame on cold nights to conserve heat, but this increases temperature by only a few degrees. There are times, however, when a few degrees can be very important. Manure generates enough heat when placed under the coldframe to prevent freezing of seedlings in all but the coldest of nights. The manure in placed in a deep bed under the spot where the coldframe will lie and a thin layer of soil is placed above it in which the seedlings will be planted.
Hotbeds and coldframes should have a southern exposure to receive the maximum amount of sunlight. To reduce the cost of heating, use a north or northwest windbreak. This may be provided by a building, bales of hay or straw, tight board fence or evergreen hedge. Bundles or bales of straw could be used on the north for temporary windbreak. Windbreaks should not shade the bed.
The site should have good natural drainage so there will not be excess moisture beneath the bed. If the hotbed is below ground level, excellent drainage is essential to keep water from entering or accumulating during heavy rains. If natural drainage is not good, use drainage tile or a thick layer of coarse gravel. If this is not practical, beds may also be built above ground level for proper drainage. However, there will be greater heat loss.
Constructing the bed
Basically, the hotbed or coldframe is a rectangular box with the back higher than the front, covered with a transparent roof. The size and complexity of the structure depend on needs or funds available. Often scrap lumber and old window sash may be used, thereby reducing costs.
Sash and coverings
The sash available will determine the dimensions for the bed. Glass sash is the conventional frame covering and generally the best. Standard glass sash for coldframes and hotbeds is 3 feet by 6 feet. When this is used, the bed should be about 5 feet 8 inches (front to back) by 3 feet or a multiple of 3 (6, 9, 12, 15 or more feet).
Used window sash is satisfactory and less expensive, but the frame dimensions may have to be adjusted. Remember, whatever the size, the slope from back to front should be 1 inch per foot
If glass sash is not available or considered too expensive, the frame may be covered with clear polyethylene (4 or 6 mil) stretched on wooden frames. Polyethylene is lightweight and allows construction of many sizes of simple frames. Such frames are lightweight and must be hinged or hooked down to prevent lifting during strong winds.
Polyethylene films must be replaced yearly. Plastics lose heat rapidly; a double layer with air space between improves heat retention.
Fiberglass may also be used as a covering. Use clear rather than colored sheets for better light transmittance.
The sides of the structure may be wood, brick, masonry blocks, concrete or metal. Masonry block, concrete or brick make excellent permanent structures but are more costly than wood. Metal must be well insulated, or heat loss is high.
The average home garden hotbed or coldframe is constructed of wood. It is easy to work with and more temporary, but also more flexible if the bed needs to be enlarged or removed as needs and interests change. For longest durability, lumber should be treated with a preservative that is not toxic to plants. Lumber that has been CCA pressure treated is suitable for this use. Don’t use wood treated with creosote or pentachlorophenol; accumulation of fumes from these materials in a closed frame can cause plant damage. Redwood and cedar are long-lasting woods.
If untreated wood is used, the frame can be painted with a white latex-base paint or white greenhouse paint for added light reflectiveness and protection.
One-inch-thick lumber may be used for the frame, but 2-inch has greater durability and insulating qualities. The back wall (generally the north wall) should be at least 18 inches tall, measuring from the top of the heat source. The front wall (generally the south wall) should be about 12 inches tall for a 6-foot frame. Weather stripping at joints helps retain heat and makes the frame more efficient. Use 2 x 4 lumber for corner posts .
Preparing the bed area
The area for the bed must be leveled. Although temporary frames may be set on the soil surface, excavation is required for more permanent structures.Most home hotbeds are heated with electric coils, but where fresh manure is available, it may be used. A deeper excavation is generally required for establishing a manure-heated bed.
For electric heating, and where some drainage material is required, excavate to a depth of about 14 inches. Corner posts should extend to the base of the excavated area, but side walls need only be extended a few inches below the level of the heat cables.
After the bed area has been excavated, place in it a layer of coarse gravel about 6 inches deep. Cover the gravel layer with screening or burlap to prevent sand and soil from sifting down into it. On this, place a 2-inch layer of sand. Sand makes leveling easy and provides a base for the heat coils
Heating the hotbed
Methods for heating the beds include manure, electric cables, light bulbs, hot water and steam. Manure-heated beds are of interest as a means of conserving energy. Temperature control in manure-heated beds is more difficult than in electric, and therefore, we can normally expect to grow better plants in electrically heated beds. The manure-heated bed is suitable for only short periods in either spring or fall. The electrically heated bed can be used for any time period or even throughout the entire winter.
Manure and Hay Bales
In areas where fresh manure is available, it is a cheap and convenient heat source. Temporary hotbeds have been made by simply placing the board frames on top of a flat pile of manure 8 or 9 feet wide and 18 to 24 inches deep. Additional manure is banked around the sides of the frame for insulation and heat retention. Since this method requires large quantities of manure, it is not generally practical. Heat can be used from smaller amounts of manure packed in a pit beneath the frame.
The manure-heated hotbed provides a means for starting plants several weeks earlier in the spring than they could be started outdoors. It is built exactly like any other frame except that the pit beneath is made 18 to 30 inches deep to hold the manure.
Hay bales also give off much heat. They can be placed directly in a pit under the coldframe and covered with dirt.
Excellent drainage is needed
If the manure layer becomes soaked with water, fermentation stops and no heat is produced. Fresh horse manure containing about one-third straw is excellent, but other manures may be used.
To prepare for the manure-heated bed, collect the manure into a pile about 10 days before the bed is to be started. Compact and allow to remain until heat starts. Then repile, compact and allow it to remain undisturbed until heat is emitted for a second time. At that time, the manure will be ready to pack into the pit beneath the frame.
Place the 6-inch layer of coarse gravel, as previously described if there are drainage problems, or run tile out of the pit to release any water as it enters. Tamp down the manure so the total depth of manure is 12 to 18 inches. Be sure to pack edges and corners as well. Next, cover the manure with a layer of good soil 4 to 6 inches deep (Figure 3). Allow to remain for several days. The soil temperature may reach well beyond 100 degrees Fahrenheit. Wait until soil temperature drops to about 85 degrees Fahrenheit before planting seeds or moving in plants. Use a soil thermometer. If temperatures start to rise, always open sash promptly.
Other heating methods
Light bulbs are less expensive but also less satisfactory as a heat source. However, they can provide a quick and easy means of adding supplementary heat on frosty nights of early spring. About four 25-watt bulbs should be adequate for a 3 x 6 foot frame if spaced around the sides. Use waterproof wiring and sockets. If more heat becomes necessary, these bulbs can easily be changed for larger wattage bulbs.
If ground-level basement windows are present, it may be possible to build the frame outside such a window, which, when open, would provide some supplemental heat from the heated basement. Hot water and steam systems are normally useful only for large commercial hotbeds and are seldom used for the average home hotbed. Soil piled around the outside of the frame provides added insulation and heat retention.
When ventilating the hotbed or coldframe, raise the sash on the side opposite that from which the wind is blowing. This prevents wind burn on young, tender plants or lifting of the sash by strong winds.
During extremely cold weather, provide extra protection and conserve heat by covering the sash with mats or a layer of straw. These must be removed when weather clears and temperatures rise. Glass must be cleaned when straw is removed to provide maximum light transmittance.
On warm, sunny days the sash may be opened wide or totally removed. A thermometer is the best guide to determine what width of opening will allow proper air movement for good temperature control. Coldframes are ventilated in about the same way as hotbeds. Since they are solely dependent on natural heat absorbed during the day, it is important to close the sash promptly in the late afternoon or evening after there is no danger of excess temperature buildup.
Plants started in a hotbed and especially those started indoors should be “hardened” before they are moved into the garden. A coldframe is an ideal place to accomplish this. The hotbed may also be used.
Taken from an article by Denny Schrock
Department of Horticulture, University of Missouri
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