from The Complete Book of Garden Magic by Roy E. Biles, 1941

A hotbed is heated from below by the fermentation of manure or otherwise. It is a miniature greenhouse. Our illustration shows the construction and much of the information offered above is applicable to it. The following instructions give brief directions but more should be obtained from garden books obtainable at your public library or from literature that any good greenhouse manufacturing company will send.

Manure and Its Handling: A good deal of the success with hotbeds is due to the careful selection of the manure. Cold manure, like that of cows and pigs, should never be used because it will not heat. Horse manure is best, but in this case at least one-third of the bulk should be straw. If pure manure is used, it will pack too tightly when firmed, so that it will not heat. If possible, the manure from grain-fed, straw-bedded horses should be used.

The manure to be used should be hauled at least two weeks before it is to be used. It should be piled in a flat, compact pile, three to five feet high, and of a convenient width and length. If the weather is cold or the manure refuses to heat, add considerable water and firm the pile. When complete, the pile should thoroughly heat in four or five days, after which it should be turned so as to put the cool manure on the inside and the hot manure on the outside. By so doing a very uniform heating will be obtained. After about another week the pile will again be thoroughly heated, so that the pit can be filled.

Filling the Hotbed: In filling the hotbed, place the manure in the bed in layers six inches deep. As each layer is put in the pit it should be thoroughly tramped, so that it is firm, especially in the corners and along the sides of the bed. It is essential that the manure be well tramped, otherwise it will not heat and the bed will be useless. When the pit has been filled to the level of the ground with well-tramped layers of manure, the sash should be placed on until the manure has thoroughly heated. The soil should then be placed in the pit to a depth of six inches, if crops are to be grown to maturity; four inches of soil will be sufficient for starting early plants The soil should be a finely pulverized garden loam, which has been well leveled and packed.

The Temperature: The temperature rises rapidly for several days, often going above 125 degrees F. After the temperature of the manure has dropped to 85 or 90 degrees, the bed will be ready to plant. To take the temperature in hotbeds, a plunging thermometer should be used. This consists of a metal, brass, or iron point which encloses the thermometer bulb. The whole is mounted on a wooden handle, which protects the glass from any possible injury. Such thermometers can be purchased for about a dollar and a quarter from supply houses. In taking the temperature, thrust the thermometer through the soil into the manure, and read the temperature when the thermometer is in place.

Care of the Hotbeds/Watering: The soil in a hotbed should be kept moist without soaking the manure. If the manure is soaked, it will cause uneven heating besides cooling the bed, so that the heat is spent sooner than it would be by judicious watering. Occasional thorough waterings are much better than frequent light sprinklings. It is advisable to water on mornings of bright, sunshiny days, and never on cloudy days or late in the day. Late watering lowers the temperature at a critical time and leaves the foliage wet for overnight. Both of these conditions foster disease especially “dampening off,” which is the most serious disease in the frames.

Ventilation: Ventilation is absolutely essential to supply the plants with fresh air, to reduce the humidity, and to control the temperature within the bed. A very good indication of the need of ventilation is the amount of moisture which collects on the inside of the glass. A little air should be given the beds each day, gradually increasing the amount until the sash can be left off during the day. A week or ten days before the plants are set in the field the sash should be removed day and night, so that the plants are thoroughly hardened.

Cultivation: If seeds and plants are in drills or rows, it is desirable to cultivate between rows and plants. Cultivation keeps the weeds down, makes a surface mulch, and lessens the amount of disease in the beds.

Advantages of Using a Hotbed:

  1. Crops can be matured in sections where the season would otherwise be too short.
  2. There is less danger from fall frosts, in the open, than there is from spring frosts.
  3. By advancing the season, two or more crops can be grown on the same land.
  4. Weeds are less difficult to handle when good-sized plants are set out.
  5. Many insect pests and plant diseases are avoided.
  6. Larger crops can be produced.
  7. Crops mature earlier.

The Electric Hotbed: Electricity is fast becoming a factor in hotbed operation. It does away with the delay and mess of using fresh manure, which, incidentally is becoming harder and harder to obtain in cities, suburbs, etc. It is easy and inexpensive to install, and it provides steady heat where most needed (just below seeds of plants) and, with the help of a simple automatic thermostat, to whatever extent is desired. Assuming that an ordinary light socket is close by the hotbed, or that you can rig up (or have rigged up) the necessary wiring, all that is needed is (1) a length of lead covered heating cable which laid back and forth on a bed of sand in the hotbed and covered with another layer of sand and then the soil; (2) the thermostat which shuts the current on when heat is needed and off when it is not; (3) a soil thermometer connected with the thermostat and inserted in the soil. The whole business can be bought at most good seed stores. dealers in electric equipment, or greenhouse manufacturing firms for a few dollars and should last a good many years.

Some Things to Remember in Operating Coldframes and Hotbeds: Be careful to keep all drains and gutters outside of frames clear and free from surplus snow. A sudden thaw may soak or even flood soil in frames with bad results.

Give ventilation frequently, even if temperature inside does not make this necessary. Fresh air is important to health of plants and it discourages disease and bugs. Too much ventilation, however, will chill your plants and retard growth; on the other hand caution is also needed in giving them too little fresh air. This error may cause you to find your plants “cooked,” which will cause them to be soft, and will also prevent that vigorous growth which is one of the delights to users. In cold weather, the sash raised a crack at the back during the middle of the day will give sufficient ventilation as a rule.

When the weather moderates, elevate the sash on short supports or else slide them down. Some experienced growers advise raising the sash on the lengthwise edge to form a protection from the wind when it blows from the top or bottom of the bed.

Slats are part of the successful grower’s equipment for hotbeds and cold frames. They serve to prevent the wilting of young plants when you are transplanting in the spring, by shading them from the direct rays of the sun, which is often too hot for them. The slats also serve to encourage the plants to make their first growth in the roots, instead of shooting up to leaves.

It is best not to rush their natural growth, otherwise you may expect to have lank and shallow-rooted plants.