This section is from the book "How To Make A Flower Garden", by Wilhelm Miller. Also available from Amazon: The Well-Tended Perennial Garden: Planting and Pruning Techniques.
Choose a sunny position protected from the prevailing spring winds by a fence, building, or hedge, where the surface drainage will be away from the site of the hotbed. Have the lower side face south, if possible.
For a permanent frame, excavate from two to two and a half feet deep, and tile-drain the bottom. For sides use a brick or cement wall, one or more feet thick, or plank from two to three inches thick. A hollow wall in either case will retain the heat longer; and if it is floored with wood, so much the better.
View in a greenhouse Persons who raise quantities of melons often plant all their seeds in splint forms or baskets made for the purpose, but the same kind of boxes may be used in a hotbed.
Remember that a single sash is three feet wide and three long, slanting lengthwise, so that the inside measurements must be multiples of these figures, first allowing a three-inch lap at all the four sides on which the sash will rest. The portion aboveground should be one foot in front and eighteen inches at the back, with the sides tapering.
If cement or brick is used, a box frame of two-inch plank should be bolted on (bolts set in the cement), and strong cross-bars run across where the sash meet. An inch strip may be nailed on these bars to divide the sash. If this is done, the width the strips occupy should be figured in the measurements. Cypress is the most lasting wood to use.
Mats made of burlaps, straw, or fiber, obtainable at the stores, are advisable to use during cold nights. Light wooden shutters further retain the heat and keep the mats dry during stormy weather. Banking up against the frame with coal-ashes or loam is commendable.
Temporary hotbeds are made by first preparing the manure as described elsewhere and spreading it out on the ground two or more feet deep and fully two feet wider all around than the frame to be used. On this set a frame one foot high in front and eighteen inches at the back and bank manure around it. Or have another frame one foot wider all around, which place outside, and fill the space between with manure.
The preparation of manure for a hotbed is a matter of great practical importance. The result aimed at is a slow, moist, enduring heat. This condition is secured only by the proper manipulation of the manure before it is placed in the frame. Often fresh manure that comes from a boxed structure is quite hot when received, and it is sometimes used at once, but the result is a quick, violent heat, rankly charged with ammonia, that soon burns itself out, and ceases to act while the weather is still cold.
Fresh horse manure is the best possible kind to use, and should have a good deal of rough, stable-soaked straw or litter in it. If this is lacking, litter or forest leaves may be added.
When it is received, shake it up most thoroughly, if it is naturally moist, and place it in a pile to heat. Protecting from rain or snow by covering with boards or piling under cover is beneficial, but not necessary. If the manure is dry and not inclined to heat, moisten it with hot water, which will soon start it.
Let it stand three or four days, then turn again, placing what was outside in the interior, thoroughly shaking each forkful, and pile up again. Let it remain a few days until thoroughly heated through, when it is ready to be placed in the frame.
Here it should be distributed evenly, and eventually be packed down firmly, especially at the sides and corners. If the gardener is not rushed for time, it is well to let the manure lie loose for a few days, during which time it will heat again. Put on the sash, but ventilate day and night until the steam passes off. During this process most of the ammonia escapes, which is desirable for this purpose, as the manure is useful for its heat alone, and not for plant food. When a thermometer, sunk in the manure and allowed to remain a few minutes, shows a temperature of less than 100 degrees, the bed is ready for use. I like to use two cubic yards of fresh manure to each sash of three by six.
If one is to hive a hotbed every year, it is usually better to use heavier lumber, and to mortise the corners together.
Seeds may be sown directly in the soil covering the manure, in which case the soil should be about six inches deep; or, if sown in shallow boxes, which are placed directly on the soil, the earth covering may be only three inches deep.
When the young plants are up, shade a little with open lath frames, or strew litter lightly over the glass on hot, sunny days, and ventilate by raising the leeward side of the sash.
During the early summer, after the plants have been removed, lettuce or radishes may be grown in the hotbed. Where six inches of soil has been used, cannas will grow to perfection; they seem to delight in the half-spent manure, either in the beds or when removed to their outside plantings in June.
When fall comes, remove the soil and manure and you have an admirable pit in which to bloom chrysanthemums; or partially fill it with sifted coal-ashes and in this bury the pots containing bulbous plants, such as Easter lilies, tulips, hyacinths, etc., which require darkness and freedom from frost in order that they may develop their roots before throwing up their tops. These may be brought into heat as required, and forced.
It is well to cover the frame with sash and shutters during the winter in order to keep the frost from the interior.