The Forcing of Plants
by Liberty Hyde Bailey
Taken from The Manual of Gardening, copyright 1932
There are three general means (aside from greenhouses) of forcing plants ahead of their season in the early spring – by means of forcing-hills and hand-boxes, by coldframes, and by hotbeds.
The forcing-hill is an arrangement by means of which a single plant or a single “hill” of plants may be forced where it permanently stands. This type of forcing may be applied to perennial plants, as rhubarb and asparagus, or to annuals, as melons and cucumbers.
In Fig. a is illustrated a common method of hastening the growth of rhubarb in the spring. A box with four removable sides, two of which are shown in end section in the figure, is placed around the plant in the fall. The inside of the box is filled with straw or litter, and the outside is banked thoroughly with any refuse, to prevent the ground from freezing. When it is desired to start the plants, the covering is removed from both the inside and outside of the box and hot manure is piled around the box to its top. If the weather is yet cold, dry light leaves or straw may be placed inside the box; or a pane or sash of glass may be placed on top of the box, when it will become a coldframe. Rhubarb, asparagus, sea-kale, and similar plants may be advanced two or four weeks by means of this method of forcing. Some gardeners use old barrels or half-barrels in place of the box. The box, however, is better and handier, and the sides can be stored for future use.
Plants that require a long season in which to mature, and which do not transplant readily, as melons and cucumbers, may be planted in forcing-hills in the field. One of these hills is shown in Fig. b. The frame or mold is shown at the left. This mold is a box with flaring sides and no top or bottom, and provided with a handle. This frame is placed with the small end down at the point where the seeds are to be planted, and the earth is hilled up about it and firmly packed with the feet. The mold is then withdrawn, and a pane of glass is laid upon the top of the mound to concentrate the sun’s rays, and to prevent the bank from washing down with the rains. A clod of earth or a stone may be placed upon the pane to hold it down. Sometimes a brick is used as a mold. This type of forcing-hill is not much used, because the bank of earth is liable to be washed away, and heavy rain coming when the glass is off will fill the hill with water and drown the plant. However, it can be used to very good advantage when the gardener can give it close attention.
A forcing-hill is sometimes made by digging a hole in the ground and planting the seeds in the bottom of it, placing the pane of glass upon a slight ridge or mound which is made on the surface of the ground. This method is less desirable than the other, because the seeds are placed in the poorest and coldest soil, and the hole is very likely to fill with water in the early days of spring.
An excellent type of forcing-hill is made by the use of the hand-box, as shown in Fig. c. This is a rectangular box, without top or bottom, and a pane of glass is slipped into a groove at the top. It is really a miniature coldframe. The earth is banked up slightly about the box, in order to hold it against winds and to prevent the water from running into it. If these boxes are made of good lumber and painted, they will last for many years. Any size of glass may be used which is desired, but a ten-by-twelve pane is as good as any for general purposes.
After the plants are thoroughly established in these forcing-hills, and the weather is settled, the protection is wholly removed, and the plants grow normally in the open.
A very good temporary protection may be given to tender plants by using four panes of glass, as explained in Fig. d, the two inner panes being held together at the top by a block of wood through which four nails are driven. Plants are more likely to burn in these glass frames than in the hand-boxes, and such frames are not so well adapted to the protection of plants in the very early spring; but they are often useful for special purposes.
In all forcing-hills, as in coldframes and hotbeds, it is exceedingly important that the plants receive plenty of air on bright days. Plants that are kept too close become weak or “drawn,” and lose the ability to withstand changes of weather when the protection is removed. Even though the wind is cold and raw, the plants inside the frames ordinarily will not suffer if the glass is taken off when the sun is shining.
A coldframe is nothing more than an enlarged hand-box; that is, instead of protecting but a single plant or a single hill with a single pane of glass, the frame is covered with sash, and is large enough to accommodate many plants.
There are three general purposes for which a coldframe is used: For the starting of plants early in spring; for receiving partially hardened plants that have been started earlier in hotbeds and forcing-houses; for wintering young cabbages, lettuce, and other hardy plants that are sown in the fall.
Coldframes are ordinarily placed near the buildings, and the plants are transplanted into the field when settled weather comes. Sometimes, however, they are made directly in the field where the plants are to remain, and the frames, and not the plants, are removed. When used for this latter purpose, the frames are made very cheap by running two rows of parallel planks through the field at a distance apart of six feet. The plank on the north is ordinarily ten to twelve inches wide, and that on the south eight to ten inches. These planks are held in place by stakes, and the sashes are laid across them. Seeds of radishes, beets, lettuce, and the like, are then sown beneath the sash, and when settled weather arrives, the sash and planks are removed and the plants are growing naturally in the field. Half-hardy plants, as those mentioned, may be started fully two or three weeks in advance of the normal growing season by this means.
One of the simplest types of coldframes is shown in Fig. e, which is a lean-to against the foundation of a house. A sill is run just above the surface of the ground, and the sashes, shown at D, are laid on rafters which run from this sill to the sill of the house, A. If this frame is on the south side of the building, plants may be started even as early as a month before the opening of the season. Such lean-to frames are sometimes made against green-houses or warm cellars, and heat is supplied to them by the opening of a door in the wall, as at B. In frames that are in such sunny positions as these, it is exceedingly important that care be taken to remove the sash, or at least to give ample ventilation, in all sunny days.
A different type of lean-to structure is shown in Fig. f. This may be either a temporary or permanent building, and it is generally used for the protection of half-hardy plants that are grown in pots and tubs. It may be used, however, for the purpose of forwarding pot-plants early in the spring and for the protection of peaches, grapes, oranges, or other fruits in tubs or boxes. If it is desired merely to protect the plants through the winter, it is best to have the structure on the north side of the building, in order that the sun may not force the plants into activity.
Another structure that may be used both to carry half-hardy plants over winter and for starting plants early in spring is shown in Fig. g. It is really a miniature greenhouse without heat. It is well adapted for mild climates. The picture was made from a structure in the coast region of North Carolina.
The common type of coldframe is shown in Fig. h. It is twelve feet long and six feet wide, and is covered with four three-by-six sash. It is made of ordinary lumber loosely nailed together. If one expects to use coldframes or hotbeds every year, however, it is advisable to make the frames of two-inch stuff, well painted, and to join the parts by bolts and tenons, so that they may be taken apart and stored until needed for the next year’s crop. Fig. i suggests a method of making frames so that they may be taken apart.
It is always advisable to place coldframes and hotbeds in a protected place, and particularly to protect them from cold north winds. Buildings afford excellent protection, but the sun is sometimes too hot on the south side of large and light-colored buildings. One of the best means of protection is to plant a hedge of evergreens, as shown in Fig. j. It is always desirable, also, to place all the coldframes and hotbeds close together, for the purpose of economizing time and labor. A regular area or yard may be set aside for this purpose.
Various small and portable coldframes may be used about the garden for the protection of tender plants or to start them early in the spring. Pansies, daisies, and border carnations, for example, may be brought on very early by setting such frames over them or by planting them under the frames in the fall. These frames may be of any size desired, and the sash may be either removable, or, in the case of small frames, they may be hinged at the top. Figs. k-n illustrate various types.
A hotbed differs from a coldframe in being provided with bottom heat. This heat is ordinarily supplied by means of fermenting manure, but it may be obtained from other fermenting material, as tanbark or leaves, or from artificial heat, as flues, steam pipes, or water pipes.
The hotbed is used for the very early starting of plants; and when the plants have outgrown the bed, or have become too thick, they are transplanted into cooler hotbeds or into coldframes. There are some crops, however, that are carried to full maturity in the hotbed itself, as radishes and lettuce.
The date at which the hotbed may be started with safety depends almost entirely on the means at command of heating it and on the skill of the operator. In the northern states, where outdoor gardening does not begin until the first or the last of May, hotbeds are sometimes started as early as January; but they are ordinarily delayed until early in March.
The heat for hotbeds is commonly supplied by the fermentation of horse manure. It is important that the manure be as uniform as possible in composition and texture, that it come from highly fed horses, and is practically of the same age. The best results are usually secured with manure from livery stables, from which it can be obtained in large quantities in a short space of time. Perhaps as much as one half of the whole material should be of litter or straw that has been used in the bedding.
The manure is placed in a long and shallow square-topped pile, not more than four or six feet high, as a rule, and is then allowed to ferment. Better results are generally obtained if the manure is piled under cover. If the weather is cold and fermentation does not start readily, wetting the pile with hot water may start it. The first fermentation is nearly always irregular; that is, it begins unequally in several places in the pile. In order to make the fermentation uniform, the pile must be turned occasionally, taking care to break up all hard lumps and to distribute the hot manure throughout the mass. It is sometimes necessary to turn the pile five or six times before it is finally used, although half this number of turnings is ordinarily sufficient. When the pile is steaming uniformly throughout, it is placed in the hotbed, and is covered with the earth in which the plants are to be grown.
Hotbed frames are sometimes set on top of the pile of fermenting manure, as shown in Fig. o. The manure should extend some distance beyond the edges of the frame; otherwise the frame will become too cold about the outside, and the plants will suffer.
It is preferable, however, to have a pit beneath the frame in which the manure is placed. If the bed is to be started in midwinter or very early in the spring, it is advisable to make this pit in the fall and to fill it with straw or other litter to prevent the earth from freezing deep. When it is time to make the bed, the litter is thrown out, and the ground is warm and ready to receive the fermenting manure. The pit should be a foot wider on either side than the width of the frame. Fig. p is a cross-section of such a hotbed pit. Upon the ground a layer of an inch or two of any coarse material is place to keep the manure off the cold earth. Upon this, from twelve to thirty inches of manure is placed. Above the manure is a thin layer of leafmold or some porous material, that will serve as a distributor of the heat, and above this is four or five inches of soft garden loam, in which the plants are to be grown.
It is advisable to place the manure in the pit in layers, each stratum to be thoroughly trodden down before another one is put in. These layers should be four to eight inches in thickness. By this means the mass is easily made uniform in consistency. Manure that has too much straw for the best results, and which will therefore soon part with its heat, will spring up quickly when the pressure of the feet is removed. Manure that has too little straw, and which therefore will not heat well or will spend its heat quickly, will pack down into a soggy mass underneath the feet. When the manure has sufficient litter, it will give a springy feeling to the feet as a person walks over it, but will not fluff up when the pressure is removed. The quantity of manure to be used will depend on its quality, and also on the season in which the hotbed is made. The earlier the bed is made, the larger should be the quantity of manure. Hotbeds that are intended to hold for two months should have about two feet of manure, as a rule.
The manure will ordinarily heat very vigorously for a few days after it is placed in the bed. A soil thermometer should be thrust through the earth down to the manure, and the frame kept tightly closed. When the temperature is passing below 90º, seeds of the warm plants, like tomatoes, may be sown, and when it passes below 80º or 70º, the seeds of cooler plants may be sown.
If hotbeds are to be used every year, permanent pits should be provided for them. Pits are made from two to three feet deep, preferably the former depth, and are walled up with stone or brick. It is important that they be given good drainage from below. In the summer-time, after the sash are stripped, the old beds may be used for the growing of various delicate crops, as melons or half-hardy flowers. In this position, the plants can be protected in the fall. As already suggested, the pits should be cleaned out in the fall and filled with litter to facilitate the work of making the new bed in the winter or spring.
Various modifications of the common type of hotbed will suggest themselves to the operator. The frames should ordinarily run in parallel rows, so that a man walking between them can attend to the ventilation of two rows of sash at once. Fig. q shows a different arrangement. There are two parallel runs, with walks on the outside, and between them are racks to receive the sash from the adjacent frames. The sash from the left-hand bed are run to the right, and those from the right-hand bed are run to the left. Running on racks, the operator does not need to handle them, and the breakage of glass is therefore less; but this system is little used because of the difficulty of reaching the farther side of Fig. q the bed from a single walk.
If the hotbed were high enough and broad enough to allow a man to work inside, we should have a forcing-house. Such a structure is shown in Fig. r, upon one side of which the manure and soil are already in place. These manure-heated houses are often very efficient, and are a good make-shift until such time as the gardener can afford to put in flue or pipe heat.
Hotbeds may be heated by means of steam or hot water. They can be piped from the heater in a dwelling-house or green-house. Fig. s shows a hotbed with two pipes, in the positions 7, 7 beneath the bed. The earth is shown at 4, and the plants (which, in this case, are vines) are growing upon a rack, at 6. There are doors in the end of the house, shown in 2, 2, which may be used for ventilation or for admitting air underneath the beds. The pipes should not be surrounded by earth, but should run through a free air space.
It would scarcely pay to put in a hot water or steam heater for the express purpose of heating hotbeds, for if such an expense were incurred, it would be better to make a forcing-house. Hotbeds may be heated, however, with hot-air flues with very good results. A home-made brick furnace may be constructed in a pit at one end of the run and underneath a shed, and the smoke and hot air, instead of being carried directly upwards, is carried through a slightly horizontal pipe that runs underneath the beds. For some distance from the furnace, this flue may be made of brick or unvitrified sewer pipe, but stove-pipe may be used for the greater part of the run. The chimney is ordinarily at the farther end of the run of beds. It should be high, in order to provide a good draft. If the run of beds is long, there should be a rise in the underlying pipe of at least one foot in twenty-five. The greater the rise in this pipe, the more perfect will be the draft. If the runs are not too long, the underlying pipe may return underneath the beds and enter a chimney directly over the back end of the furnace, and such a chimney, being warmed from the furnace, will ordinarily have an excellent draft. The underlying pipe should occupy a free space or pit beneath the beds, and whenever it lies near to the floor of the bed or is very hot, it should be covered with asbestos cloth. While such flue-heated hotbeds may be eminently successful with a grower or builder of experience, it may nevertheless be said, as a general statement, that whenever such trouble and expense are incurred, it is better to make a forcing-house. The subject of forcing-houses and green-houses is not discussed in this article.
The most satisfactory material for use in hotbed and coldframe sash is double-thick, second-quality glass; and panes twelve inches wide are ordinarily broad enough, and they suffer comparatively little in breakage. For coldframes, however, various oiled papers and waterproof cloths may be used, particularly for plants that are started little in advance of the opening of the season. When these materials are used, it is not necessary to have expensive sash, but rectangular frames are made from strips of pine seven-eights inch thick and two and one-half inches wide, halved together at the corners and each corner reinforced by a square carriage-corner, such as is used by carriage-makers to secure the corners of buggy boxes. These corners can be bought by the pound at hardware stores.
MANAGEMENT OF HOTBEDS
Close attention is required in the management of hotbeds, to insure that they do not become too hot when the sun comes out suddenly, and to give plenty of fresh air.
Ventilation is usually effected by raising the sash at the upper end and letting it rest upon a block. Whenever the temperature is above freezing point, it is generally advisable to take the sash off part way, as shown in the central part of Fig. j, or even to strip it off entirely, as shown in Fig. h.
Care should be taken not to water the plants at nightfall, especially in dull and cold weather, but to give them water in the morning, when the sun will soon bring the temperature up to its normal state. Skill and judgment in watering are of the greatest importance in the management of hotbeds; but this skill comes only from thoughtful practice. The satisfaction and effectiveness of the work are greatly increased by good hose connections and good watering-pots (Fig. t).
Some protection, other than the glass, must be given to hotbeds. They need covering on every cold night, and sometimes during the entire day in very severe weather. Very good material for covering the sash is matting, such as is used for covering floors. Old pieces of carpet may also be used. Various hotbed mattings are sold by dealers in gardeners’ supplies.
Gardeners often make mats of rye straw, although the price of good straw and the excellence of manufactured materials make this home-made matting less desirable than formerly. Such mats are thick and durable, and are rolled up in the morning. There are various methods of making these straw mats, but Fig. u illustrates one of the best. A frame is made after the manner of a saw-horse, with a double top, and tarred or marline twine is used for securing the strands of straw. It is customary to use six runs of this warp. Twelve spools of string are provided, six hanging on either side. Some persons wind the cord upon two twenty-penny nails, as shown in the figure, these nails being held together at one end by wire which is secured in notches filed into them. The other ends of the spikes are free, and allow the string to be caught between them, thus preventing the balls from unwinding as they hang upon the frame. Two wisps of straight rye straw are secured and laid upon the frame, with the butt ends outward and the heads overlapping. Two opposite spools are then brought up, and a hard knot is tied at each point. The projecting butts of the straw are then cut off with a hatchet, and the mat is allowed to drop through to receive the next pair of wisps. In making these mats, it is essential that the rye contains no ripe grain; otherwise it attracts the mice. It is best to grow rye for this especial purpose, and to cut it before the grain is in the milk, so that the straw does not need to be threshed.
In addition to these coverings of straw or matting, it is sometimes necessary to provide board shutters to protect the beds, particularly if the plants are started very early in the season. These shutters are made of half-inch or five-eights-inch pine lumber, and are the same as the sash – three by six feet. They may be placed upon the sash underneath the matting, or they may be used above the matting. In some cases they are used without any matting.
In the growing of plants in hotbeds, every effort should be made to prevent the plants from growing spindling, or becoming “drawn.” To make stocky plants, it is necessary to give room to each plant, to be sure that the distance from the plants to the glass is not great, to provide not too much water in dull and cold weather, and particularly to give abundance of air.