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 The mold is then withdrawn, and a pane of glass is laid upon the top of the mound to concentrate the sun’s rays, and to prevent the bank from washing down with the rains.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.