A very good temporary protection may be given to tender plants by using four panes of glass, as explained in Fig. 193, 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 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 season by this means.
One of the simplest types of coldframes is shown in Fig. 194, 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 greenhouses 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. 195. 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 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.