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.
A necessary adjunct to the flower and vegetable garden is a coldframe. In it the early plantings of cabbage, cauliflower and lettuce (raised from seed sown in the fall) are kept over during the winter. Hardy annuals and biennials, such as pansies, daisies, violets, chrysanthemums, auriculas, cowslips, forget-me-nots, hollyhocks, carnations, etc., are best grown from seed sown in August or early September, transplanted into a coldframe, and again transplanted in spring to a permanent situation.
A coldframe is easy of construction, being simply a box of the desired length on the surface of the ground, and covered with sashes when cold weather sets in. If possible, the frames should be constructed so as to run northeast to southwest, or east to west if the former is not feasible. Calling the side facing northwest or north the back of the frame, the board forming the back should be ten or twelve inches high - the width of a hemlock board; the front boards should be six or eight inches high. This will give a slope toward the sun, the better to catch its rays, and will also quickly shed rain. The frame is made by putting posts in the ground and nailing the boards to them, one at the center of each board and one at each end. The posts are generally made from hemlock joists two inches by three inches, and should be sunk about two feet in the ground, first giving them a good coat of tar. Where the boards join, each can be nailed to one post, the wide surface of the posts being faced to the boards, the posts to be on the outside of the frames.
The standard length of the sashes is six feet, so that the boards should be five feet eight inches apart, thus allowing the sashes to project two inches over the boards, an inch at each end, for convenience in giving ventilation and in taking them off and putting them on.
Various devices are used to so fasten the sashes as to prevent them from being blown away by heavy winds. The simplest is to prepare small wooden wedges about six inches long, which are driven in between the sashes and so bind the whole frame securely. A safe way is to put a screw-eye in the end of each sash and a hook in the board and fasten each sash in that manner. These fastenings should be used on the north side of the frame, if the prevailing winter winds come from that quarter. Another method is to stretch a stout wire the length of the frame over the sashes and along the center, anchor it securely at each end to stout posts, fasten it at one end to a half-inch iron rod which is threaded and which passes through the post, an iron plate being on the outside of the post. Then, with a key screwed on the bolt, the wire can be made as taut as desired. Shelter from the cold northwest winter winds is very important. The market gardeners put up a six-foot fence behind their frames as a windbreak. The south side of a house, barn, or row of evergreen trees can be taken advantage of on small places.
It will be necessary, too, to put a bank of barnyard manure against the outside of the frames, both sides and ends, as additional protection from cold.
Azalea shed and pits.
The soil in coldframes should be well manured and well dug to get the best results. It is intensive culture, and the soil must be rich and mellow. Care should be taken, too, to see that it is well drained, and the frames guarded against any outside surface flow of water. Nothing is more harmful than a surplus of water in coldframes during the cold winter and early spring.
When the crop is out of the frames in the spring or early summer, it will be found beneficial to plant a crop of potatoes in them occasionally, also to seed them down to a green crop, such as red clover or millet. These can be dug in later in the season, and will be valuable to renovate the soil.
The uses for coldframes are many, and are indicated in the short list of plants given to be grown in them. Before violet culture reached the high state in which it is now, violets were very largely grown in coldframes for the New York market. One florist in Jersey City, who had an exceptionally favourable location, a sharp southern slope protected from the north, made a very comfortable living from about five hundred sashes entirely devoted to violets. Greenhouse culture of violets has, however, practically forced the abandonment of coldframes by florists. They can be and are still used by amateurs for their own use, and possibly to market the flowers if a surplus is produced. If violets are to be grown in coldframes, they must not be allowed to freeze hard at any time. Care must be exercised, therefore, to cover the frames during cold weather at night with straw mats or the new burlap mats, and over them close board shutters made out of half-inch pine boards, and the size of the sash. Heavy weights should be put on these to keep them from blowing away. These coverings will be found useful, but not indispensable, for plants in coldframes which are simply being carried over the winter.
If care is taken to properly temper the plants in the early part of the winter, no covering but the sashes will be necessary. When snow covers the glass it should be removed as soon as possible, provided the ground in the frames is not frozen hard and the plants are consequently growing. If it is frozen hard, the snow may be allowed to remain on for weeks.
The most important point in handling coldframes is ventilation. With a frame tightly closed and the sun shining, the temperature in the frame, even in the coldest weather, will rise rapidly, and air must be admitted. The usual way is to have small blocks of wood prepared and laid on the sashes ready for use. With the wind blowing briskly from the north and the thermometer showing twenty degrees or less, give ventilation on the southern side of the frame. The blocks should be about four inches high, sawed out of furring strips. By inserting these flat, on edge, or upright, three gradations of ventilation can be given, as desired. Sometimes it will be found desirable to ventilate by tilting the entire sash and inserting the block either flat or edgewise at the middle of the sash, the block resting on the adjoining sash. With a strong wind blowing along the frame, this method is desirable, as the sashes can be tilted so that the angle of elevation is in the same direction that the wind is blowing. During fine days in winter, when the sun is shining and there is not more than two or three degrees of frost, alternate sashes may be removed.
When this is done several days in succession, be sure that the sashes thus removed and placed on top of the next ones are alternated, so as not to have the same plants covered each day with the double sash. During the fine days of late winter and early spring the sashes should be removed entirely, piling them at the end of the frame.
Should the soil become dry at any time, so as to impede growth, then water, but this is not likely to happen during the winter months, and rarely even in the spring.
Many of these details may seem superfluous, but it is only by close attention to details that success can be achieved.