Coldframe Gardening: Important Tips On Building Coldframes and Hotbeds

The place of the cold frame and hotbed in the culture of plants is second in importance only to the garden itself. 

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Managed correctly, no piece of garden equipment will yield more per dollar invested. A cold frame and hotbed have many purposes. 

The Function Of Garden Aids

Although the primary function of these garden aids is to supply plants for early crops, long before it is possible to sow seeds outdoors, the cold frame can be kept in operation throughout most of the year as an auxiliary to the garden.

Beginning in late winter or very early spring, when seeds are started, through late spring, plants are being prepared to withstand early outdoor temperatures, and then successively through the following months. 

Devotion Of Cold Frames

The cold frame can be devoted to the following: 

  • Raising perennials, increasing plants from cuttings
  • Protecting tender plants
  • Prolonging the growing season
  • Sowing late vegetables which mature around Thanksgiving
  • Growing such plants as violets and pansies for out-of-season flowers

Distinction Between A Cold Frame And Hotbed

There is a marked distinction between a cold frame and a hotbed. The cold frame is a frame-like structure, usually made of wood, set on top of or enclosing a small area of prepared soil. 

The roof, or top, is of glass sash through which the sari supplies light and heat. When this same structure is supplied with artificial heat and the sun’s heat, it becomes a hotbed. 

In the past, this heat was usually supplied by fermenting horse manure placed in the bottom, under the growing soil. 

More recently, an electrical heating cable buried under the soil has served as a substitute for the manure. 

This gives a cleaner appearance, and the heat, controlled by a thermostat, can be kept at any desired temperature, which is impossible when manure is used for heating. 

Titus the same structure, can function as a hotbed early in the season and a cold frame later on when the heating element is turned off.

Size Of The Frame And Glass Sashes

The overall size of the frame is determined by the size and number of glass sashes used; Until recently, the 3- X 6-foot sash was the standard unit; commercially, it still is. 

The width is 6’ feet, but the length depends upon the tile number of sash units used. For example, a two-sash frame would be 6 feet square.

Since a sash of this size is difficult for one person to handle and it is impossible to reach across the 6-foot space without stepping inside the frame, a smaller and lighter 2- X 4-foot frame has recently been introduced.

Home Gardening Use

This is much easier to handle and well suited to home gardening use. The introduction of plastic substitutes for glass also makes frames lighter and easier to handle. 

Some of these are quite practical and are gaining favor for cold frame use. The future will see greater development along this line.

Frame Construction

Frames and sash can be purchased ready to be assembled. Or, the sash can be purchased and the frame made from any suitable material. 

It is possible, however, to use any kind of glass sash available, whether it is a window, storm sash, or any other kind that a handy person can put together.

Sometimes a temporary frame will serve just to start early seeds. This could be made by digging a 12”-inch pit and covering it with a glass sash of any size, placed on a level with the soil surface.

Construction materials will vary with the purpose for which the frame is to be used. Light shelving lumber is suitable, but a more permanent structure is possible from 1 ¼”- to 2”-inch lumber.

Most Durable Woods For Frame

Pecky cypress and redwood are the most durable, but pine, when treated with a wood preservative and then painted, will last a long time. 

Preservatives such as Cuprinol are the safest to use. Materials such as creosote are injurious to plants and should be avoided.

Essential Things in Frame Construction

Tight construction and good fitting are essential, especially if the frame is used as a hotbed. 

More permanent structures may be made of brick or poured concrete.

Cross Pieces For Small Frame

When planning a frame in which several sashes are to be used, cross pieces made from 2”- by 4”-inch lumber are needed to facilitate sliding the sash on and off and to cover the slit between each of the two sashes.

In the small frame, however, this crosspiece can be eliminated by fastening a 2”-inch wide strip of wood on one sash to overlap the other sash when in place. 

This could also be used on the heavier sash but would make handling them more difficult.

Cross Pieces should be removable so digging, transplanting, and other operations can go unimpeded.

A frame composed of several sashes, stakes, or pegs made from 2” by 4” inches lumber may be driven into the ground a foot deep and the boards used for the sides attached to these. 

One stake is used inside the frame on each corner, and others are spaced at intervals along the structure on the outside to hold the boards in place.

Sunlight Must Enter the Frame

So that the most sunlight possible may enter, the frame should be built with the rear 6” inches higher than the front so that the sash is at an incline toward the sun. 

The best direction for the frame to face is south or southeast, and protection is desirable on the north from a wall, building, or hedge. 

If none of these situations is available, a mound of soil or manure should be placed outside the frame for protection.

Depth Of The Frame

The depth inside the frame (or the air space between the soil surface and the glass) influences the temperature within the frame. The deeper it is, the greater the protection from cold. 

The depth may vary from 8” to 12” inches for seedlings to over 2′ feet, where larger plants are to be set out from flats or pots.

A convenient depth ranges from 8” to 12” inches at the front and 14” to 18” at the rear. 

Any depth above this would require a greater degree of slope to the glass sash, and since a high sun is low on the horizon in early spring, the front board would throw a shadow halfway across the frame to the detriment of the plants.

Well-Drained Site For Frames

The site for the frames must be well-drained. Water lying at or near the tile soil surface, especially in spring, is fatal to seeds and seedlings and will play havoc with whatever heating method is used for a hotbed. 

If the soil is heavy and holds water, excavate to a depth of 18” inches and put several inches of cinders or gravel on the bottom. 

If the bottom is hard clay, slope it in one direction to throw the water away from the frame. The drainage area should extend several inches beyond the structure.

Best Soil For Cold Frame

The best soil for a cold frame is composed of 3/4 good garden loam mixed with 1/4 peat moss. leafmold. or well-decayed compost. 

If the loam is heavy, mix it with enough sand to give good porosity. 

The mixture should be passed through a 4” inches screen before spreading it in the frame to a depth of 6” inches. Here it should be firmed, flown slightly, and raked level.

44659 by P. J. Mckenna