Whether or not you have a greenhouse, every serious gardener should have a cold frame, hotbed, or both.

A hotbed is nothing more than a cold frame with some means of heating it. A hotbed, for example, can be used in place of a greenhouse to start early, cool-season crops, such as tomatoes, peppers, eggplants, and many flower plants that require a long growing season.

Simply, it gives the plants a head start.

Some people even use hotbeds in which to grow salad vegetables all winter long because the double-paned panels now available make hotbeds very heat-efficient even when the snow blows.

A cold frame, on the other hand, can be used to protect crops in the fall from frosts or for carrying chrysanthemums and other perennials through the winter.

A movable cold frame is also great for setting out rhubarb plants, perennials , or bulbs to make them burst forth much earlier than your neighbors' plants.

The feature we find most helpful about a cold frame is its use for toughening up young plants before setting them outdoors in the garden.

Started indoors, the plants are not acclimated to the drying outdoor winds and sudden drops in temperature. Putting them in a cold frame for a week or two before planting outside gives the succulent tissues a chance to mature. This process is called ''hardening off.''

A cold frame or hotbed cover should be raised on mild, sunny days, but left down on cold nights and on cold, cloudy days. If temperatures are expected to fall below 40 degrees F., a blanket can be draped over it.

Anyone who can saw a board or pound a nail can make a cold frame since it is nothing more complicated than a box with a hinged top made of glass or some type of plastic. It should have a sloping roof so that rain and melting snow can drain off. Too, it can be an ''A'' type which can be opened on both sides or it can be a slanted box that is hinged at the back.

If you are a rough-and-ready gardener, you may omit the hinges and just slide the cover on and off.

Should you prefer a portable structure, make the base of lightweight wood and the cover of fiber glass or a plastic such as polyethylene.

Locate the hotbed or cold frame where it will get full sun most of the day. A southern exposure, next to a building (which will shut off the wind and give added protection), is ideal.

Should you prefer a portable structure, make the base of lightweight wood and the cover of fiberglass or a plastic such as polyethylene.

Locate the hotbed or cold frame where it will get full sun most of the day. A southern exposure, next to a building (which will shut off the wind and give added protection) is ideal.

Be sure it will not be shaded by nearby trees or buildings.

Some gardeners build them so the back is against their house foundation, thereby eliminating the need for building the back wall. By digging out an area the size of the planned hotbed and setting the base (made of cement blocks or treated wood, or both) below ground, you can take advantage of the earth's heat.

If you use wood below ground level, it is good to treat it with one of the zinc or copper preservatives. Don't use wood that has been treated with mercury, creosote, or pentachlorophenol as it will burn tender plants if they come in contact with it.

Don't hesitate to make use of scrap lumber or used window sashes. A friend of ours, Robert Mann of Victor, N.Y., made a cold frame for less than $8 by using material that was headed for the scrap heap. All he had to buy were nails and hinges.

Later on he made it into a hotbed by adding a large-size heating cable for less than $10.

Mr. Mann starts seeds indoors in late March. When the seedlings are large enough to transplant, he moves them into plant boxes (8 1/2 by 4 1/2 by 3 1/2) which he made from scrap wood. You also can use wooden fruit boxes. Then he sets them in the hotbed to grow for about a month or six weeks before setting them into a cold frame to harden off.

Even though we have two small greenhouses in which we grow our bedding plants each spring, we still set up a cold frame made with cement blocks and old storm sash. Then we move the plants into it, not only to harden them off, but because we need the extra space for the overflow.

A hotbed can be heated with incandescent light bulbs, heating cable, or by manure, an old-fashioned method that now has come back in style.

Light bulbs, however, are not as efficient as a heating cable but they will keep the plants from freezing at night. Yet they might not keep the structure warm enough for starting seeds since this practice calls for a constant temperature of 68 to 75 degrees F., day and night.

If used, light bulbs can be placed on a bar across the center of the frame at the top or they can be spaced around the sides and ends about 8 inches above the plants.

Waterproof porcelain sockets should be used, by all means.

Eight 25-watt bulbs will heat a 3-by-6-foot hotbed if it is set down into the ground. A pit about 1 1/2 feet deep should be dug for an electrically heated hotbed whether or not you use a heating cable or light bulbs.

It is advisable to put about 6 inches of cinders or small crushed stones in the bottom for drainage. It also is helpful to add leaves before adding the soil so that the soil will not sift down in the crevices of the box.

Six or eight inches of soil over the leaves is quite adequate, either for starting seeds or setting plant boxes on top. A similar layer of cinders or sand is put into the pit when you use a heating cable, but a couple inches of sand should then be spread over the layer of leaves. That job done, loop the cable back and forth, 3 inches from the sides and 6 inches between loops. It must not touch.

If a piece of hardware cloth (one-quarter-inch mesh) is laid over the top, it will conduct the heat more evenly and keep temperatures more uniform. Four or five inches of soil should be put over the hardware cloth.

Seed boxes resting on the soil won't dry out so quickly, and once the medium is heated it requires little electricity to keep an even temperature.

Almost all heating coils have a built-in thermostat.

A similar procedure is followed for a manure-heated hotbed. Fresh horse manure is best because it is relatively dry. Most other kinds of manure are too wet. Remember, the manure will generate heat only if it is fresh. Dig a 2 1/2 -foot pit and put about 3 inches of crushed stones or cinders on the bottom. Then add manure until it is 1 1/2 feet thick after it is tamped down.

After puting about five inches of soil over the manure, you're ready to set any plant boxes on top -- either started plants or seeds.

It is best to sow only the seeds of plants that can tolerate cool temperatures, such as lettuce, radishes, or those in the cabbage family. A manure-heated hotbed is liable to have fluctuating temperatures, especially on cold nights.

We do not sow in the soil directly since the fresh manure would be too ''hot'' as a fertilizer for the growing roots of the plants. As the season progresses, however, even tender seedlings can be placed in the manure hotbed if you keep an eye on the temperature.

Plant areas heat up fast on sunny days. If it gets too warm, raise the sash. Most seedlings do well at 65 degrees F.

Avoid watering plants on dull days unless you do it sparingly. Water in the morning so that the leaves will be dry when you close up the frame at night. Water temperatures are often in the low 40s during cold weather so you may need to draw it the night before and let it sit in the water can overnight at room temperature. Otherwise, it may slow down the plant growth.

If the temperature of the water is below 65 degrees F., it definitely will inhibit the germination of the seeds.

So even though you may not have a greenhouse, you still can widen your gardening experience by buying, or building, a cold frame, hotbed, or both. You'll be glad you did.

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