A greenhouse, of course, would be the ideal location for starting seeds. But if you are not starting vast amounts of seeds, you can create greenhouse conditions in other, less elaborate, ways. Starting seeds indoors provides room temperature warmth; covering the container with plastic film maintains moisture for sprouting. Some seeds sprout better with added warmth; those begun outdoors need protection from cold and from drying out that a cover provides.
Artificial light. Since all seeds are covered with some soil at planting, light from the sun or from fluorescent lamps serves mostly to provide heat. Light becomes necessary for photosynthesis and growth as soon as the first sprout shows above the soil.
Covered seedbeds. A number of useful devices are based on the principle of the coldframe. European gardeners have long used cloches, interlocking A-frame glass canopies (with closed ends) over rows of seedlings (mainly frost-tender leafy types) to add growing time to both ends of the season, and to protect warmth- loving vegetables. Most cloches are held in wire carrying frames and can be taken apart for storage. Modern cloches employ sheet plastic over wire hoops. Bury the edges of the plastic in the soil. Open both ends of the shelter on warm days to prevent vegetables from literally becoming cooked.
Individual, heavy, waxed paper bonnets can be placed over individual seedlings. The caps trap solar heat and speed growth, as well as protect plants from frost damage. No covering can protect-against extreme cold, though, so don’t set warmth-loving plants in the garden until all heavy frosts are past.