Greenhouses provide an extra layer of insulation against variable outdoor temperatures, capturing and maintaining heat from the sun for a longer growing season than would otherwise be possible in your natural climate. This makes it possible to start growing plants earlier in the season, and to grow plants that are not otherwise frost-tolerant after the first fall frost. Greenhouses also extend the degree of control you have over your plants' environment.
Types of Greenhouses
Greenhouses can be built to any scale--from a tiny cold frame box in your backyard for growing lettuce in the winter and spring, to acres-long structures dedicated to growing tomatoes throughout the year. Permanent greenhouses, such as seed-starting greenhouses, can be built out of glass, which is more durable than plastic and will last for a very long time. Less permanent structures can simply be built with sheets of plastic laid over metal or wood frames and can be moved with relative ease to prevent the soil's exhaustion.
Greenhouses protect the plants grown inside them both from the extremes of outdoor temperatures, and from possible exposure to pests and insects. Greenhouses absorb heat in cooler temperatures and climates, storing it in the soil and in the plants growing inside. In the summer they make shading the plants inside possible by covering the greenhouse with fabric or by spraying the outside with wet chalk dust to limit the amount of sun hitting and potentially burning your plants.
Greenhouses are sealed environments which help protect the plants inside from the stresses of the natural environment, but they also close out some of the necessary benefits. Irrigation and pollination are the two most important factors that must be managed in greenhouses. Smaller greenhouses can be watered by hand and opened up to allow bees in for pollination, but larger greenhouses must have built-in irrigation systems, and pollination must be accounted for either by building regular vents into the greenhouse walls, or by bringing beehives directly into the greenhouse.
What to Grow
The best plants to grow in greenhouses are those with long growing seasons and those which are most particular about temperature and irrigation since greenhouses extend the growing season and even out temperature extremes. The most popular greenhouse crop today is the tomato, which needs a very steady supply of water and plenty of indirect sunlight and warmth. Other fruiting plants such as eggplants, peppers, melons and cucumbers also do very well in these conditions since they can be kept warm without burning the leaves, and they can be carefully irrigated. In the winter greenhouses are excellent for planting cold-tolerant leaf crops such as lettuce and spinach, or root crops such as beets and turnips.
If you don't regularly move your greenhouse, the crops must be rotated between cash crops and cover crops. Every three to five years the soil in the greenhouse must be left dormant or planted with a cover crop to replenish the soil nutrients that have been depleted through successive plantings. You will see that your plants become smaller and less productive as the soil is depleted, and this can be slowed in the short term by mulching and fertilizing, but eventually you will have to plant in cover crop legumes or buckwheat.