There are two basic kinds of greenhouses that you can use to grow vegetables. A hothouse greenhouse is a heated greenhouse used to grow warm-season vegetables and tropical plants in winter. Unheated greenhouses are known as cold houses. You can use cold houses to grow vegetables in winter as well. The key to using a cold house is to select cold-tolerant vegetables such as broccoli and lettuce, or waiting until just before warm-season vegetables are in season to grow them.
Add warmth to an unheated greenhouse by filling black rain barrels with water and placing them throughout your greenhouse. The barrels will absorb heat from the sun during the day and create areas of thermal mass by radiating the heat back outward at night. This practice is primarily done in winter because summer temperatures are warmer.
Cover vents and windows in greenhouses at night with a lightweight insulation such as bubble wrap. This will prevent heat loss or invasion by cold at night. During daylight hours, the insulation can be removed to allow the warming greenhouse to vent air that grows too warm.
Determine your USDA hardiness zone. This will help determine which plants you can grow and at what time. For example, warm-season plants like tomatoes cannot grow year-round in unheated greenhouses in many areas, but cold-season vegetables such as beets or carrots can grow throughout the winter. Tomatoes can be started in cold season greenhouses after the last hard freeze of the year and can be transplanted into the ground outside once the weather is warm enough. This procedure will help you get a jump on short growing seasons. You can grow your plants in containers or raised beds.
You can provide temporary heat to an unheated greenhouse during unexpected cold temperatures by moving a portable heater into the greenhouse. Be wary of heating with portable gas heaters. These heaters can vent ethylene gas into the greenhouse. Ethylene is the gas that causes fruits and vegetables to ripen.