The proper setting for a greenhouse thermostat depends on the type of plants inside the greenhouse as well as the level of sunlight shining on the greenhouse. Temperatures should range from 45 to 80 degrees Fahrenheit.
Winter crops such as leafy greens tolerate temperatures between 45 and 70 degrees Fahrenheit. Warm-weather crops such as tomatoes grow well in temperatures between 55 and 80 degrees Fahrenheit. A greenhouse thermostat should be adjusted down at night and up in the day to emulate the natural climate a plant would encounter outside the greenhouse. In addition to a proper thermostat setting, a greenhouse should be ventilated at least once a day to cycle out carbon dioxide.