I just adore this greenhouse made from plastic bottles. There are some amazing and inspiring structures on this site! Check it out Green Houses, Gardens Ideas, Greenhouses Built, Recycled Bottle, Greenhouses Free, Recycled Plastic Bottles, Wine Bottle, Plastic Bottle Greenhouses, Diy
Have a lean-to greenhouse! They have the advantage of getting some heat from your house in winter, either heat that would be lost, or if it is very cold, you could open windows into the greenhouse to let your house heating system heat it. Much simpler and less dangerous than any greenhouse heater. On warm sunny winter days, the greenhouse will actually add heat to your house. And Humidity. And Oxygen.