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.
In climates with cold winters, you can successfully keep potted citrus trees, ferns, succulents and perennial herbs (like rosemary) alive by bringing them in once nighttime temperatures dip into the Some plants can stay out longer than others; check w