GIMME SHELTER- Greenhouse effects: Put it off until tomorrow
Q: We just moved into a house with a small greenhouse attached. We'd like to start using it but have never grown plants in a greenhouse. Any tips?
A: Recreational greenhouses are great for cold-sensitive plants you want to "over winter" or for getting a head start on your spring planting– like getting your tomatoes started in February! They can also be handy storage areas for your gardening supplies and tools, as well as a quiet place to sit and enjoy some greenery during the bleak winter months.
However, you may want to wait before you start work on your greenhouse. In the summer, the heat and light factor is going to do you more harm than good. Even with proper venting, greenhouses can get as hot as 130 degrees in the summer, causing plants to wither. In addition, the excess humidity can cause mold and mildew growth. While it's no trouble heating a greenhouse, as the sun helps with that, you can only cool it so much. That's why most people in this area typically shut down their greenhouses in the summer.
September is usually a good time to start cleaning out your greenhouse and deciding which plants you want to over-winter. It's also a good idea to make sure the greenhouse environment is sterilized, especially if is attached to your house, as soil-born diseases can get a foothold.
How you heat the greenhouse depends on what plants you choose. For cold-sensitive tropical plants such as orchids, you'll probably need a more economical gas or propane heater to make sure temperatures don't get below 65 degrees. If you want to grow carrots, lettuce, radishes, and other vegetables through the winter, you'll need to make sure temperatures don't go below 50 to 55 degrees. If you simply want your greenhouse to be a winter home for spring plantings, a small electric heater to keep night temperatures between 40 and 45 should be sufficient.
You might also consider lighting options, as the days are shorter in the winter and it can be fun to work on your gardening in the evening or at night. For germinating seeds and coaxing seedlings, fluorescent and incandescent lamps are sufficient, but to get plants to flower or bud over the winter, you'll need the more powerful metal halide lamps.