Growing vegetables in a greenhouse during the winter season is a bit different than growing them in warm weather. The greenhouse must be kept warm enough to prevent freezing and an inexpensive way to do this is to utilize passive solar storage techniques.
One of the best passive solar structures are plastic bottles filled with water and painted black to absorb the heat of the sun. During the day the bottles absorb heat and at night, the stored energy is released, to help keep the temperature above freezing. An additional heat source may also be needed in extremely cold climates, but even if it is, a good passive solar system will help reduce the cost of heating a greenhouse.
Planting vegetables in the greenhouse during winter is similar to planting them in the summer; the biggest difference is the plants need some extra protection from cold temperatures. Cool weather vegetables like broccoli, carrots, chard, cauliflower, broad beans, asparagus, peas, winter lettuce, garlic, onions and spinach are good candidates to grow inside greenhouses during the cold weather season. They do not require as much sun and warm temperatures that hot weather vegetables require. Broccoli, cabbage and cauliflower should be planted at least ten inches deep and five gallon containers work best as passive solar units work best for these vegetables.
It is important to pay attention to the frost maps and choose seeds that will be able to withstand the temperatures inside the greenhouse during winter. The container sizes that are most commonly used for passive solar units are the 2, 3 and 5 gallon sizes. Additionally mulch can be used to keep the pots insulated against drafts or excess cold. A watering system on a timer or hand watering is still required to ensure the vegetables receive enough moisture to grow. Ideally the first crop should be harvested in October which gives the grower enough time to replant crops or seedlings to raise a crop that can be grown for the rest of the winter months.
As with any garden or greenhouse plantings, be sure to keep the plants inspected for signs of disease or other problems. If an infestation is found take quick action to prevent it from spreading to other plants and destroying an entire crop. Clean any pots before you use them and make certain the soil being used is weed and disease free.
Growing the correct plants can make greenhouse gardening during the winter just as much fun as it is during the warmer months.