Many gardeners seem to shut down operations in the winter entirely and start completely anew each season. With proper overwintering techniques, which are made much easier with the use of a greenhouse, many plants can be preserved only to be more vigorous beautiful producers the next season. In addition to perennial flowers and ornamentals most berries including Black, Blue and Raspberries can be grown for more than one season as can most herbs including Basil, Chives and Parsley. Several vegetables are also perennials including Asparagus, Garlic, Shallots, Sweet Beets, Sweet Potato and many Peppers.
There are some distinct advantages to using a greenhouse for overwintering. In many areas rodents can be a significant source of damage to plants during the winter months when other food is scarce. Another major cause of damage and loss of plants during winter is caused by freezing and fluctuations in soil temperature, and this is an even bigger problem for plants in containers. Plants out in the open also often sustain tissue burn from the moisture loss caused by cold dry air. Proper overwintering in a greenhouse makes it much easier to protect against these and other winter hazards to plants. It is possible to keep many plants from one year to the next. They will not continue to bloom profusely and grow over winter as they did during the summer, but in many cases they will have better production than they did in year one and will give the gardener a great head start on the next season. Depending upon the climate, late August (Hardness zones 3-6) to late September (Hardness zones 7-8) is generally the window for beginning the overwintering process. In most areas it is practically impossible to keep plants thriving and producing outside past October. The low sun and shorter days tend to induce 'semi-dormancy', a period of reduced functioning in the plants. For this reason the beginning of the overwintering program will start by gradually tapering off watering and fertilization in October. In the winter the soil should not become bone dry or dusty, but should be watered only occasionally during the plant's dormancy. It is best to hold off on moving plants into the greenhouse until just before the first freeze of winter sets in. Moving the plants inside too early will slow down the hardening off process and may result in undesirable out of season growth.
Once plants are in the greenhouse temperature control is very important to overwintering success. Temperatures of 45°-55°F are ideal. The higher the temperatures above this range, the more likely the plants will produce weak growth out of season which can make them susceptible to various infestations. Greenhouse temperatures can generally be controlled using ventilation during the day to keep the greenhouse from overheating. In very cold climates a heat source may be necessary to keep the temperature close to the ideal range, particularly at night.
Most plants will benefit from some pruning back in mid-March in preparation for the vigorous growth during the new season. This is also a good time to consider transplanting plants that grew a lot during the past summer into the garden or larger containers. Plants adjust best to gradual change. At this time, the greenhouse can be allowed to reach slightly warmer temperatures (65°F days, 55°F nights). Resume fertilizing with balanced blend mixed ½-strength. When all danger of frost is past, the plants can be moved outside again for the summer, and the greenhouse temperatures can be raised for plants that will remain in the greenhouse or for germination of new plants for the growing season.
For successful overwintering in the greenhouse keep these tips in mind: Begin overwintering by reducing water and fertilizer in late August to late October, depending on Hardness Zone. Take care not to allow soil or air to become overly dry. Wait until just before the first freezing weather of the season to move plants into the greenhouse. During the winter dormant period maintain greenhouse temperatures as close to 45°-55°F as possible.
Prune plants back in mid-March in preparation for the new growing season. After pruning, increase greenhouse temperature to 65°F days, 55°F nights and resume fertilizing with balanced blend mixed ½-strength. Move plants to desired outdoor locations when all danger of frost has passed.