A portable greenhouse, characterized by its lightweight frame and thin polyethylene or plastic covering, can function during winter months, but only under specific, managed conditions. These temporary structures operate by trapping solar radiation to create a warmer microclimate, similar to larger, permanent greenhouses. The success of using a portable model in winter depends on proactive modifications and strategies a gardener employs to enhance insulation and manage internal temperatures. Without these additions, the structure will struggle to maintain a temperature significantly higher than the ambient environment, especially after sunset.
Inherent Limitations of Portable Greenhouse Designs
The primary challenge for portable greenhouses in cold weather is their inability to retain heat. Most portable units use thin, single-layer polyethylene, which offers minimal thermal resistance compared to twin-wall polycarbonate or glass panels. This poor insulation causes heat loss through conduction, transferring warm air through the plastic to the exterior. Furthermore, seams, simple zipper closures, and air gaps contribute to heat loss through convection and air infiltration, allowing warm air to escape.
Portable units also face structural weaknesses that become pronounced in winter weather. The lightweight, often tubular metal or plastic frames are susceptible to wind uplift and collapse. A sudden snowfall can accumulate rapidly, and the structure may not be engineered to withstand the weight of a heavy snow load, leading to structural failure. This lack of structural integrity means any heat gained during the day is rapidly dissipated during long winter nights.
The high surface area relative to the small volume of a typical portable greenhouse further exacerbates heat loss. A greater surface area allows for more contact with the cold external air, meaning heat transfers out much faster than in a large, well-insulated structure. While the temperature inside might spike on a sunny winter afternoon, it can plummet back to near-freezing levels within hours of the sun setting, endangering unprotected plants.
Strategies for Maintaining Internal Temperature
To make a portable greenhouse viable for winter, gardeners must focus on reducing heat loss and introducing supplemental thermal sources. Enhancing insulation involves lining the interior with horticultural bubble wrap, which traps a layer of air to create a secondary barrier. The bubble wrap should be applied securely to the inside walls and ceiling, using the largest bubble size possible while still allowing light transmission. Sealing any unintended gaps, such as around zippers, seams, or ground-level edges, with tape or weighted objects is necessary to prevent air infiltration.
Introducing thermal mass provides a passive way to stabilize the environment by absorbing solar heat during the day and releasing it slowly overnight. Dark-colored objects, such as black-painted water barrels, stones, or bricks, absorb radiant heat most efficiently. Placing these materials directly on the floor allows them to act as a heat battery, helping prevent the internal temperature from dropping below freezing after dark. Even a large container of water placed on the lower shelves can contribute to this effect.
For colder climates or for plants requiring temperatures above freezing, supplemental heating may be necessary. Small electric fan heaters, controlled by a thermostat, are the safest option if electricity is available. In the absence of a power source, a paraffin or kerosene heater can keep a small greenhouse frost-free, though this requires careful monitoring and ventilation to manage exhaust fumes. For localized heat, a soil-warming cable or incandescent string lights placed under plant trays can provide direct warmth to the roots, which is important for young seedlings.
Managing humidity and air circulation is also important, even in winter. Heating the space can increase humidity, leading to condensation and promoting fungal diseases. Brief, controlled ventilation on sunny days is required to exchange air and lower the moisture content before closing the structure tightly before sunset to trap the accumulated heat.
Best Uses and Suitable Crops
Portable greenhouses are best utilized for season extension, meaning they extend the fall harvest or allow for an earlier start to spring planting, rather than supporting true mid-winter growth in severe climates. They excel at providing protection from harsh wind, heavy rain, and sudden frost events for plants that are already cold-tolerant. In milder climates, the protected environment can allow for continuous, slow growth throughout the winter months.
The most suitable crops for a portable winter greenhouse are those classified as cold-hardy, which can tolerate temperatures close to freezing.
- Leafy greens like spinach, kale, and Swiss chard are excellent choices.
- Certain varieties of lettuce and Asian greens thrive in cooler conditions.
- Many of these plants, particularly kale and carrots, develop a sweeter flavor after being exposed to a light frost.
Root vegetables such as carrots, beets, and turnips can be successfully overwintered in the protected soil, and hardy herbs like parsley and chives will continue to provide fresh leaves. Conversely, warm-weather crops like tomatoes, peppers, cucumbers, and basil require consistently high temperatures, typically above 50°F. Growing these plants in a portable structure during winter would require a significant amount of supplemental heating.