Greenhouses allow gardeners to grow plants in colder temperatures by trapping heat from the sun during the day and releasing it slowly at night. However, there is a limit to how cold it can get inside a greenhouse before plants suffer damage. Knowing what temperature is too cold for your greenhouse plants can help you take steps to keep them healthy through winter.
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How Greenhouses Trap Heat
Greenhouses work by allowing sunlight to enter through transparent glazing, usually glass or plastic sheeting. The sunlight warms objects inside the greenhouse during the day, including the soil, plants, and other structures. These objects then release heat as infrared radiation at night.
The glazing material traps this infrared radiation, preventing it from escaping the greenhouse. This creates a warmer microclimate inside the greenhouse than outside. The difference between the inside and outside temperatures is called the heat retention capacity.
Factors that affect heat retention in a greenhouse include:
- Number of glazing layers – Multiple layers trap more heat.
- Insulation – Extra insulation around the walls and roof retain more warmth.
- Air tightness – Sealing drafts prevents warm air from escaping.
- Thermal mass – Objects like masonry or water tanks store and release heat.
- Size – Larger greenhouses lose less heat relative to their volume.
With proper design and construction, greenhouses can stay significantly warmer than the exterior even on cold nights.
Minimum Safe Greenhouse Temperatures
Different plants have different minimum temperature requirements. Some key thresholds for common greenhouse plants include:
| Plant Type | Minimum Safe Temperature |
|---|---|
| Tomatoes | 45°F / 7°C |
| Peppers | 55°F / 13°C |
| Lettuce and greens | 25°F / -4°C |
| Cucumbers | 60°F / 15°C |
| Herbs | 50°F / 10°C |
| Root crops | 32°F / 0°C |
Temperatures below these thresholds can damage crops. Common symptoms of cold damage include:
- Wilting, shriveling, or collapse
- Water-soaked areas on leaves and stems
- Darkened or blackened foliage
- Stunted growth
- Discolored, damaged buds and flowers
- Poor fruit set or deformed fruits
The exact temperature a plant can withstand depends on factors like the length of exposure, wind conditions, humidity, and how cold-tolerant a particular variety is. But in general, keeping the greenhouse above the minimum thresholds for each crop is recommended.
Nighttime Temperature Management
The biggest greenhouse heating challenges often come at night when the sun’s radiation is absent. Nighttime lows can plunge far below comfortable levels for plants.
Here are some tips to help maintain warmer nighttime temperatures in your greenhouse:
- Use thermal blankets – Also called frost blankets, these insulated cloth sheets can be deployed overnight to provide 5-10°F of extra protection.
- Add portable electric heaters – These can boost temperatures on very cold nights when passive heat retention isn’t enough. Choose greenhouse-safe units with thermostats.
- Insulate the greenhouse – Add extra insulation around the perimeter foundation, walls, and roof framing to reduce nighttime heat loss.
- Seal any leaks – Use caulk and weatherstripping to seal gaps where warm interior air could escape to the cold outdoors.
- Add thermal mass – Include materials like stone, concrete blocks, or water barrels inside your greenhouse to absorb and release heat.
- Group plants together – Arrange planting beds and containers close together to conserve heat.
Venting and cooling systems may also be needed during sunny winter days when temperatures can spike inside the greenhouse. Proper ventilation and air circulation prevents overheating when solar gain is high.
Additional Winter Greenhouse Tips
In addition to maintaining adequate temperatures, there are other considerations for winter greenhouse growing:
- Maximize sunlight – Remove shade cloth and any other obstructions blocking light transmission through the glazing.
- Provide supplemental lighting – Use grow lights or reflectors to increase light levels during shorter winter days.
- Monitor soil moisture – Watering requirements are lower but don’t let plants dry out.
- Adjust fertilizer – Use lower concentration, nitrogen-only feeds in winter.
- Allow dormancy – Some plants benefit from a natural winter rest period.
- Remove dead plant material – This prevents pests and diseases from overwintering.
- Check glazing for leaks – Seal any cracks in glass panes or plastic sheeting.
With extra preparation, greenhouses can remain productive year-round even through cold winters. Heating and insulation are key to maintaining temperatures suitable for your particular greenhouse plants.
Conclusion
There is no one-size-fits-all answer to what temperature is too cold for a greenhouse. Different plants have different sensitivity to cold. However, most common greenhouse crops will suffer damage if exposed to temperatures below 45-50°F for extended periods.
Greenhouse design, insulation, heating, and proper cultivation techniques allow gardens to stay productive through cold weather. Monitoring nighttime temperatures and maintaining the ideal zone for each crop is critical. With adequate heat retention and thoughtful plant choices, greenhouses can continue yielding fresh produce even during frosty winters.
References
- Nelson, P. V. (2012). Greenhouse operation and management. Upper Saddle River, NJ: Prentice Hall.
- Aldrich, R. A., & Bartok, J. W. (1994). Greenhouse engineering. Ithaca, NY: Natural Resource, Agriculture, and Engineering Service.
- Boodley, J. W. (2002). The commercial greenhouse. Clifton Park, NY: Delmar Cengage Learning.
- Wang, S., & Boulard, T. (2000). Measurement and prediction of solar radiation distribution in full-scale greenhouse tunnels. Agronomie, 20(5), 557-565.
- Hanan, J. J. (1998). Greenhouses: Advanced technology for protected horticulture. Boca Raton, FL: CRC Press.