Growing tomatoes in a greenhouse through the winter is possible, but it requires significant investment in controlled systems. The winter environment naturally lacks the two primary components tomatoes need: consistent heat and sufficient light. Growers must introduce artificial systems to supply these needs, successfully forcing the plants to fruit year-round. Achieving a winter harvest depends on precisely replicating the long, warm, bright conditions of a summer day, which goes beyond the protection offered by a standard cold-frame greenhouse.
Maintaining Required Temperatures
Tomatoes are warm-season crops requiring specific temperatures for optimal growth and fruit set. The ideal daytime air temperature is between 70 and 82 degrees Fahrenheit, and the minimum overnight temperature should not fall below 60 to 64 degrees Fahrenheit. Consistently lower temperatures slow growth, inhibit flowering, and prevent proper fruit set, making a stable heat source necessary during cold months.
Achieving the required warmth begins with minimizing heat loss through robust insulation. Adding a second, inner layer of material, such as UV-resistant bubble wrap or polyethylene film, creates a thermal barrier that significantly reduces heat loss. Insulating the foundation perimeter with foam boards or mulch also helps prevent ground chill from seeping into the root zone.
Active heating systems are necessary to maintain minimum overnight temperatures. Propane or natural gas-fired unit heaters offer powerful heating but require proper ventilation to exhaust combustion byproducts safely. Electric heaters are highly efficient and require no ventilation, but they are often a more expensive option for large spaces due to electricity costs. Incorporating thermal mass, such as large water barrels painted black, helps stabilize temperatures by absorbing solar energy during the day and radiating heat at night, reducing the demand on the primary heater.
Providing Adequate Supplemental Light
Even with perfect temperature control, natural winter light is insufficient for fruiting tomatoes due to short day lengths and low intensity. Tomatoes are high-light crops requiring a specific amount of photosynthetic light to develop fruit. To achieve continuous production, supplemental lighting must be used to extend the photoperiod and increase the light intensity reaching the leaves.
Usable light is measured using Photosynthetic Photon Flux Density (PPFD). Fruiting tomatoes require a high intensity, typically 500 to 800 micromoles per square meter per second. This intensity must be delivered for a sustained duration of 12 to 16 hours daily to mimic summer conditions and ensure energy accumulation for fruit development. Light-emitting diode (LED) grow lights are the preferred modern choice due to their high energy efficiency and ability to customize the spectrum for flowering and fruiting. High-Pressure Sodium (HPS) lamps are another common option, though they produce more heat and consume more electricity than comparable LED systems.
Choosing the Right Varieties and Managing Growth
The confined space and artificial conditions necessitate careful selection of tomato varieties and strict management of plant structure. Varieties with a determinate or semi-determinate growth habit are often favored because they grow to a manageable height and set fruit over a shorter period. Smaller-fruited types, such as cherry or cocktail tomatoes, are also excellent choices, as they set fruit more reliably under lower light conditions than large slicing tomatoes.
Since natural insect pollinators are absent in a closed winter greenhouse, manual intervention is required to ensure fruit set. Growers must hand-pollinate by gently shaking the tomato trusses or using an electric toothbrush to vibrate the flowers, releasing pollen onto the stigma. This action should be performed daily during peak flowering, ideally when humidity is around 70 percent. Aggressive pruning and training are also necessary to maximize light penetration throughout the dense canopy. Regularly removing suckers and training the main vine up a stake or string ensures the plant directs energy into fruit production rather than excessive vegetative growth.
Addressing Pests and Diseases in a Closed Environment
A consistently warm, enclosed greenhouse environment is a breeding ground for pests and fungal diseases. High humidity, a common side effect of heating and plant respiration, quickly leads to fungal issues like late blight or gray mold. To prevent this, growers must run circulation fans constantly and use ventilation, even in winter, to exchange humid air with drier outside air. This keeps the relative humidity between 60% and 70%.
The most common indoor pests are soft-bodied insects like whiteflies, spider mites, and aphids. These pests reproduce rapidly and can quickly destroy a crop. Management strategies involve:
- Regular scouting to detect infestations early.
- Placing yellow sticky traps near plants to capture adult whiteflies and thrips.
- Maintaining good sanitation practices.
For established issues, biological control using beneficial insects, such as releasing parasitic wasps or predatory mites, is highly effective and avoids chemical sprays.