I’ve been reporting on a greenhouse that grows tomatoes in January. In July, you can expect red, fat, and structurally flawless tomatoes. For the past two seasons, a different grower with a similar setup and climate has struggled with root rot and yellowing leaves. They are not separated by money. It doesn’t even require effort. It’s harder to see and quieter.
Most greenhouse builders believe they are building a shelter. A bit of heat, some glass, and a roof. After losing one or two crops, the successful ones realize they are creating a climate. A living, adjustable system must maintain a particular tension among heat, water, light, and air. Plants can perform amazing feats when tension is just right. When you make a mistake, the building becomes less of a haven and more of a slow trap.
Usually, people ignore the science of light, or more specifically, the type of light that plants can use. We tend to believe that greater brightness equals better growth. No, it doesn’t. For photosynthesis, plants use light in the 400-700 nanometer range, known as Photosynthetically Active Radiation, or PAR. Plants receive the wrong spectrum of light when raw sunlight floods a greenhouse. Today, successful businesses use spectrum-specific supplemental lighting and diffused glazing, treating PAR more like a dial than a switch. A tiny difference can have a huge impact on yield.
Another issue is air pollution. It is not ventilation in the broad sense, but what happens within the thin, imperceptible layer that surrounds each leaf. The boundary layer is essentially a pocket of still air that adheres to the leaf surface and slows down gas exchange. As long as that layer remains unaltered, the plant begins to work against itself, humidity concentrates, and carbon dioxide uptake decreases. Active airflow breaks up that layer. To be significant, it seems almost too easy. In contrast, growers who use horizontal airflow fans report that their plants are not only healthier, but also structurally stronger, with thicker stems and deeper roots. During thigmomorphogenesis, the plant physically reinforces itself as a result of constant mechanical stimulation from moving air. It’s the kind of information that only someone who has read a lot of literature or has seen enough failing greenhouses to ask uncomfortable questions would know.
Among serious growers, Vapor Pressure Deficit, or VPD, is the metric that differentiates them. Most inexperienced greenhouse owners consider humidity. Do you think this place is too damp? Is it too dry? Humidity, however, is not a reliable indicator of a plant’s transpiration rate. In VPD, the difference between the current moisture content of air and its potential moisture content at a given temperature is calculated. A large gap causes plants to shut down since they lose water faster than they can absorb it. When their systems are too narrow, water cannot move through them, stunting growth and resulting in warm, damp conditions that fungi love. Most casual growers are unprepared for the equipment and attention needed to monitor VPD. This attention gap is at the root of most greenhouse failures.
Most failures revolve around a few common errors when you examine them in detail. Overwatering is the most common cause of plant death in enclosed growing environments, rather than underwatering. Water loss causes soil to lose oxygen, resulting in anaerobic conditions where roots practically suffocate. Yellow leaves are present. It appears that the plant is thirsty. Water is used more by the grower. It’s frustrating to watch the spiral because the solution is so illogical. Likewise, improper site selection results in long-term problems that no amount of equipment can resolve, such as greenhouses oriented without considering sun exposure or situated where drainage is inadequate.
In addition, growers are caught off guard by seasonal blindness. An indoor greenhouse that performs flawlessly in the winter can turn into a breeding ground for mold and pests during the summer if the ventilation system isn’t designed with heat management in mind. Most seasoned growers will tell you that summer cooling should be addressed before winter heating. In contrast to cold nights, hot days in an under-ventilated greenhouse can kill a crop in hours.
A greenhouse that thrives-those that produce in January and supply restaurants all year round-is distinguished by its operators’ shift from thinking like gardeners to thinking like systems managers. Temperature variations are recorded by them. The VPD is monitored throughout the day. The airflow is treated as infrastructure instead of an afterthought. In the best-run operations, plants are essentially free from environmental stress, allowing them to focus all their energy on growth. There is a clinical tone to it. While standing inside one of those greenhouses and observing row upon row of robust plants in the middle of winter, you sense that this is what the building was meant to do. It just took some people longer to figure it out.