The smell of warm plastic and stressed foliage fills the air when you enter a sealed backyard greenhouse at eleven in the morning on a sunny July day. The paper is heavy, dense, and almost wet. On the staging bench, the temperature is 38 degrees Celsius. The temperature outside is twenty-three degrees. In perfect condition, the tomato plants along the back wall stand upright with uncurled leaves. They aren’t doing well. At 35 degrees, tomato pollen becomes unviable. The flowers will open, and if the door is cracked, the bees will come. A few weeks later, the grower will wonder why the trusses are bare.
During the warmer months, this scene occurs in backyards around the world, but it is nearly impossible to avoid. Ventilation isn’t difficult; it’s not. It is disappointing how often it is ignored. Greenhouse buyers spend a lot of money on building, staging, grow lights, and heating propagators. When the sun shines, they rarely consider what will happen to the trapped air. Some of that responsibility might fall on manufacturers. Hobby greenhouses typically have one roof vent measuring about 0.5 square meters. The vent space should cover about 20% of the floor, according to horticultural authorities. Every warm day from April to September, the difference between those two numbers matters a lot on a six-by-eight structure.
Physics underlies the problem fairly simply! As short-wave light, solar radiation warms everything inside the building it comes into contact with before re-radiating as long-wave infrared heat. Unauthorized outward escape is prevented by glazing. There is an increase in temperature. Due to their large glazed surfaces, small greenhouses heat up more quickly than large ones. The interior temperature can exceed the outside air temperature by 20 to 30 degrees in an hour on a calm day with no vents open. Six-by-fours heat up more violently than ten-by-twelves. The plants in the smaller structure face odds in a very literal sense.
Despite taking center stage in discussions, heat is just one aspect of the problem. Humidity is the other factor. Every plant in a greenhouse releases moisture through its leaves throughout the day; this process is known as transpiration, and it takes place continuously in a closed environment where water vapor can’t escape. On the glass, condensation accumulates. Moisture remains on surfaces. Leaves never dry out. These kinds of environments are required for gray mold, botrytis, and powdery mildew to flourish, and once they do, the damage spreads rapidly. A tray of seedlings developing white fuzz overnight is not unlucky. This indicates that the air has stopped moving. Another problem that receives scant attention is carbon dioxide. During photosynthesis, plants can use up CO2 supplies faster than most people realize in sealed greenhouses. There has been a slowdown in growth. The grower wonders about the variety, checks the soil, and modifies the watering. The problem was with the air.
There is one principle that underpins the solution: hot air out high, cool air in low. When rising warm air is allowed to escape by convection through roof vents at or near the ridge, cooler air is drawn in through any lower opening, such as a door, a louvre panel, or a gap near the base. To achieve this stack effect, just enough openings are required in the right locations; neither electricity nor fans are needed. If a louvre vent is added to the windward side wall in conjunction with a ridge vent, most small greenhouses are significantly closer to the recommended ventilation area. Standing inside the building on a warm day, the grower feels immediately and clearly better after this relatively inexpensive adjustment.
Automatic vent openers should be given more consideration than they usually are. When heated, a cylinder of mineral wax expands, pushing a piston to open the vent. As the temperature drops, the cylinder contracts, allowing a spring to close the vent once again. The vents are attached to an existing roof vent. There is not a single wire. There is no power source. There is no programming at all. A basic model begins to react at about 16 degrees Celsius and costs about £46. When the grower is at work, at school pickup, or just not paying attention, they make the difference between a crop that survives and one that doesn’t. It takes fifteen minutes or more for the wax to move enough to open the vent fully. Experienced greenhouse growers know how to fit them. Up until the first time it saves a planting, it seems like a nice extra.
Seasonal management shifts the computation throughout the year. At noon it can be too hot for overheating, at night it can be too cold for frost, and sometimes both within a twelve-hour period. Keeping vents closed for safety causes the midday spike that causes quiet damage. Summer is easier: open everything, leave the door open on nights above 15 degrees, and water the floor twice or three times on sunny days instead of watering the plants. Through evaporation, that final step cools the air by several degrees, in addition to discouraging red spider mites, which thrive in hot, dry, and still conditions caused by insufficient ventilation. During the winter, a quick vent opening at noon on dry days is still necessary to remove damp, stagnant air that contributes to grey mold growth.
When growers see failed crops and struggling seedlings season after season, it’s hard not to think that a fifteen-pound thermometer and one extra vent panel would have revealed the whole story sooner. Checking the maximum reading on a digital min-max thermometer every morning is the easiest diagnostic. The ventilation is inadequate if the temperature rises above 30 degrees. At temperatures above 35, tomatoes lose fruit set for days. Plants appear to be typical. Harvest will not take place.