Fresh Air Matters: Greenhouse Ventilation is Vital
February 3, 2025
Fresh Air Matters – Greenhouse Ventilation is Vital!
Mid summer can feel like kick-back time in the tunnel house. Fruit is ripening, flowers are being pollinated, we’re enjoying a daily harvest of produce, and it’s too soon to be thinking about sowing the next round of plants for our undercover haven. But don’t be deceived, because if we’re to keep everything on track, the task of ventilating the tunnel house is crucial!
Unhelpful heat
It’s in mid-summer, more than at any other time of year, that temperatures in the tunnel house can rise dramatically. And when they do, it can spell disaster to our plants. When temperatures rise above the 24°C to 29°C that most tunnel house plants are comfortable with, fruit can spoil, flowers may drop and fail to set fruit, and if we’re not careful, rapid evaporation can cause soil to dry out to a detrimental level, even over a 24 hour period. Water should be used early in the morning or later at night if there are high temps, as this will also help reduce evaporation.
Humidity hurts!
Excess heat accompanied by poor ventilation can also result in humidity levels rising to an unhelpful level. Humid air is filled with water vapour and provides perfect conditions for fungal diseases such as mould and mildew to flourish. When these diseases set up home on foliage and in cracks in stems, plants quickly succumb to the damage they do. High humidity can also hinder transpiration – the process whereby moisture evaporates from a plant, helping it cool down and, at the same time, encouraging nutrients to move through it. As well as this, high humidity can interrupt a tunnel house plant’s ‘breathing’ by lowering the level of carbon dioxide available, and reducing opportunities for photosynthesis or ‘plant food-making.’
Pest pile-up
What’s even more infuriating to the mid-summer undercover gardener is how pests take advantage of a poorly ventilated tunnel house. The moment a plant loses condition, pests are in for the kill, burrowing into damaged stems, sucking sap from weakened foliage, and attacking vital root structures. And many insects thrive in high-humidity settings!
So, if you’re thinking of taking a break from the tunnel house over mid-summer, it’s time to think again. By attending to ventilation, a multitude of problems can be overcome simply and easily – here’s how to go about it:
Ventilation tips and tasks
Air vents, doors, and windows
While commercial greenhouse growers have recourse to heat extraction fans, home growers need to use manual methods to keep their tunnel house to an optimum temperature. The first step in doing this is to hang a greenhouse thermometer in your growing space. While ventilation is required at all times, a thermometer will take the guesswork out of gauging when extra cooling is called for (note: some greenhouse thermometers also come equipped with humidity readers).
Next, consider the ventilation available to you: all tunnel houses will have a doorway, and some will have two – one at each end of the structure, roof vents and windows. Your aim is to manipulate these vents/windows/doors so that cool air enters the tunnel house., and warm air escapes. The most effective way to have this happen is through cross ventilation whereby cool air enters through one side of the tunnel house and exits out the other. However, also effective is to have cool air entering through one door, and warm air passing out another. If you have only one door, opening it will allow cool air to enter and warm air to pass out through the roof vent. At night, temperature is controlled in the same way but vents are best kept only partially open to always allow ventilation unless a gale wind is imminent, then they should be closed.
One of the biggest drawbacks to using vents is that they admit birds and other animals/pests. These animals/pests have their eyes on your tunnel house and produce as much as you do! Keep them at bay with screen doors. Available from Morrifield, these mesh doors can be purchased at the time of buying your tunnel house, or can be fitted retrospectively. They will also keep out insect pests such as white butterfly. Also available are Morrifield shade cover kits which reduce heat but they also cover vents, still allowing ventilation but keeping out the birds!
Foliage control
Dense foliage restricts the circulation of air in the tunnel house., as do fruit and weeds. Wherever possible, once plants are producing fruit, snip off excess lower leaves (in tomatoes and cucumbers, this can be to a height of around 30 to 40cm). Pick fruit as soon as it is mature, and remove any weed growth from the tunnel house. bed. Most importantly, support plant growth so that it doesn’t topple down on itself, crowding the lower region of the tunnel house and restricting air movement.
Keep your cool
The less heat build-up in your tunnel house, the less ventilation will be required. Morrifield now offer 30% shade cover kit protection in the form of mesh covers that can be removed during the cooler seasons when all available heat is welcome.
Ventilation isn’t rocket science, and the home gardener can expect great results when they take a keen interest in using it effectively. By tailoring ventilation to your climate and by adjusting it with a day/night routine, your plants will flourish and your produce will be plentiful!