Ventilating the greenhouse
Glass and foil
Since greenhouses and polytunnels consist almost entirely of glass or thick foil temperatures can rise surprisingly quickly, simply caused by the fact that greenhouses and polytunnels are sheltered environments; next to that, glass and foil transform sunlight into warmth-energy, creating a highly humid, tropical climate. Most plants revel in these circumstances but when it's too moist and hot, diseases and moulds may develop swiftly.
High humidity in warm greenhouses and polytunnels
Warmth makes plants grow but also induces moisture to evaporate from leaves and soil, causing high humidity. All sorts of moulds, fungal infections and diseases love this, providing them with opportunities to spread rapidly and affect your plants. Naturally you'll wish to avoid that at all costs. Ventilating your greenhouse is the answer to a healhty environment for your plants.
How to ventilate your greenhouse and/or polytunnel
Simply open all doors and windows on a regular basis. From May onwards open at least one window and a door, day and night. In case of a polytunnel with two doors: always open these during the day (as tunnels usually don't have windows). In order to allow warm air to escape rapidly greenhouses with a skylight or large roof window are best. Lacking that, you might try using a ventilator to do the job for you, as fresh air blowing in is crucial for all plants; they absorb carbon dioxide (CO2) from the air which stimulates growth. Regular fresh air supply also lowers humidity levels which obstructs the development of moulds, fungal infections and diseases.
Ventilating in spring and autumn
In spring always open both a door and window on mild, frost-free days. Close all windows and doors at night, unless it's mild outside. All this applies to similar weather conditions in autumn.
Ventilating in winter
Although your greenhouse will be almost empty of plants in winter, do ventilate from time to time, for moulds developing in winter can harm plants you wish to grow in spring. So open both a door and window on sunny, frost-free days. Even in winter this the best possible way to control air humidity,refraining moulds and diseases from developing.
Automatic window openers
In case you forget to open windows and doors in your greenhouse, or when you plan to go away for a prolonged period of time, automatic window openers might do the trick for you! They operate on a straightforward system of heat-sensitive oil, a piston and a cylinder. When temperatures in your greenhousel rise, the oil expands and automatically causes the window to open. And when it gets colder the window closes automatically too.
Beware of vermin and cats
In summer, when your greenhouse is ventilated night and day, it's hard to check (especially at night) which animals visit your greenhouse, or even eat your fresh vegetables. Sliding screen doors or screen windows, or netting doorways and windows, form a perfect solution for carefree ventilation: they do not hamper air circulation whilst keeping all visitors out.
Tip: Always water your plants at their roots or through any watering system in the soil to avoid risks of increasing high humidity levels in the greenhouse.
High temperatures in the greenhouse
Ventilating your greenhouse allows fresh air to flow in which lowers temperatures inside slightly, but on a windless warm summer's day temperatures will remain high. A possible solution is to cover the greenhouse roof with any type of shadow screen, or whitewash its windows to stop direct sunlight from coming in. Do read our 10 tips for your garden in summer!