At its most basic level, a greenhouse absorbs sunlight inside the structure, making for warmer growing conditions than those outside. A greenhouse can provide the permaculture gardener with another type of growing environment on their plot, to compliment the natural climatic condition ns and the microclimates they have created. It can help to start new seedling, giving them protection from the elements during their fragile juvenile stages. A greenhouse can also serve to extend the growing season of plants, giving them a ‘head start’ on spring, before they are planted out. Gardeners can also use the greenhouse conditions to grow species that require warmer conditions than those their plot provides outside (although these are unlikely to be native species so care must be taken to avoid cross-pollination).
Having an efficient, effective greenhouse is dependent on several factors, including choosing the right plant species and siting it in the best location for capturing sunlight. Within the greenhouse, variations in temperature, moisture levels and airflow are also important. One of the key aspects of a greenhouse that impacts upon these variables, and others, is ventilation. Arguable, getting the ventilation right is the way to ensure a productive greenhouse. Here are the reasons why.
While the purpose of a greenhouse is to be hotter than outside conditions, it is important that it not get too hot. Plants are sensitive to temperature, and getting too hot can cause them to wilt, stop growing or producing crops, and even to die. They also prefer to avoid extreme swings in temperature. Good ventilation in a greenhouse avoids both these problems by allowing hot air to escape and cool air in, maintaining an even, optimum temperature.
Plants give off moisture vapor into the air through transpiration. If this moisture is not allowed to escape then conditions within the greenhouse become detrimental to plant growth. This is because excessive humidity causes the temperature to rise considerably (as water absorbs a lot of the heat from the sun) and provides the conditions that pathogens tend to favor. Molds, fungi and mildew all become more profuse in humid conditions. If the humidity is not controlled, the air in the greenhouse can reach the dew point – when the air no longer has the capacity to hold the water vapor, and it condenses back into liquid. This can cause problems with condensation and saturation of plants. Proper ventilation allows the wet, warm air to be replaced with dry, cooler air and so keeping humidity at the optimum level.
As we know, plants utilize carbon dioxide from the air in the process of metabolizing energy in photosynthesis. One of the by-products of this process is oxygen. If a greenhouse is not well ventilated, the air inside becomes ‘saturated’ with oxygen and the plants cannot get the carbon dioxide they need to photosynthesize and grow.
Plants also require oxygen – it plays a key role in their ability to take up nutrients – and primarily get it via their roots from the soil, so having a good supply of fresh air (combined with a well structured soil, rich in organic matter and with plenty of pore space to allow for aeration) is essential for plant growth in this respect as well.
The movement of air that greenhouse ventilation provides also allows your plants to pollinate. In nature, many plants depend on the wind to disperse their pollen to other individuals in order to propagate. In a closed greenhouse, this method of transport is not available. Ventilation allows airflow to gently shake the plants, releasing their pollen and carrying it to other specimens.
Air circulation is linked to all of the above factors. Movement of air within a greenhouse helps to regulate temperature and humidity, contributes to oxygen exchange and encourages pollination. It also serves to provide a stable environment for the plants, who tend to prefer consistency, as well making the greenhouse more uniform, so that all plants receive the same atmospheric conditions, and no area is allowed to become over-hot or moist. The air circulation provided by ventilation is the interior version of wind. Wind is essential to cultivating strong plants, because as they bend under the pressure of the wind, plants grow stringer cell walls to protect themselves. This helps grow hardy, healthy specimens, particularly important of they are to be transplanted outside into the garden.
Poor ventilation is a greenhouse will result in weak plants, and so more susceptible to insect pests. Many insects also favor hot and humid conditions for laying eggs. Good ventilation will help prevent blooms in pest populations by inhibiting breeding, as well as allowing beneficial predatory species to enter and leave the greenhouse. By allowing insects into the greenhouse, the ventilation system also affords the plants another means of pollination.
There are a few methods available to the permaculture gardener to ensure that their greenhouse gets sufficient ventilation, and each requires planning at the construction stage to incorporate them into the design of the building. Fan ventilation is common in many commercially available greenhouses, with the fan exhausting the hot air to create a vacuum into which cooler air is drawn through louvers in the sides of the greenhouse. However, as permaculturists we should avoid using this method unless we can link it to a solar or wind-generated source of power to drive the fans. There are also ways to ventilate the greenhouse that do not require the input of energy.
Side vents are one option. They are best place higher up on the walls of the greenhouse to take advantage of the way warm air naturally rises. This is also the reason why greenhouses with roofs that can open are another option. Instituting a design that has removable side panels could be an alternative (although make sure your chickens can’t then get into the greenhouse), while simply leaving the door open will have some effect as long as there is one at each end of the greenhouse to allow for airflow.