Air Circulation

Winter growing in a greenhouse, especially if you use hydroponics exclusively, will require extra air circulation and ventilation. Air circulation is simply moving the existing air around inside the greenhouse; whereas ventilation is moving air from the outside into the interior of the greenhouse, and moving greenhouse air out.

Fans will move the interior air around, and ventilation fans will bring in (intake fans),or expel air out (exhaust fans). These are both important to have in place for good plant production.

Plants utilize Carbon Dioxide (CO2) that is present in the air. When humans breathe, they expel CO2, and the plant can use it. Trees do the same thing. Trees and plants expel Oxygen (O2) which humans need to survive. This is called a symbiotic relationship, where two living things benefit each other.

In a greenhouse environment, you can create symbiotic situations that will benefit the plants, even when you are not present. The fastest and easiest way to incorporate CO2 into the interior of the greenhouse is with a wall mounted intake fan that brings outside air inside. You can also heat this air before it hits your plants, which is helpful in the cold winter months. If your intake fan is not going to heat the air, you will not want it bringing in winter air if you are gardening inside. You can use air circulation fans to stir the air, which will make what CO2 that is present more available to the plants. A completely air tight or sealed greenhouse will have a limited amount of CO2 present, so you will need to find a way to bring more CO2 into the greenhouse.

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Here is a greenhouse term you will want to remember: CO2 Enhancement. It refers to increasing the amount of CO2 that is present in the greenhouse to speed up the growth of plants and fruiting or flowering. Too little CO2 present will actually slow down the plant growth. To enhance, or increase, the amount of CO2 present inside your winter greenhouse when cold outside air is not desired, you simply need to put into place the practices that will emit more CO2. A greenhouse that is attached to a house is not as likely to have a CO2 shortage because of increased human activity and opening of doors, gas stove fumes, pets present, etc.

One way to add CO2 into your greenhouse interior, is to put your working composter inside it. As the composting process is going on, the microbes emit CO2. When manure is decomposing, it gives off CO2. Putting your composter inside your greenhouse will help the composter work faster, and it will actually heat up, and give off a small degree of heat into the greenhouse, which benefits the greenhouse plants. It is also very convenient for putting plant debris into it when you harvest, dead head, or trim them. That is a benefit for the greenhouse grower.

Another factor involved in CO2 levels is compost in soil. The billions of tiny microbes that are present in compost emit CO2. Therefore, the more organic compost you have in the pots, the more likely they are to be obtaining all the CO2 they need.

Modern greenhouses are so well made and airtight, they tend to need CO2 enhancement. Fans are the most critical way of moving the air around and making the CO2 that is there available. In a totally still environment, a plant leaf will use up all the CO2 around it, and not be able to receive more. When you move the air, it pushes CO2 onto the leaf surface where the leaf can take it in. Even if you have no other CO2 Enhancement program in place, a fan will increase the plants growth by as much as 40% compared to without air movement.

If you are burning propane or natural gas inside your greenhouse to heat it during the coldest months, you are recieving ample CO2 enhancement.

Plants utilize and convert CO2 only when there is good light available. On days that are dark and cloudy, not much photosynthesis is going to happen, so you can turn off the fans. They are not needed at night-time either.

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If you are growing plants in raised beds inside your greenhouse in the cold winter months, use plenty of good organic soil amendments in the beds with plenty of compost, and mulch your plants with natural materials, and you won't need to worry about carbon dioxide levels at all.

If your greenhouse has an exterior wall mounted intake or exhaust fan, you want to make sure cold air is not entering through it. If you have very mild days and want to bring in outside air, be sure the ambient temperatures are warm when the air reaches the plants. Check it with a thermometer placed near the plants on the shelves.

Carbon Dioxide in the air is essential to photosynthesis, which is the process of the leaf exchanging light into energy to grow.