October 2013 Newsletter (vol.22)   >>   Importance of Ventilation Year Round

 

 

Apple Orchard

When thinking about a greenhouse, the first thing that comes to mind of most consumers is beautiful flowers and fresh produce. On the opposite end of the scale, most greenhouse growers think about temperature, water, ventilation and the logistics of running a greenhouse. One of the most important, and hardest to regulate, aspects to proper greenhouse growing is ventilation. Ventilation not only controls the temperature of the greenhouse, but also the humidity control, carbon dioxide/oxygen replacement and air circulation.

When a grower first looks into building a greenhouse, they have two types of ventilation they can choose from: natural or mechanical:

  • Natural ventilation is a system that has no motorized fans but instead relies on wind and thermal buoyancy for air movement. Thermal buoyancy is based on the physical properties of air; as air is heated it has the natural tendency to rise. This can be accomplished by using sidewall and ridge (roof) vents. This process will make a vacuum that draws cool air in through the sidewall vents and as the temperature raises the warm air will rise and escape out of the ridge vents.
  • Mechanical ventilation systems use exhaust fans and other mechanical devices to produce airflow and circulation. If you use this system, you will have more control than with natural ventilation. You will be able to automate your system to keep your greenhouse’s environmental conditions consistent regardless of the conditions outside, creating a micro environment. With this type of system, you will also be able to have a relatively sealed environment which reduces the entry points for unwanted pests, and the amount of insect screen required.

Ventilation systems play many roles in a greenhouse operation. A system can directly influence a plants ability to perform photosynthesis, uptake essential elements and successfully complete reproductive cycles. Ventilation affects many factors within a greenhouse such as:

  • Temperature Control: The greenhouse effect causes solar radiation that gets trapped and as a result raises the temperature in the greenhouse. If this heat is not removed it can cause undesirable conditions for plant growth. Both types of ventilation are effective at removing this heat buildup in the greenhouse.
  • Humidity Control: Humidity, like heat, can become trapped inside a greenhouse without proper ventilation. While some plants thrive in this condition, most plants can become crippled by the pathogens associated with high humidity levels. Most molds, fungus, and mildew thrive in high humidity conditions. If more pathogens are present, then more chemicals are required to treat your plants. On the other hand if you live in a desert area humidity is something that you need to add to your greenhouse and a fogger would be a great way to do that.
  • Carbon Dioxide/Oxygen Replacement: Plants “breathe” carbon dioxide and produce oxygen during photosynthesis. The carbon dioxide needs to be replenished in any greenhouse that is not artificially supplying it. To artificially supply carbon dioxide in your greenhouse you can use a carbon dioxide generator. If carbon dioxide levels decrease the rate of photosynthesis and all other functions associated with it will also decrease. It is important to maintain an adequate amount of fresh air in your greenhouse to replace the carbon dioxide that the plants require.
  • Air Circulation: Air circulation or (air movement) in a greenhouse serves many purposes. It creates uniformity in temperature, humidity, carbon dioxide and oxygen in a greenhouse environment. The plants within a greenhouse respond better to consistency and this also helps create a uniformed crop. Air movement is also a way for us to simulate the wind that the plants will encounter after leaving your greenhouse. Wind strengthens the cell walls of the stems of your plants and creates structural integrity of the plant. Air movement also serves as a way to pollinate plants, which can be the determining factor to whether a plant will be able to carry out their reproductive cycle. A great way to get the air circulation that you need in your greenhouse is to use horizontal air flow fans. They are installed in a greenhouse to circulate the air when the vents are not open.

It is important to monitor all of these factors year round, but there are also factors that need to be monitored seasonally.

  • Winter Season: During the winter months, heaters will be needed to maintain the environmental conditions inside the greenhouse conducive for plant growth. Ventilation systems still need to be on when heating systems are running at full capacity. Fresh air must be ventilated to remove warm moist air from the greenhouse. If the warm moist air is not removed, condensation will occur, causing humidity damage. Ventilation is also required to remove any gases of combustion that may be present as a result of leakage around the heater and duct work when a direct-fired heating system is used.
  • Summer Season: The main purpose of a ventilation system in the summer is to prevent the air temperature inside the greenhouse from rising too high above the outside air temperature. The ventilation system has to move the air directly through the plants and over the soil to prevent excessive temperature buildups around the plants. No matter how effective a system is, the inside air temperature of a greenhouse will never be as low as the outside air temperature in the summer. If you want to lower the inside air temperature below the outside, then an evaporative cooling system must be used.
  • Spring/Fall Seasons: The spring and fall seasons have sunny and warm days as well as cloudy and cool days. Because of this, there are no special recommendations for your ventilation system. Temperature and humidity will be the determining factor for the amount of ventilation needed.

Customers might not notice if your system is not clean or running smoothly, but they will notice if your plants do not look healthy. It is a great time to check and make sure that your ventilation system is working properly to ensure healthy growth for any season.

Sources:

Greenhouse Ventilation:
http://www.clemson.edu/extension/rowcrops/tobacco/crop_management/greenhouse_ventilation.html

The Basics of Greenhouse Ventilation:
http://www.gardenandgreenhouse.net/index.php?option=com_content&view=article&id=1398:the-basics-of-greenhouse-ventilation&catid=121:july-august-2012&Itemid=18

Greenhouse Ventilation:
http://edis.ifas.ufl.edu/ae030