Keeping your cool: Growroom ventilation tips and tricks

Modern commercial glasshouse climate control for production horticulture is a mammoth task; it includes the control of vents, windows, fans, heaters, shades, misting, humidity systems, carbon dioxide (CO2) dispersal and the list goes on. Recent technological advancements in machine automation involve precise computer control and regulated growing environments that are remotely monitored by external experts on a regular basis. For hobby and personal horticulturalists it can be a daunting and expensive task to set up an optimized and controlled environment but hopefully we can help to simplify.

Ventilation is as important as water, light, heat and nutrients (Cervantes, 2015). If we understand the basic requirements of our chosen crop it’s actually quite a simple and affordable process to optimise the environment. We need to recognize the required hardware capabilities and the desired ranges to select the appropriate ventilation and environmental control equipment.

The most important factor for hobby indoor or small greenhouse growers is selecting the appropriate fan(s) and vents to control airflow and temperature. Fans are predominantly used to control growroom climate by extracting the stale warmer air and by providing your plants with a source of fresh, cool air. Calculating the ideal fan size for small growrooms in the Australian climate generally states that we require a complete exchange of the air in the room every 60-120 seconds (Palmer, 2015). This provides enough air movement to remove most of the heat from HID lighting and prevent stagnant pockets of stale air forming. With stale air, stratification around the leaves is likely to slow growth and cause problems with mould and rot. The internal air must be moved via the use of natural currents or mechanically to imitate outdoor climates and environmental conditions.

Calculating the fan size for a small growroom is a fairly basic formula. It’s important to remember that we should balance the air intake and extraction to create atmospheric negative pressure. It’s important to use a larger exhaust fan than intake and the generally accepted ratio is 1:4 (Cervantes 2015). This ensures that all extracted air passes through a carbon filter for the removal of pathogens and undesirable odours. This can put some pressure on the grow tent stitching and wall lining. If the intake fan is more powerful than the exhaust, your growroom will balloon and leak odour. Most common indoor grow tents have lower mesh vents that lightly filter the inward airflow and these importantly reduce the strain on the tent if an appropriate intake fan is not used.

To determine the cubic size of a growroom, we must calculate the length x width x height of the area. For example, if you have a 1m x 1m x 2m tent – the cubic volume is 2m3. Once we have established the volume of the room we add 25% to factor in the reduction from ducting, friction and filters (Palmer, 2015). For our ‘example tent’ the required air exhaust would be 2.5m3 per minute (2 + 25% of 2). To allow for a complete air exchange every 60 seconds we multiply our 2.5m2 x 60 = 165 CMH (Cubic meters per hour) The recommended corresponding intake fan would be ¼ – ½ the size of this exhaust fan. It’s better to purchase a fan with a larger CMH capacity and run the fan at a lower setting for better longevity and quieter audible fan noise.

It’s also worth remembering a useful lesson from high school, hot air rises, so our exhaust will ideally be pulling air from the top of the room and our intake ducting pulling fresh, cold air into the lower expanse of the room. A vent fan will pull air much more efficiently than it will push it; try to arrange your fan location accordingly. Additionally, indoor gardens can often take advantage of existing air-conditioning, household heating or ventilation systems but be careful of expensive operating costs for these units!

Once we have installed the exhaust and intake fans our plants should be receiving a plentiful amount of clean air. Delivering fresh air to plants ensures they will have adequate CO2 to continue plant growth. An internal air mover and intake fan can assist with providing fresh, CO2 rich airflow. Without CO2, plant growth ceases and a plant is unable to produce the sugars/fuel for growth and metabolism.

In commercial greenhouses one industry trick is to capture the CO2 generated by flue emissions from the heating boilers, then vent it back into dispersing mechanisms for the plants to use. As most home horticulturalists lack this equipment, there are a number of innovative products for small-scale CO2 dispersal using chemical reactions, decomposition or compressed gas that can all be implemented effectively. Using a CO2 tank with a regulator and solenoid valve is the most cost-effective means of improving your growroom CO2 levels during the day. Plants cannot use CO2 during the night, boosting during dark hours is a waste of money, natural resources and can be harmful to the plant.

For small indoor tent environmental control ventilation is also fundamental for temperature and humidity management. Always plan your temperature management to be able to cope with extreme high and low temperatures expected in your climate. Grow room temperatures should always be kept above 13oC and below 30oC as temperatures outside these parameters will slow/stop plant growth.

If we control sudden temperature fluctuations drastic changes in humidity are reduced, which diminishes the chances of rot and mould forming. Smart dimming switches and controllers for HID lighting are beginning to replicate the sunrise and sunset in nature with the intention of reducing drastic temperature and humidity fluctuations. If humidity is too high it will slow evapo-transpiration, reduce water movement in the plant and diminish the plant’s cooling ability (Cervantes, 2015). High humidity also encourages disease spores that can attack during the day or night!

In Australia, heat management is generally the biggest issue in small growrooms. HID lamps and ballasts radiate warm temperatures, however with smart reflector selection the heat generated by your lamp(s) can be directly exhausted outside the room. Coolcells or Cooltubes directly attach ducting and fans onto your luminaire and can be linked together for superior heat control efficiency. It’s worth investigating if you can dissipate the warm air into the roof or outside the building (through a chimney or into the walls) but always ensure you consider any unwanted fragrances and your neighbours.

When regulating the temperature and humidity, it’s important to remember that accurate monitoring is essential. Modern technology has provided digital thermometers, hygrometers, thermostats, humidistats and much more advanced sensory and regulating equipment. Ensure you are testing the temperatures and humidity readings above and below the canopy level for an accurate interpretation of your growroom climate.

The last tip for growroom environmental optimisation comes in the form of appropriate crop management and pruning techniques. De-leafing in commercial production horticulture encourages easier crop management and considerably improves air circulation around the plant. Depending on your crop, it’s recommended to prune out lower spindly branches and foliage that are not receiving much light.

Calculating ideal environmental conditions isn’t overly difficult, it just requires a little planning and preparation. Always plan for the entire duration of your crops lifespans, especially coming into summer or winter extremes of our harsh and unforgiving Australian climate.

Prepare for the worst, so you can provide the best and give your plants an immaculate environment!

Tom Forrest