The most overlooked environmental factor affecting plant growth inside a growing environment or growing structure is airflow. Getting just the right degree of air movement across a leaf surface is vital to good production and yields and can mean the difference between high rates of photosynthesis occurring or none at all. Good air flow also assists temperature control, CO2 replenishment, reduces humidity and lowers the occurrence of certain diseases.
Boundary Layers and the Leaf Flutter Effect
A small amount of air movement – just enough to gently move or ‘flutter’ the leaf – has the effect of removing the stale, humid air from the boundary layer that lies just above and just below the leaf surface. This boundary layer of air supplies the leaf with CO2 and also holds much of the moisture transpired by the plant. If there isn’t any air movement, diffusion of water vapour out of the leaf and CO2 into the leaf begins to slow as the boundary layer air mixes too slowly into the rest of the environment. Air movement across the foliage creating a `flutter effect’, also assists photosynthesis and transpiration which plays a major role in calcium transportation, preventing blossom end rot and tipburn in certain plants.
One of the most effective methods of cooling a growing environment is simply by having adequate ventilation and airflow drawn in from outside and vented out again. Obviously the amount to ventilation required to maintain the ideal temperature range will depend on such factors as the temperature of the incoming air, the heat load from lights in the growing area and the amount of air drawn into and flushed out on a regular basis. Optimal growing temperatures for warm season, high light crops differ depending on the level of CO2 provided in the growing environment. Where CO2 enrichment is used to maximum levels (in excess of 800 ppm), plants are able to maintain higher rates of photosynthesis under good light conditions where the temperature is run higher than normal. Temperatures in the range 80 F (27 C) to 92 F (32 C) with CO2 enrichment are recommended where light levels are high for warm season crops. Where CO2 enrichment is not being used, or is only being applied to prevent CO2 depletion by the plants and provide ambient levels (265 ppm), temperatures should be set lower to prevent ‘plant stress’ which can occur when conditions become too warm and stomata shut down to prevent excessive water loss. Temperatures for non enriched crops with good light levels are best kept in the range 75 F (24 C) to 85 F (29 C) for warm season crops. Night temperatures, when no CO2 enrichment should be carried out are best run at lower levels – this assists the plant to restore turgor pressure with an increased uptake of water at night. Temperatures in the range 65 F (18 C) to 75 F (24 C) are ideal for night or `non lighting’ periods.
Humidity and Disease Control
High humidity levels can become a major problem where plants with large leaf areas in a warm but restricted growing environment are continually transpiring and releasing water vapour into the air. Ideal humidity levels for flowering plants are in the range 30 – 50%, however the higher the humidity level, the greater the risk of certain plant diseases such as mildew and botrytis as well as bacterial infection where moisture forms on the leaf surfaces. Rapidly transpiring plants, with no air replacement can raise humidity levels within a very short time – conditions ideal for most disease to take hold. High humidity levels also slow the rate of plant transpiration (moisture loss from the leaves). Since transpiration is essential to not only cool the leaf surface but creates a suction effect resulting in water and mineral uptake and transportation within the plant, it is essential to keep the process going.
Fans in a grow room, not only vent out humid air but bring in drier air to the growing environment and this is essential for not only good plant growth but also disease prevention. When humidity levels are high, condensation at night when it is generally cooler can become a major problem. Condensation on plant surfaces provides the perfect environment for many fungal spores and bacterial diseases to infect the plants. It takes only a few hours of high moisture levels for most diseases to infect plant tissue and take hold, so reducing humidity and preventing condensation are one way of protecting plants from disease outbreaks.
Fan Types Required
Airflow patterns should be considered in the design of any growing environment. The placement of fans, vents and air mixers needs to be arefully planned to create good air movement in through the inlet vents, over and under the plants and out again. For odd problem areas where still, moist air is collecting, small mixer fans can be installed.
Intake and Exhaust or Vent Fans
There are two types of fans commonly used in growing areas – intake and exhaust or vent fans. Intake fans pull air into the growing area, exhaust fans push it out. Exhaust or extraction fans which are positioned to extract warm moist air from the crop are the most useful, however an intake fan which draws in sufficient fresh air with an adequate vent system to allow stale air to be vented out works well, provided the fan is large enough for the area to be vented. Whatever sort of fan is being used to vent out and draw in fresh air, inside the growing area, air needs to be mixed and circulated over the plant surfaces.
Osculating Wall Mounted and Pedestal Fans
Circulation or mixer fans, which may be wall mounted, pedestal, or osculating types carry out the essential function of mixing the cooler, drier fresh air being brought in, as well as any CO2 enrichment to create a uniform temperature and prevent cold drafts from stressing the plants. These fans also carry out the role of gently moving the stale, humid boundary layer of air from around the leaf surface and replacing it with fresh, CO2 enriched air which stimulates both photosynthesis and transpiration. Mixer fans can be wall mounted to save space, but need to be carefully positioned and angled to get the greatest mixing and air movement effect. Stand up fans and osculating fans also need to be positioned with air movement in mind – and if any area of stagnant air (perhaps areas where fungal disease seem common) is found, small fans can be positioned to deal with these problems. The main objective is to not only get air circulating and mixing in the lower levels of the crop to reduce humidity and disease problems, but also over the tops of the plants where the most light is falling and maximum rates of photosynthesis are occurring. Spot checks on CO2 levels, temperature and humidity around the growing area and in the crop will help discover where air flow is not occurring sufficiently.
Ideally, fans should be linked to a thermostat – triggering increased air flow and ventilation when temperatures start becoming to warm, and the CO2 enrichment system if one is used. CO2 injectors which are designed to enrich fresh new air with CO2 as the inlet fan comes on are one way of making sure high levels of CO2 are always present when the lights are on. Fans should also be triggered to vent out warm, humid air and high CO2 levels just after the lights switch off at night. High levels of CO2 are not required at night when the plants are respiring and need to use oxygen only from the air. Condensation, can be a problem at night when temperatures cool and humidity in the air result in water forming on plants and other surfaces. Getting good air replacement or air changes in the first couple of hours after lights go off is one way of preventing diseases such as mildew and botrytis whose spores need very high humidity or free water on the leaf surfaces to germinate and infect the plants. If drier, fresh air is continually brought in so humidity is lowered and condensation does not form, then fungal and bacterial pathogens can’t attack the plants.
Fans can also be linked to a thermostat and dehumidistat controller, which does the same as thermostat but adds on dehumidistat. Pre-set to desired humitity level. When grow room becomes to humit, the exhaust fan will turn on and suck the wet air out until preset levels are reached. By having control, our fan is not needed to be on full time without the expense of continual running. This type of system is ideal where the outside air temperature is cool and needs to be rapidly mixed and warmed when it enters the growing area. Where cool outside air temperatures exist, which might be many degrees below what is being maintained in the growing environment, continual air changes will result in sudden and continual drop in temperature resulting in ‘thermal stress’ on the plants.
Getting the size of the intake and exhaust fans right for the growing area is important for plant growth and development and disease prevention. The best set up is a system of two ‘vents’ – an intake vent set relatively low down at one end or corner of the growing area, with the exhaust or extractor fan set higher up at the opposite end of the room. The idea behind this is that cool, drier air sucked in from outside will flow up, through and over the crop (assisted by mixer fans in the room), and warmer, moist air which rises will be extracted by the fan at the other end.
The first step in working out the size of fan(s) required is to calculate the amount of air in the growing area. This is done by multiplying the length x the width of the room x the height of the room; this will give a value in cubic feet. For example, a 12 foot by 12 foot room with a height of 8 foot:
12 x12 x 8 = 1152 cubic feet of air inside the growing area.
Ventilation fans are rated in the number of cubic feet of air they can move per minute.
Work out how fast one complete ‘air change’ needs to be carried out under warm conditions (i.e the maximum you will ever need the fan to operate).
If excess heat in a certain growing environment is a common problem, or there is a large volume of plants growing in a very restricted space you will need more air flow per hour than for a larger growing area which doesn’t suffer from too much heat build up with smaller plants.
Growers commonly underestimate just how much ‘air exchange’ is required to remove excess heat and humidity, bring in fresh CO2 and generally create fresh air movement over all of the plant surfaces. As a comparison to greenhouse crops growing in full sunlight – one air change per minute or 60 air changes an hour are often aimed for with large, mature crops growing under warm, humid conditions. However, in a grow room situation, one complete air change obtained in 4-5 minutes is acceptable. Obviously this needs to be more frequent (one complete air change in 2-3 minutes) where lighting is creating a lot of extra heat to be removed or when a CO2 generator is being used.
Divide the air volume of the growing area by the number of minutes required to get one full air change:
If the room is 1152 cubic feet of air divide by 4 minutes (that’s one air change can be carried out in 4 minutes)
Fan capacity required is 288 cubic feet per minute (for just one extractor fan).
Add on at least 1 medium sized mixer fan (either wall or stand mounted) for each 200 cubic feet of air, make sure these are equally spaced in the growing area. More smaller fans will be beneficial to increase air flow up and under plants in any ‘stale air pockets’ which may be prone to fungal or bacterial disease attack.
While its relatively simple to work out the size of fan required for a certain size of growing area, other factors should be taken into account. If insect screens are installed over inlets, this reduces the rate at which air can be drawn in and both inlet size and fan size need to take this into account. If the inlets or outlets are not directly drawing in from or venting to the outside, but using long ducts, then a larger capacity fan or correct type of ‘ducting fan’ will be required, the size of which will depend largely on the distance air has to be pulled or pushed from outside.
Air movement with the correct sized fan, well placed mixer fans to displace stale boundary layer air around leaf surfaces and fan controllers to get maximum climate control are vital to the success of any indoor crop. Air movement is often over looked, but an essential part of maintaining optimal growth conditions by modifying temperature, humidity and CO2 levels at the leaf surface where the important plant process of photosynthesis and transpiration are occurring. Getting fan size and air movement calculations right means plants have the best conditions for growth, development and supreme yields.