When growing indoors one must, for all intents and purposes, provide the essentials for plant life. When taking a brief survey most will pay heed to providing proper nutrition, Carbon Dioxide (CO₂) and light, the basis for photosynthesis and consequently plant growth.

Growrooms are always a development in progress and as such many indoor gardeners have pondered what new piece of equipment or additive to experiment with next.   Often an equipment  upgrade or additional HID lamp will head the list.  Many times it is the wrong choice.   Man is often guilty of thinking, 'more is better.'  In this case, any of the aforementioned should not hold consideration if proper attention has not been given to the garden environment.  Yes, it is the exhaust fan that is one of the most essential and most often ignored pieces of equipment within the growroom.    

Air movement, through exhaust, can help maintain ideal temperature, humidity and CO₂ levels in the growroom.  There are a number of problems that can easily be prevented by taking control of temperature and humidity ranges indoors.   Air movement has a direct effect on a number of plant processes.  For instance the effect temperature can have on transpiration; a process that is shut down when temperature is excessive or causes condensation when temperatures reach too low a value.   Complete control can be a formidable accomplishment during the winter months,  however, any effort put toward  the cause will be rewarded handsomely. 

Most indoor gardens are often set up with regard to the winter season, with the air conditions outdoors being drier and cooler than those in the summer.    In the winter, for the grower, this is a valuable resource, free of charge.  Well, almost. In fact, all that is needed is a quality exhaust fan that is able to remove the volume of air in the room within three to five minutes.  This may have to be accompanied by an additional intake fan,  

depending on the conditions, the number of lights involved or the amount of heat created.

A fan will easily facilitate the removal of hot, humid air inside a growroom replacing it with cool, dry air.   

In the summer months the outdoor conditions are reversed, making temperature control infinitely more difficult.    Intake air will likely be as hot and humid as that from the outdoors.  Many times a gardener can escape this by having the lights on in the middle of the night, taking advantage of cooler temperatures.    But the relentless summer heat of the Northern hemisphere will usually catch up with them in the end.   This hot, humid air can have a devastating effect on indoor plants.  This problem can be exacerbated by the increase in temperature from having several lights. 

In the summer it often becomes imperative to control the heat created more effectively.   Many times an air-cooled reflector with a separate exhaust fan is the answer.  This will remove the heat from the bulb before it is able to increase the temperature of the garden area.  An additional benefit of this method is the ability to bring lights closer to 

the plants increasing the total number of lumens available. 

At other times, air-cooled systems are not enough, and it becomes necessary to introduce an air conditioner or heat exchanger.   These options come at a significant cost.  However, these costs can be deemed inconsequential when considering the amount of grief that can be prevented by buffering your indoor gardens from high temperatures and humidity, conditions that can have a detrimental effect on the plant's ability to function. 

There are a number of reasons how the plant is affected by the gardener's ability to remove air effectively.   Chief amongst them would have to be the effect it has on CO_2.  Not only in relation to the amount available within the environment, but also to both the amount that can be taken into the plant and the rate at which it is processed.   

It is common knowledge that CO₂ and light must be present in order for plants to photosynthesize, the process it uses to create energy.  It is a naturally occurring compound in the air, around 300 ppm.   However, with adequate lighting a garden can easily consume the CO₂ available indoors within a few hours.  By controlling temperature  the CO₂ depleted air is removed and cool, carbon dioxide rich air is added.           

When considering how CO₂ uptake is effected by temperature, a brief  examination of the leaf structure is necessary.   CO₂ is taken in through millions off microscopic openings located on the undersides of the leaves known as stomata.   It is here that carbon dioxide is absorbed by the plant and taken within the interior of the plant in order to be combined with chloroplast and water to form Adenosine Triphosphate (ATP) the major source of usable chemical energy in metabolism.  ATP is a compound that can be transported and broken down to be used for energy needed for development.         

In respect to the stomata humidity and temperature ranges are of great consequence,  but it is the latter that is of a primary concern.   Just, as it can speed up the metabolic rate in animals, so too can it affect plants. Air temperatures within the range of 65-80º Fahrenheit are usually good parameters to seek within an indoor garden.   The upper daytime limit can be raised to 85ºF or more when CO₂ is supplemented.  In fact, the processing of CO₂ is directly affected by temperature.  Some experiments have shown a rise of 20-30ºF can increase the rate of photosynthesis dramatically by increasing the speed at which carbon is taken from the CO₂, thus increasing the amount of energy available.  Of course this relationship is not infinite!  A limit is reached, not too far above the 90º F mark.    Once core leaf temperature rises to this point, the stomata will close in order to curtail excessive transpiration.   This effectively starves the plant of CO₂ consequentially having a disastrous effect on yield.   

Temperature in many respects can be seen as linchpin.  If kept within range transpiration will occur keeping stomata open, which will allow the plant to absorb the much needed CO₂.   

When considering transpiration CO₂ is not the only concern.  Most simply put, transpiration is the evaporation of water through the plant.  Water is taken in through the roots because of osmotic pressure and sent up into the body of the plant, into the leaves, and in the end released through the stomata.   It is through this process that nutrients taken into the plant and sugars created through photosynthesis are cycled throughout the plant. With this process occurring throughout the day a number of gallons of water can be evaporated into a growroom having a direct effect on humidity.  Plants that are reacting to higher temperatures attempt to cool themselves through transpiration.   Hence, the temperature will increase the rate of transpiration directly affecting the humidity of the environment as well.

Most plants indoors would prefer relative humidity ranges of  40-60 % because it is within that optimum CO₂ absorption occurs.   As relative humidity grows beyond the 60% level, the stomata's ability to absorb it is retarded.  It is mentioned above why CO₂ is important to plant development, but because of the effect high humidity has on stomata, it is also a concern.

A far more serious issue arrives when moist warm air is cooled to low temperatures.  This occurs when the light(s) go into the off-cycle, eliminating the heat created by the bulb.   When the temperature is left to drop more than 10-15º F in a humid environment condensation occurs.  Basically, this temperature change affects the relative humidity or how much water the air may hold.   When the drop is too sudden, the volume of air's capacity to hold water vapour is lowered and water vapour becomes liquid ending up covering the surfaces of the garden room.  These water droplets allow a number of fungi and moulds to colonize, powdery mildew being the most common.  These reproduce by releasing spores that can spread throughout the foliage and if left unchecked will decimate the plants.  Once these populations are present, a number of different products can be used to control them.  These will, however, only limit the damage and sometimes a fresh start is what is needed.  The removal plant material and wash down with a bleaching agent may be necessary.  The best approach is to nip the problem in the bud and ensure all hot air is exhausted from the room. 

It is by moving air that one can take control over the humidity in the room.   It can be done in a number ways with various rates of efficacy.   Arguable the simplest is to purchase a humidistat and a fan or if warranted a dehumidifier, allowing for establishment of upper humidity controls.  By not allowing the humidity to build one escapes excessive condensation.  Removing this air is essential, but equally important is moving fresh air throughout the garden canopy.

The foliage of the plants' is the area where all the aspects mentioned above come into play, and so the air within must be oscillated.  By bringing in an oscillating fan or two the gardener will help to mix the air within the room, helping to create more uniform temperature and humidity.  By mixing the cooler air from outside the area of the canopy with that within will reduce the humidity around the plants keeping the stomata open.  There is additional benefit here, in that this new air is rich with carbon dioxide.

Oscillating air will also have an effect on a number of garden pests that become uncomfortable under a breeze.  There are too many varieties of pests that can reek havoc on an indoor garden to discuss in full here, however, there is space to explore one example, perhaps the most common and devastating:  The spider mite.  

This microscopic spider's metabolism is increased with temperature reducing the time it takes for them to reach sexual maturity.  When one is dealing with a population that grows exponentially, it can become beyond control in a short period of time.  To shed a little more light on it, a spider mite living in conditions around 45º F will take around 25 days to produce an egg from the time it born.  If the temperature is doubled to 90ºF the number of days will fall to about five. As well, the number of eggs that a female can lay will increase as the temperature increases.

It all comes back to the temperature/humidity issue.  That is the primary reason for moving air in any garden.   The above is no more than a brief synopsis, listing some of the benefits gained from moving air.  It is therefore imperative not to ignore the climate within your growroom even if at times it is tempting to add another light or more additives with any extra money one might have.  

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