Article 3-4 Growing Environment

by Erik Biksa

Often, the primary focus of indoor garden enthusiasts is on crop nutrition. This plays a vital role in the results achieved, however, nutrition also includes the growing environment. Carbon makes up the largest portion of the dry weight of a plant, yet typically, is not taken up by the roots, but supplied in the air. Temperature plays an important role in determining plant functions (such as evapotranspiration,) and life cycles (hormonal concentration and balances within the plant). Humidity can dictate how fast a plant will take up nutrients and water. Further yet, the above mentioned also play a role in the life of beneficial and detrimental organisms in the growing environment, which in turn, influence overall plant characteristics such as yield and quality.

Controlling the indoor environment is a sure way to increase the results of your next crop. It may also help you to reduce or avoid the use of pesticides and fungicides on your favorite crop. This will cost you less money and allow you to breath a little easier. For optimum results you must meet and manage the following environmental factors:

  1. Temperature
  2. Humidity
  3. Carbon Dioxide (CO2)

Others might include moisture, light, nutrients, etc. However it is not within the scope of this article to discuss all areas influencing plant growth, rather, to offer some suggestions for the management of the air volume in the growing area.

Ventilation is a general term for controlling the air quality in the growing environment. Fans are a most valuable tool, and can be better put to use with understanding the different types of fans available to the grower and where each type is best suited. The most critical fan in the growing area is often the exhaust fan. It is capable of removing heat and moisture from the grow room, as both accumulate rapidly in indoor growing situations. Indoor growers typically require an exhaust fan capable of exchanging the volume of air in the room within five minutes. A 10 wide X 10′ long X 10′ high has 1000 cubic feet of volume (10x10x10=1000). So, a 200 cfm exhaust fan is required (1000 cu ft / 5 min = 200 cfm). This is the ideal situation. In reality, there are often several lights in this same room, and due to remote locations, static pressure from extensive ducting, and the incredible amount of heat generated, greater capacity is required. Greenhouse growers often multiply their square footage by eight to determine fan capacity for summer cooling, a requirement similar to indoor growing conditions. For example, a 10′ wide X 10′ long grow room would require 800 cfm fan capacity (10 X 10 X 8=800).

After determining your cfm requirements (cubic feet per minute of air flow), you will need to determine your intake requirements. As a rule of thumb one square foot of opening to the area is required for every 700 cfm of exhaust output. Otherwise static pressure (the room sucking into itself), will severely limit the exhaust output. It is not often feasible for the indoor gardener to have openings for intake air in the grow area, so intake fans are required, and greater attention must be paid to the type of exhaust fan chosen.

Inline, centrifugal, fans and shaded pole blowers (squirrel cage fans) are best suited for indoor applications due to some of the reasons mentioned above. Do not confuse inline, centrifugal fans with duct booster fans. The above example called for an 800 cfm exhaust fan. An “Elicent AXC 315″ 12” inline centrifugal fan or “Grotek Hurricane Series GHI 12800” will help to successfully ventilate the grow area. At 0.5 SP (static pressure) the exhaust fan has an output of 677 cfm (from an initial 800-865 cfm at 0 SP). A “Dayton 815 cfm shaded pole blower” will perform at about 537 cfm at 0.5 SP, as would a “Grotek 800 cfm shaded pole blower”. If no opening or intake fan was also installed these fans might only be capable of an output less than 25% of their initial cfm ratings (at 0 SP). Axial fans, muffin fans, duct booster fans, etc, are not well suited to exhausting grow rooms. However, they can be used to exhaust smaller grow chambers, cabinets, etc, providing adequate intake opening or fans are also supplied. Axial fans and muffin fans are wonderful for spot cooling H.I.D. lighting or used in conjunction with air-cooled lamp shades.

Air intake can be supplied through motorized shutter openings, which are not always practical in indoor situations, as light would escape the grow room. Air for intake to the grow room is best drawn in from outside, if no central air conditioning or heat exchanger is available. Ideally, all intake air should pass through an activated charcoal filter to remove spores, insects, etc. Keep in mind that any bend or item such as filter in the ducting will restrict airflow, thereby increasing fan capacity requirements. For our example, a fan of equal or greater capacity to the exhaust fan is required to deliver air so that exhaust output is not restricted. This is the ideal situation, however by using an exhaust fan with higher output than required, the intake fan need not be as large because static pressure has been accounted for. Ventilation requirements can be reduced with modifications to the growing environment such as air and water-cooled lights. In this regard, most of the heat generated by the H.I.D. lighting doesn’t make it’s way into the grow room, as it is directly removed versus, being removed from the grow room as it is being dissipated. They also reduce “hotspots” in the grow room, allow for closer light to plant tolerances, and more efficient use of CO2 enrichment, all of which will increase results.

The exhaust cycle can be determined by use of timers (mechanical or digital), cooling thermostats, and de-humidistat. Some growers have their fans wired to all three. Somebody with a little experience can integrate the three via relays. Commercially manufactured units are also available. For example, calculations have determined that the CO2 burner needs to be on for 15 minutes every hour to supply 1500ppm of supplemental CO2 in the growing area. We don’t want to run that all at once, but disperse the amount more gradually over a period of one hour. When the lights are on, the CO2 timer is on, so once every 20 minutes, the generator cycles for five minutes, providing for 15 minutes of combustion every hour. The cooling thermostat is set at 80 degrees, and the de-humidistat at 60%. If during the one-hour period conditions rise above these set points, the fan will cycle until they are below. If neither of these set points are exceeded during the one hour period, a cycle timer will turn the exhaust and intake fan on for five minutes every hour. This type of control is difficult to attain without using central air conditioning, or heat exchangers. Air-cooled lighting is just shy of necessity with this type of set up. Often the temperature rises rapidly from the H.I.D. lighting, and even further with CO2 burners, activating the exhaust fan and intake while the CO2 burner is on. If the price of propane or natural gas is not an issue, your plants will still benefit from some of the additional CO2 traveling over the leaf’s surface as the air is exhausted. Commercially manufactured units, and advanced “hydro-electricians” can allow the cooling settings to override the CO2 burner, keeping it off anytime the air in the room is being exchanged.

If you are able to successfully achieve this level of control in the growing environment, you will be able to use temperature to manipulate plant responses to the growing environment to your favor. To keep shorter plants with tighter internodal spacing you can experiment with maintaining warmer night temperatures than day temperatures during vegetative growth and up to the first two weeks of flowering. Having warmer temperatures during the day and cooler nights is most common with indoor grow rooms. This increases cell elongation, for more loosely branched and farther internode spacing on the plants. Some varieties are more sensitive than others, and cooler nights can bring early flowering even with longer days. By maintaining the room 20 degrees cooler during dark cycles for the last two weeks of flowering, you can help to speed up ripening. Take care, as humidity will also increase in with a sharp drop in night temperatures which can lead to blights and mildew on densely packed flowers. Plants that are still actively growing, or when cut for processing will convert stored starch to sugar in warmer temperatures, and convert stored sugar to starch in cooler temperatures. This type of growth manipulation has been coined “D.I.F.”, as the differential in day and night temperature is influencing plant characteristics.

Humidity can also play a role in managing plant growth. Growers know that unrooted cuttings require high levels of humidity to prevent dehydration. Plants absorb water through their roots. The water then travels through the plant through tiny vein like structures called xylem, then travels through the leaves and exits as a vapor through tiny pore like structures on the leaves called stomata (also used for CO2 assimilation). The rate of water intake through the roots versus release rate through the stomata determines turgor pressure in the plant. This is what gives a plant it’s rigidity. If the plant has no roots the humidity around the plant must be higher so that water uptake requirements are low due to reduced evapotranspiration rates. In more mature, actively growing plants lowered humidity will result in an increased uptake of water and nutrients. This rate is further increased with CO2 enrichment. For un-rooted cuttings, humidity levels of 85%-95% are optimal. Different strains will prefer different levels. Desert plants tend to be shorter and more tightly branched with broader leaves and will prefer lower humidity levels in the area of 40%. If kept at higher humidity levels they are more susceptible to molds. Tropical and equatorial varieties will thrive in slightly higher levels. These more laxly branched plants will perform well in ranges of 55-75% humidity. Theses plants tend to have a longer life cycle, perhaps this is an adaptation to their native higher humidity levels.

Carbon makes up the majority of a plants dry weight. It is absorbed through tiny openings in the leaves called stomata from the atmosphere in the form of CO2. Carbon dioxide is as essential to plant growth as the air we breathe is to us. Many plants will benefit from raised levels of carbon dioxide in the grow room. It is especially beneficial for newly rooted cuttings and vegetative growth. Some growers use CO2 enrichment throughout all stages of plant growth. Carbon dioxide can impede maturation in ripening plants, and can result in diminished production of terpines (what gives the plant it’s flavor and smells) and other essential oils. Typically fresh air has about 350 ppm (parts per million) of carbon dioxide in the air. Increased levels from 1000-3000ppm will generally increase yields by 20-30% and take a couple of weeks off a three month growing cycle. For smaller areas bottled CO2 is best. In fact if it were economical, it is the best choice even for larger areas. Bottled CO2 will sink fast to the floor when released, it must be circulated through the plant canopy via carefully positioned oscillating fans to keep it from sinking. CO2 as a product of the combustion of fossil fuels such as propane and natural gas will increase the temperature and humidity in the grow room if not past through cooling coils (such as those in an air conditioners). Because the warm moisture released with the carbon dioxide, it will travel towards the ceiling of larger grow rooms. If vertical clearance permits, ceiling fans can be installed to direct it towards the crop. Most plants only benefit from increased carbon dioxide levels during the light cycle. Orchids are an exception. The least expensive way to control CO2 enrichment is by using timers. A “COMPUGAS” device is available from some hydroponic suppliers. The grower simply types in the dimensions of the grow room and plugs in the solenoid from the flow meter and fans into the unit. Other wise you have to make some calculations of your own.

In a 10′ X 10′ X 10′ area there is 1000 cubic feet. One cubic foot of CO2 in this room should provide an additional 1000 ppm of Carbon dioxide, two cubic feet would supply 2000ppm, etc. If you wanted an additional 2000ppm, and set the flow meter to 20 cubic feet per hour, the tank would have to run for six minutes every hour. It would be best to release it four times for one and a half minutes every hour. An Intermatic C8865 one hour cycle timer can be plugged in only to run when the lights are on. It can be wired so that when the exhaust and intake fans and CO2 regulator are plugged in the fans will run on the “off” period for the tank, and will shut-off while the CO2 is activated, all running on the same timer. The wiring for this might take some practice though.

When using CO2 burners to produce carbon dioxide you should make your calculations based on the above numbers, but with the cubic feet per hour rating of the burner versus the flow meter. For example a CD-12 produces 12 cubic feet of carbon dioxide per hour. This would need to be on for 10-15 minutes (burner efficiency varies) to produce an additional 2000ppm of CO2 in a 10′ X 10′ X 10′ grow room. CO2 is used more efficiently when all cracks in the room have been tightly sealed.

Infrared sensors are also available (and costly) and activate solenoids from tanks or burners when the preset level drops. These can also be integrated into commercial grow room controllers.

Carbon dioxide gas can also be used to preserve dried flowers if the gas is released into mason jars before tightly closing the containers loosely packed with the product. Carbon dioxide enrichment should only be supplied when all other conditions are optimal. The plants will only grow as fast as their most limiting factor allows, so if oxygen levels at the roots are low, the plants can only grow as fast as the amount of oxygen that is made available. In contrast, many greenhouse growers will add carbon dioxide in winter months when light and temperature levels are lower and find that it increases production.

The above are just a few tips that will help you to achieve better results by better managing the growing environment for growth manipulation. This is a valuable technique that is often overlooked due to growers focusing primarily on other growth factors such as light and nutrients. It is also important to remember that if all factors are optimal and managed accordingly, the strain itself becomes the limitation. If any readers have any tips or insight in to a particular area relating to intensive plant production, please feel free to write or e-mail so that other growers might also benefit.