Chapter 16

      Carbon dioxide (CO2) is a colorless, odorless gas found in the air. Under normal circumstances, including the conditions growers deal with, it is totally harmless. Each molecule consists of one part carbon and two parts oxygen. CO2 is often generated in the home. When a stove or water heater bums gas it produces water vapor and CO2.
      Plants use CO2 as a raw material during the process of photosynthesis. CO2 is quickly used up in a well lit enclosed space Until it is replaced, the process cannot continue. The availability of CO2 to the plant can be a limiting factor in photosynthesis and plant growth.
      Keeping the door or curtain of a small grow room open helps tremendously because a whole side of the grow space is exposed to external air. An open door in a large a room gives a much smaller ratio of interface, since the percentage of the perimeter serving as a vent is much smaller.
      CO2 constitutes about .03%, or 300 parts per million of air in country areas and about .035 - .04% in industrialized regions. Photosynthesis and growth could proceed at a much higher rate if the amount of CO2 available were increased to about .15% or 1500 parts per million instead of the .035 - .04% found in urban areas. Higher concentrations of CO2 can increase the growth rate up to 300%. Usually though, growers report increases of under 100%. Either way growth rate is increased significantly. When plants grow faster, it takes less time to yield a bigger crop. Once CO2 enrichment is added to the grow space, light will most likely be the limiting factor.
      The most practical method that a closet farmer has to enrich the garden with gas is a CO2 tank with a regulator. The regulators are sold by all of the high-tech garden supply companies. These devices control the number of cubic feet of gas released to the garden. CO2 gas refills are available from companies listed under the Bottled Gas or Industrial Gas sections of the Yellow Pages. The largest tanks hold 50 pounds of gas, but they weigh 170 pounds filled. A 20 pound tank is much smaller and weighs about 50 pounds filled. At room temperature there are 8.7 cubic feet in a pound of gas. Refills are inexpensive.
      CO2 enrichment reduces ventilation requirements considerably for several reasons. First, the CO2 in the air is being replenished and the plants function more efficiently at a higher temperature when CO2 is at high levels. Rather than trying to draw in CO2 from the surrounding atmosphere, the aim is now to stop the gas from dispersing into it.
      Growers figure out how much gas to use by finding the number of cubic feet (ft3) there are in the grow space (Length x Width x Height). For instance, a closet 6 feet long, 2.5 feet wide and 8.5 feet high contains 127.5 ft3. Then they multiply that number by .0015. In this case the figures look like this: 127.5 x .0015 = 191 ft3.
      One grower had a closet 3 feet by 3 feet by 10 feet. He figured that its area was 90 ft3. To find the amount of gas to inject he multiplied 90 x .0015 = .135 ft3.
      For each one hundred ft3 of space about .15 ft3 of gas is required.

1 lb. CO2= 8.7 cubic feet

      Small unventilated closet areas are sometimes set up with a constant flow of CO2 enrichment when the lights are on. Well designed ventilated rooms are re-enriched every time the ventilation stops. Unventilated rooms need a full replenishment of CO2 every one to two hours.
      A room 6 x 3 x 9 = 162 ft3. The lights are on continuously and the air is enriched with a steady flow of.25 ft3 of CO2 per hour. Six feet of gas is used per day. A 20 lb. tank holds 20 x 8.7 = 174 ft3 6 = 29 days of use per refill.
      Growers often ventilate the hot air out of the space to disperse heat. They found that it does not do much for the plants to run the CO2 enrichment system and the ventilation system at the same time, since the gas is drawn out. Instead, the CO2 unit goes on after the ventilation system has stopped and quickly re-innoculates the area with CO2. Some high-tech garden companies sell devices designed to regulate the systems automatically.
      CO2 is heavier than air, and when it comes out of the tank it is being depressurized, which makes it cold. Subsequently, the gas sinks as it enters the space. In gardens with little internal ventilation the tubing is usually suspended just over the tops of plants. In large spaces the gas is sometimes dispersed using laser drilled irrigation tubing or released in front of the internal fans.
      Exhaust gas emitted from a stove or water heater is suitable for the garden. A garden in a room with a water heater will be enriched every time the burners light. Of course, anytime a person works with natural or LP gas or with fire, they must be very careful.

A CO2 tank and regulator is used to enrich the air. CO2 laden air increases the growth rate phenomenally.

Step By Step

      Plants do best in indoor gardens when they are supplied with CO2. Growers usually choose:

  1. An open door or curtain is often the best solution for small spaces which have a large surface-to-air ratio.

  2. External ventilation to blow out the used air and draw in new air. This is usually adequate for small rooms.

  3. A CO2 enrichment system. This consists of a tank and regulator flow meter and either a timer or other automatic valve. This increases the growth rate of the plants phenomenally.

  4. A water heater or gas stove may supplement the garden with CO2.


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