This is the sixth of a multi-part series on plant growth and liming factors (previous
articles can be found at the htgsupply.com website ). This month I will discuss carbon and ventilation
and next month will conclude the limiting factors series with temperature and water.
As you know, the most important thing when considering plant growth is understanding limiting
factors. Plant growth is determined by limiting factors, plants that don’t receive enough of any one (1)
factor won’t grow at the maximum rate no matter how much of any other factor you give them. To grow
better, find your limiting factor which will increase growth, sometimes dramatically, without having to
adjust or do anything else. There are several major factors that can limit plant growth, in order of
importance they are, light, water, temperature, carbon dioxide and nutrients.
1. Carbon dioxide uptake
2. Airflow to increase CO2
3. Other affects of airflow on plants
4. Carbon dioxide supplementation indoors
5. How much CO2 to add to your grow room
Carbon dioxide uptake – Concerning plant growth, everyone thinks about light and the amount of
water, but an invisible, colorless odorless gas is often overlooked, even though it is of the utmost
importance to good growth, and is so easy to add to an indoor garden. From previous articles you know
carbon dioxide (CO2) is needed for plants to conduct photosynthesis.
** Light Energy + 6CO2 + 6H20 ---> C6H1206 + 6 02 **
Carbon dioxide diffuses (moves) into the plant via tiny holes/pores called stomata. Diffusion is a passive
process meaning the plants can only get as much CO2 as there is in their immediate environment.
Stomata must be open for CO2 to get into the plant. Stomata open and close based on plant physiology
and in response to environmental factors. If something keeps the stomata closed, the rate of
photosynthesis will slow and eventually stop as CO2 becomes limiting. Indoor growers should know that
dust or other particulate matter may clog stomata. It might be a good idea to occasionally (monthly or
more if there is smoke or dust in the room) mist your plants with water on the top and bottom of the
leaves (especially the bottom of leaves where in many plants the majority of stomata are located). Use
enough mist so that the water beads up and runs off the leaves. The mist, like rain will wash off the leaf
surface and clean out the stomata. This should be done during vegetative growth. If you are producing
fruit/flowers that are susceptible to mold, you should not mist the plants once flowers begin to develop.
High humidity and misting plants can lead to powdery mildew or other fungal infections. You should
mist the plants in the beginning of the light cycle so that the light will help dry the plant leaves. I am
sure you know water and electricity don’t mix. BE SURE NOT TO GET ANY MIST OR WATER ON
AN H.I.D. LIGHT BULB (ie: MH Metal Halide or HPS High Pressure Sodium bulbs) as it may cause
the bulb to crack or break.
Although you can’t see individual stomata with the naked eye, most plants, when you look closely have
tiny “hairs” on their stems and/or leaves. These protuberances are to trap air around the plant and create
a microenvironment trapped air is usually warmer and has more humidity than the surrounding air. In
the natural environment this microenvironment is replenished periodically with wind. Without air
movement as the plant uses up the CO2 from this microenvironment CO2 can easily become limited. If
the air around the plant is not moving, the growth rate will slow with limited CO2. I recommend using
some type of circulation fan to move air around the plants (see: Circulation Fans ).
Airflow to increase CO2 – Carbon dioxide availability can first be increased indoors via air flow, so
buy a fan before a CO2 tank. Adding light airflow will increase CO2 input and may increase H2O output
so hydroponics growers should pay extra attention to the reservoir when adding a fan to the grow area.
Experienced growers know that CO2 can still be limiting even with a fan. Increasing the amount of CO2
with a CO2 release system can result in big yields when used properly (see below)
If you use an HID like many do, you should move the air around the bulb and plants with a fan and/or
ventilate your grow area to remove heat. All the greenhouses I have worked in have had ventilation or
air exchanges with the outside. Ventilation was needed to remove the heat and moisture coming from
the lights and plants (See temperature 10/08 ). Since ventilation can supply fresh CO2 it seems like a
good idea, and it can be. However, keep in mind that if you bring in air from the outside, or even pump
hot air outside, you risk getting pests into your garden, which can bring disaster. (look for future articles
on pests). You should always screen air intake and exhaust if you are venting to the outside. I use a
carbon filter fan combo ( see: Carbon Filters ) to move the air around the room; I direct the outflow air
via flexible duct across the bulb and the top of the plants. Doing this keeps the plants safe from the heat
of my HID bulb, increases the CO2 via airflow and also keeps mold spore numbers down. A carbon
filter could also greatly reduce pollen, so if you are growing fruits which need pollination, or do not
want pollen in the room, keep that in mind.
Concerning airflow, like any factor you need to keep in mind that too much of a good thing, is not good.
You can over stress your plants with wind. Your plants should jiggle in the wind, not be blown over.
You can also limit the time the wind (fan) is affecting the plants with a timer. Using a fan to simulate a
slight breeze will help increase CO2 and O2 exchange, which will increase your plants photosynthesis
rate and yield. I’ve seen some plants placed too close to circulation fans that have suffered “wind burn”
whereas the leaves of the plants were damaged by the excessive air flow.
Other affects of airflow on plants – Plants grown outdoors develop thick sturdy stems due to
environmental stress. Wind causes tiny tears on the stem which are repaired, making the stem stronger.
It is the same basic idea behind lifting weights to build bigger muscles. Indoor plants benefit from a lack
of most environmental stresses, but not having to cope with wind stress results in weak stems.
Thigmomorphogenesis (10 cent word eh!) is the term for a stress response to mechanical stimulation
that causes most plants to have increased radial stem growth (thicker stems). This mechanical stress can
be shaking the plants, or simply having a fan gently blow on the plants. This stress promotes thicker
stems and shortened plants. In the wild, being shorter and thick stemmed makes the plants sturdier so
they can better stand up to wind. Thus, Thigmomorphogenesis has a beneficial effect, it results in shorter
thick stemmed plants, the opposite effect of HPS (yellow/red) lights and high temperatures. I always put
a small circulation fan on young seedlings to promote sturdy stems. This helps them hold up the big
flowers later in life.
Carbon dioxide supplementation indoors – CO2 is often limiting before nutrients. Almost everyone
spends thoughts and money on nutrients, consider a CO2 tank. (see: CO2 Systems ) If you ventilate your
grow area, only vent your room if your CO2 system is off and make sure you let the plants sit in the CO2
for a while. There are many different environmental controllers that turn CO2 on or off with the
ventilation system and some that even measure in CO2 the air and regulate the release of CO2. (see:
Environmental Controls). In larger areas, you could use a carbon filter to keep down the fungal spores
and bacteria, and you might not need to ventilate the area resulting in a self contained high CO2 room.
One concern would be the build up of humidity so even rooms with CO2 may need ventilated to some
Normal atmospheric CO2 levels (in the air) are between 300 to 500 ppm (parts per million), depending
on where you live (urban or rural) with an average of 387 ppm. The earth’s atmosphere contained more
CO2 when plants evolved, and plants have not lost their ability to process CO2 at higher rates. In other
words, plants with ample light and water will grow faster with higher CO2 in the atmosphere. The
amount of CO2 in the atmosphere for maximum plant growth is around 1500-2000 ppm. You should add
additional CO2 to increase your growing areas PPM to these levels. CO2 is lighter than air, and is cooler,
so it should always be released above the plants. If you let CO2 go at the soil level, it may just seep out
of your room and be unavailable to your plants. The best way to distribute CO2 is directly to each plant
with a hose. With a tank and multiple hoses you can have a hose positioned above each individual plant.
If you have one hose, you can string it above the plants and put small holes in the tubing so the CO2
seeps out over all the plants. Keep in mind if you make big holes all the CO2 will be exhausted before it
gets to the end of the tube. An easier but LESS EFFICIENT possibility is that you can put a single hose
behind a fan and spread the CO2 over the area the fan blows. If you do this, you should seal your grow
area with caulk in the corners and weather stripping around the doors. The faster the CO2 escapes the
grow area the less effective it will be.
I have heard of a lot of kooky ways to add CO2 to your grow room, from composting in the grow area,
to brewing beer (using yeast) in the grow room to exhausting a gas dryer into a grow room. I understand
the desire to use organic or recycled CO2 and honestly any increase in CO2 should increase you plant
growth rate. There are products on the market which natural gas or propane in order to release CO2 (See:
Environmental Controls ). You could even use a gas or kerosene space heater in your grow area if heat is
not a problem. Please be careful to not create any fire hazards by doing so. Burning fossil fuels does
release CO2 and water vapor as part of the combustion process. On average, one pound of fuel will
produce about 3 pounds of CO2 and 1.5 pounds of water. Good heaters should not produce any residue
or carbon monoxide (CO) which is a deadly gas. It is the CO in car exhaust that kills people. If you use a
fossil fuel heater to add CO2 you should vent the room before you go in to be safe. There are other
natural ways to produce CO2 in your garden as well. While I have not used products like CO2 boost
(which basically brews sugar and yeast to produce CO2) I believe they will add CO2 but I do not know
how long the shelf life is when the CO2 will be generated. Any increase in CO2 can be beneficial, but
you need to add it above the plants, most efficiently with a hose. I would suggest you use a CO2 tank
(see: CO2 Regulator And Tank Combo ) with a regulator and release the proper amount into your room
at the proper time. I think that for ease, safety and efficiency having a CO2 tank with a regulator is
the way to go.
How much CO2 to add to your grow room – When you advance to using a CO2 tank with timer and
valve, you can regulate the amount of CO2 that you have in your grow room. If you increase the amount
of CO2 to 0.2% of your rooms atmosphere, that is 2000 ppm. The first thing you need to do is figure out
how much CO2 you need to release to bring your grow room to 2000 ppm. To do this determine the
volume of your room (length x width x height). You then multiply your room volume by 0.002 (that is
0.2%: if you want 1500 ppm that is 0.0015% multiply your room volume by 0.0015). For example, if
your room is 5 feet by 5 feet by 8 feet, your volume is 175 cubic feet (5x5x8=175) and 175 x 0.002 is
0.35. You need to release 0.35 cubic feet of CO2 into the room. Step two, in order to figure out how long
the valve should be open the number of cubic feet of gas required (0.35) is divided by the flow rate. If
the flow rate is 10 cubic feet per hour divide 0.35 by 10 = 0.035 hours or (0.035x60min/hour) 2.1
minutes an hour (two minutes) That is not a lot of CO2 for a potential doubling of yields. Many timers
don’t regulate below 5 minutes (see: Timers ) but you could start off having the valve open for 5 min.
every hour and then increase or decrease the amount of CO2 until you see no more increases or reduced
growth. As you know by now, giving your plants too much CO2 will not increase growth. You should
keep the room sealed as best as possible when the plants are being given CO2. Carbon Dioxide is not
toxic to plants, so if you can’t be exact, err on the side of too much. In humans, symptoms of high or
prolonged exposure to carbon dioxide include headache, increased heart rate, dizziness, fatigue, rapid
breathing, visual and hearing dysfunctions. Exposure to higher levels may cause unconsciousness or
death within minutes of exposure. Sound scary? High levels for humans are above 2%, while what I am
describing for your plants, a large increase in yields with CO2 at 0.2%, is not even close. Carbon dioxide
poses no real danger to you.
So, buy that CO2 tank and regulator or generator and send me an E-mail at:
firstname.lastname@example.org I enjoy hearing about your experiences.