Apr 13, Easy Grow Room Building Advice

I have an indoor area I would like to turn into a grow room. It is in a room that is never in use … totally dark. I am building an area inside this room 39″ wide x 49″ deep x 84″ high. One side will have a mylar access. I want to grow one tomato plant and maybe a green pepper plant in a stratum method using hydroton rocks, coconut coir and Fox Farm Organic planting mix. I will be using a 400 Watt MH light and was wanting some help with the ventilation. I’m on a limited budget.

I was hoping to use the inexpensive in-line duct furnace fans, maybe the 4″ size. Do I need two of these fans? Is 4″ size enough or do I need 6″ size? Do both the fans need to be the same size? Do I locate the exhaust fan up top and the intake fan at the bottom? The light will be on for 14 – 16 hours … Do I need the fans on 24/7? I don’t expect any odors. Will the MH light grow the tomatoes all the way to fruiting? I do have two CFL 105 Watt lights I could use instead, one is blue, one is red. Would I be better off just using them instead of the MH light?  Thanks Jason!

Answer: Personally, to light your grow room area I would use the 400 watt metal halide light. Using the CFL’s by themselves would probably not provide enough light for a bountiful harvest. Many plants, like tomatoes, require a considerable amount of light in order to produce well. A metal halide light is perfectly capable of growing tomatoes and other plants in the vegetative stage, as well as in the flowering stage, all the way to completion (with good results). For more help selecting the proper grow lights for your indoor garden, check out my light selector tool page.

Before you try to fruit your plants, you need to make sure they have been grown to a “mature” point under 18-24 hours of light. This is known as vegetative growth….some plants only need a week or two of vegetative growth before flowering, while other plants need 4-8 weeks of vegetative growth before they will flower properly. In any case, your plants need a minimum of 18 hours of light a day until they are at least 8 to 12 inches tall.

In order to get your plants to produce fruits, they will require 12 hours of in-interrupted darkness at the same time each day (or night). This is generally required for plants that produce fruits and vegetables as well as plants that produce flowers. My page on flower forcing goes into greater detail about this part of the process. In general, your grow room must remain completely dark for twelve hours each night, at the same time each night. Most plants will finish growing their fruits or flowers to maturity after 8 to 12 weeks of this treatment.

Before I get to your most important question, I would like to say a few things about the use of fans in the grow room…or in your case the grow box. The use of a circulating fan is just as important as the use of an exhaust fan in the grow box. The use of exhaust fans is important to keep the temperature in the grow box from getting to high. Exhaust fans also keep the humidity from getting to high as the plants breathe out water vapor. Thirdly, by bringing fresh air into the grow room, exhaust fans keep CO2 levels from falling too low….

Plants constantly use CO2 in the air as a source of Carbon atoms, which the plant uses to grow more leaves, stems, and flowers. A small amount of air movement is required around the surface of every leaf in the garden….this helps the plant get rid of it’s exhaled Oxygen and water vapor, and also helps bring a fresh supply of CO2 in contact with the leaf. Learn more about the proper use of CO2.

Usually, this kind of air movement is not provided for adequately enough by the exhaust fan(s). This is why, in addition to your exhaust fan(s), you should also have a small oscillating fan or two in the grow room for proper air circulation. Small oscillating fans are cheap, $10 at WalMart, and you would be surprised how important they are to the healthy growth of your plants.

Your exhaust fans do not need to run when the lights are off to control the temperature. However, I would recommend you leave one on to keep the humidity in the grow room from getting too high. Warm air holds more moisture than cool air. As soon as your lights go off for the night, you will likely close up the grow box so it is light-proof. As the temperature cools off inside the grow box, the air inside is no longer able to hold as much moisture as it did when the grow light was on.

The humidity in the air begins to condensate out of the air…..and onto your plants. If you do not have an oscillating fan and exhaust fan running, this extra moisture will probably encourage the growth of a fungus or powdery mildew that will kill the fruits/flowers you are trying so hard to grow.

So, you know you need an oscillating fan, and you know you should leave an exhaust fan on all the time. Now let’s consider how to run your exhaust fan(s) exactly….

The smaller the space you try to grow in, the more difficult the temperature will be to control. In general, 80*F or above (measured at plant height directly below the grow light) is too warm. 70 to 75*F would be ideal. With your setup, as I understand it, you will have a volume of air space INSIDE the grow box, and you will have a much larger volume of air inside the ROOM in which the box is located.

If your light reflector has glass in the bottom, you could use one in-line duct fan to draw air from the room, through the light, and exhaust it back into the room. Eventually, all of the air in the grow room will warm up….another option would be to exhaust the warm air out of a window. Air cooling the light in this way will make it easier to control the temperature inside the grow box.

The second in-line duct fan can be used to exhaust hot air from a hole in the top of the grow box (hot air rises). This will create a negative air pressure inside the grow box, which will naturally suck air from the room back into the grow box (through holes made in the bottom of the grow box, or possibly by leaving the front door of the grow box open). Keep in mind, these ventilation holes may make your grow box less than light-proof.

The chances are good that you will still have a problem controlling the high temperatures in the grow box. I have had the same problem myself in the past. You may want to try keeping the temperature of the entire room on the cool side by keeping a window in the room cracked open. Another possibility is to run a 4 inch duct from the hole in the bottom of the grow box to a window, so the air you pull into the grow box will actually be coming from outside. This can be very effective, but if the air outside is too cold you run the risk of stunting your plants.

Instead, manufacture a cardboard box with two holes (and cardboard flaps for the holes)…one hole connected to the window with a 4 inch duct, and the other hole left open to draw air from inside the room. In this way, you can open or close the holes in different amounts (how much outside air is being mixed with the inside air) to control the temperature of the air coming into your grow box. You may also find that you need to close the window every night to keep the temperature from getting too low when the lights go off.

Using the outdoor air, it is much easier to control the temperature of an indoor garden during cold months. In fact, many indoor gardeners ONLY garden indoors during the cold months for just this reason. Take it from my experience, your money is best spent buying a cheap indoor/outdoor digital thermometer/humidity meter. I found one at a local hydroponic shop for about $20.00. Indoor/outdoor thermometers come with a remote probe (usually on a 10 foot cord) which you can place right in your garden’s hot spot, which your meter records as the “outdoor” temp.

The thermometer itself can be placed somewhere else, inside or outside the grow box, and will record an “indoor” temp. The highest and lowest readings are usually kept in the memory of the thermometer until you reset it. Same with the humidity. In this way, you will be able to tell if your garden is staying in the ideal temperature range, when the lights are on during the day as well as when the lights are off at night.

Using in-line duct fans, in my opinion, you will not be able to keep your grow box or grow room in the ideal temperature range without using some cold air from outside. However, using two in-line duct fans, a digital indoor/outdoor thermometer, a little air from outside, and a little experimentation, you should have no problem working out a setup that will control the temperature in your garden. Also check out my exhaust setup page for more information. I hope this helps, and happy growing!

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