Article 4-5 Why Hydro?

As I mentioned earlier, the most important tasks facing the indoor gardener are copying and improving upon nature. Okay, now that you are a god, you have to make a sun. Poof! Are you done yet? Probably not.


There are four basic terms that are used to describe light and how it affects your plants. These terms are Lumen, Foot-Candle, Watt, and Lumens per Watt.

The term “Lumen” is the basic unit of light. If light were tangible and you could somehow grab it in your hands, the amount of light that you would be holding would be described as lumens. If you took that handful of light and stuffed it into a sandwich bag, or let it loose in a dark room, the amount of lumens would be the same. Lumens do not decrease or increase as that light you were holding condenses into a smaller space, or expands to fill a room. For that particular amount of light, regardless of the space it is filling, the amount of lumens remains constant.

The term “Foot Candle” measures the amount of light intensity, or how much light you manage to shine on a given area. The foot-candle is based on how many lumens of light you distribute on a given area, which are measured in square feet. To illustrate this, if you shone one lumen of light on one square foot of space, you have achieved one-foot candle of light. You can use this formula to measure the foot candle output of any bulb. What if you were operating a 1000 watt Metal Halide bulb over a garden which measured five feet by five feet? Well, first you need to find out the amount of lumens that your bulb produces. The packaging that your bulb came in should inform you of this. A 1000 watt metal halide bulb produces 120,000 limens. The next step you need to perform is to find out the square footage of your area. The area we are using for this example is five feet by five feet, so we know that our square footage is 25′ (Length (5′) x width (5′) = square footage (25 square feet)). Now we have to divide the amount of lumens (120,000) by the amount of square feet (25). Doing this, we come up with 4,800 foot candles. The amount of light intensity or foot candles that you produce is the most important measurement of light to a hydroponic gardener because it tells you how much light is available for your plants to “drink”.

The “Watt” is probably a term that you are already familiar with. It is a common term used to measure the amount of energy a bulb requires electrically.

“Lumens per Watt” is another term, like the Watt, which is applied strictly to artificial lighting, whereas Lumen and Foot Candle can also be applied to natural light. This term refers to how many lumens of light that a bulb can produce per watt of electrical usage. The higher the ratio, the more efficient and economical your lighting system will be. Ideally, you would like to create the desired amount of lumens with the lowest wattage cost possible.


Well, direct outside daylight in the summertime averages somewhere in the neighbourhood of 10,000 foot candles. This is equal to 10,000 lumens per square foot. If you compare this to the output of our thousand watt metal halide bulb earlier, we can see that the sun does a pretty good job.

However, the sun is subject to annoying little changes in weather that our artificial lighting is not. On an overcast day, the amount of lumens that the sun produces drops to about 1,000. If you were standing under a large, tall tree, the amount of light falling on your head is referred to as open shade. Open shade produces about 300 foot candles. In the deep shade, you would be experiencing around 50 to 100 foot candles, and under the light of a full moon you would be subjected to .02 foot candles.

So, artificial light is not subject to the interruption of inclement weather, and we can run our light for the optimum period of 12 to 18 hours per day on our plants, whereas the sun only produces six or seven hours of useable light.


There are quite a few choices of lighting for the hydroponic gardener. However, not all lights are acceptable, even though by looking at them you might think that they are bright enough.

Following is a list of different bulbs that are suitable and unsuitable for use in your hydroponic garden.


A regular household incandescent bulb is not very efficient. It only produced somewhere in the range of four lumens per watt. This means that if you were using a 100 watt incandescent bulb, you would only be producing 400 lumens. So, if we could reflect all of the light into a one square foot area, we would have achieved 400 foot candles. However, for this example, and all of those to follow, we will assume that we are only reflecting 75% of the light that our bulbs are producing. Even with reflective material and reflectors, some light is going to be “spilled” where it is not needed. So, our 100 watt incandescent bulb is only shining 300 foot candles on our one square foot area (400 foot candles multiplied by 75% = 300 F.C. (Foot Candles)).

Earlier we figured out that the sun produces 10,000 F.C. on a perfect day, whereas a single 100 watt incandescent bulb produces 300. If we hung that 100 watt bulb over the five feet by five feet garden we used in one of our earlier examples, we would see that that garden is receiving only 12 F.C. (300 foot candles divided by 25 square feet = 12 foot candles). If we were shooting for 1,000 foor candles for our garden, we would need to install 83-100 watt bulbs. Outside of the fact that your hydro bill will be higher than that of the Griswald’s around Christmas time, you just turned your grow room into an oven. Because of this, incandescent bulbs are really unsatisfactory for hydroponics.


Halogens are much more efficient than incandescent bulbs. They weigh in at about 20 lumens per watt. Halogens are available in a 1,000 watt bulb, and since we are trying to produce as much light as possible, we will use this for our example. A 1,000 watt halogen bulb, at 20 lumens per watt, will give us a total of 20,000 lumens of total light energy. As we did with our last example, we will multiply this number by 75% to adjust for out “spilled” light. Therefore, our 1,000 watt halogen bulb is producing 15,000 useable lumens. Our sample garden that we have used before measures five feet by five feet, giving us 25 square feet. When we shine 15,000 lumens on our 25 square foot area, we end up producing 600 F.C., or foot candles of light intensity. This is getting us closer to a useable light source, but it is still barely adequate. Also, a major drawback of the halogen bulb is that it produces a large amount of HEAT! This heat would cause foliar burn, and would be a welcome invitation for infestations. Because of this, we can conclude that halogen bulbs are also inadequate…leave them in your fog lamps where they belong.


Fluorescent bulbs come in varying wattages, and spectrums. Ultimately, you would like to use the highest output, fullest-spectrum bulbs that you can find. When a plant is growing, it not only requires enough light, but that light should be rich in both ends of the light spectrum. There are two definitive stages in a plant’s growth and they are the vegetative and flowering stages. The initial stage is vegetative. This is where the plant performs most of its vertical growth, and strengthens the main stalk. The flowering stage takes over as the plant begins to get “busy” and starts to produce its flowers. When a plant is in its vegetative state, it focuses its thirst mainly on the blue-violet end of the spectrum. When it is flowering it focuses on the red-orange end. Ultimately, you would want to incorporate a 40 watt full-spectrum tube. This would provide you with the greatest results.

These tubes produce 68 lumens per watt, for a total of 2750 lumens. Fluorescents are ideal for initial propagation because they produce almost no reciprocal heat. You can hang a four tube fixture six to eight inches above your plant canopy, and then just keep raising it as your plants grow, without fear of burning your leaves. In this example, we will scale down the size of our test garden. Earlier, we have been using a five feet by five feet garden as a reference point. For this example, we wil use a two feet by four feet garden. A two feet by four feet fixture can hold four bulbs. This will give us a total of 11,000 lumens (4 bulbs multiplied by 2750 individual lumens = 11,000 lumens). Allowing for “spilled” light, we are probably generating about 1031.25 F.C. (11,000 lumens multiplied by 75% = 8250 lumens…8250 lumens divided by eight square feet = 1031.25 Foot Candles). This is assuming that the lights are placed DIRECTLY over your plants. As you raise your lighting, your light intensity drastically drops. When you double the distance between your light and your plants, you cut the light intensity by four times.

As I mentioned earlier, fluorescents are ideal for initial propagation. They provide a good supply of full spectrum light for your seedlings or cuttings, without fear of burning them when they are at this fragile stage. A two feet by four feet area can support up to 400 plants. Because of this, even though fluorescents are adequate for the full life-cycle of a plant, most indoor gardeners use them as an initial propagation area, then move the plants to another area as they begin to mature.


Metal Halide Lamps are the most widely used hydroponic bulbs. They carry an efficiency range of 80-120 lumens per watt, depending on the bulb. Although the initial cost of a metal halide bulb is quite high, they are two to 20 times more efficient than any of the earlier listed bulbs because of the amount of lumens they produce per watt.

Some companies even produce super high output 1,000 watt metal halide bulbs that produce anywhere from 10 to 15% more total lumens, without costing you any more on your electrical bill.

The metal halix output is very economical, when compared to their output and the light they produce is far superior to any of the other bulbs we have looked at so far. They also have a very long lifetime. A 1,000 watt bulb will last for about 12,000 hours, whereas a 400 watt bulb will last for about 20,000 hours.

As for the cost of operating a metal halide bulb, you can find that out by contacting you local electrical company.

A “kilowatt hour” is a measurement used for the consumption of electrical energy. It refers to the cost of running 1,000 watts of electricity for one hour. Take the total wattage of the bulb or bulbs that you are using (example: 400 watts) and divide that number by 1,000 (400 divided by 1,000 = .4). Multiply this number by what your electrical company is charging you per kilowatt hour ( .4 multiplied by 5.7 cents = 2.28 cents). So, a 400 watt bulb will cost you 2.28 cents an hour to run. Plants thrive with the infusion of between 12 to 18 hours of light per day. So, if you were running a 400 watt bulb for 18 hours per day, that would cost 41 cents per day to run. Taken further, the consumption of your 400 watt bulb would be $12.30 per month. There, you’ve built your artificial sun and it really isn’t costing you an arm and a leg to run!


High Pressure Sodium lamps are even more efficient than Metal Halide lamps. They produce somewhere between 90 to 150 lumens per watt, depending on the size of the bulb. The only drawback to a sodium vapor bulb is that the spectrum is shifted severely towards the red-orange end of the spectrum. The most ideal use of a high pressure sodium is to either replace, or ultimately, accompany your metal halide bulb once your plants reach their flowering stage. There have recently, however, been some amazing advances with sodium vapor bulbs. One such advancement is the Son Agro bulb.

The Son Agro bulb has been designed to utilize 30% more of the blue-violet end of the spectrum than traditional high pressure sodium bulbs. Also, the light that they produce is much more economical because their output is 430 watts of light, while running off a 400 watt ballast (transformer). This is actually like getting 30 watts of free light…thank you, Son Agro!

There is also a hybrid bulb that is produced by Eye Sunlux. Every bulb that you use in your indoor garden requires its own ballast, and a normal high pressure sodium bulb that will not work in conjunction with a metal halied ballast, and vice versa. However, the Eye Sunlux bulb is a specially designed high pressure sodium bulb that Will work when plugged into a metal halide ballast. This saves you the expense of buying a new ballast for your lighting when your plants reach their flowering stage. These conversion bulbs, however, do lose a portion of their output. They are available in 3760 watt and 940 watt sizes, which plug into 400 watt and 1,000 watt metal halide ballasts respectively.

Please keep in mind that the light that the standard sodium bulbs produce is severely limited to the red-orange end of the spectrum. It is mostly used for your plant’s flowering stage.


The answer is yes! Throughout this chapter I have mentioned “spilled” light. This term refers to the amount of light that is generated, but not focused on the growing area. As light travels, it funnels out from its source. If you could draw an outline around the light generated from any bulb, the resulting diagram would look like a cone. The best way of directing the light that is traveling out of your growing area is to use something with a reflective surface that will “bounce” it back to where it is needed.

There are many things that you can do to clean up your “spilled light”. If your garden is completely contained by surrounding walls, painting those walls with a flat, white paint will reflect your light. If your garden is only taking up part of a room, hanging sheets of material with a reflective surface can accomplish the same effect.

A special two-sided poly, with one side black and the other white is one option. The white side faces your plants to reflect your light while the black side faces out to make sure that no light escapes. Mylar is another option. It reflects light with up to 95% efficiency, but is brittle, easy to tear and hard to clean. Aluma-Glo is like Mylar, but is veined with a re-enforced mesh so that it is much stronger and easier to maintain. Aluma-Glo reflects with about 94% efficiency. The king of reflective materials is a product called Foylon. It strongly resembles Aluma-Glo, but reflects light with an amazing 97% efficiency! This will take care of redirecting the light that is funneling downward from your bulb, and a reflector, or hood, will redirect the light that travels upward.

Reflectors or hoods, come in many sizes and configurations. Two of the most common are a “flat-cone” and a “parabolic” reflector. The standard size for these reflectors are two feet and four feet, with the 2 foot model being suitable for a 400 watt bulb, and a four foot being suitable for a 1,000 watt bulb. Of these, the parabolic is the most efficient because besides just the upward traveling light from your bulb, the angles of the reflector actively bounce the light where it is most needed.

There are also some new self-contained hoods that are air-cooled. Any bulb produces head as a by-product as it is producing light. These hoods are attached to a blower via air hose, so that fresh air is rushed through the hood thus cooing the bulb and dissipating any reciprocal heat that could be harmful to your plants.

Another way of improving the efficiency, and even the range of your lighting, is to install a light mover. One of the most popular methods is to use a Light Rail .

A Light Rail is powered by a small motor that continuously travels over a straight six feet track, and kept in motion by small, rubber wheels. The obvious advantage of using a light track is that it basically doubles the area that your light can affect. It also ensures that all of the plants in your garden receive the same amount of light from all angles, so that their growth will be uniform. As I mentioned earlier, light funnels as it travels towards your plants, and the distance that it travels is greater reaching the plants at the end of your garden than it is reaching the plants directly below your bulb. Because of this, the plants directly below your bulb will grow at a more accelerated rate than those at the edges of your garden. A light rail alleviates this problem.

The standard light rail is 6′ in length, but you can buy extenders that can increase the range of your light as long as your garden demands!

That should hopefully answer most of your questions concerning lighting for your indoor garden. If you are still confused, or have any concerns that have not been addressed, please contact your favorite Hydroponic Store.

Stay tuned for part three of “Why Hydroponics” in the next issue.