By Thomas Valentine

If you’ve ever talked to anyone about growing their own greens, the subject of lighting has surely come up. Chances are they said they couldn’t get enough of it! It’s true . . . the more light you have per square foot, the more plant you will get. This is a scientifically proven fact, and it makes sense. Let me expand on why that is.

Lighting Options 

There are four general lighting options: fluorescent lighting, metal halide lighting, mercury vapor lighting, and high-pressure sodium lighting. Each has its own pros and cons, and you should be familiar with them.

The intensity of light is measured in lumens. Each type of lighting has different light intensity values per watt of power, and in different spectrums. By spectrum I mean the color of the light that is produced. Peruse the following to get a feel for what each lighting option offers.

A fluorescent bulb at 40 watts will put out 3,000 lumens. The light is pure white, with very little heat generated.

A mercury vapor bulb at 175 watts will put out 8,000 lumens. The light has a tinge of blue to it, and there is a lot of heat generated.

A metal halide bulb at 400 watts will put out 36,000 lumens. The light has a lot of blue to it, much higher than the mercury vapor. The metal halide bulb puts out less heat than the mercury vapor, which is a good thing.

A high-pressure sodium bulb at 400 watts will put out 45,000 lumens. The light has a visible orange color to it. The high-pressure sodium bulb puts out about the same amount of heat as the metal halide.

See the differences between your lighting options? Let’s look at each in more detail, and in which situation you would use them.

Fluorescent Lighting

Fluorescent lighting has been around pretty much since the advent of electricity, and is a very efficient way to light a large space like an office building. For growing, though, it isn’t what you’re looking for. The spectrum is pure white and isn’t very intense for the amount of power it sucks up. Even with a bank of four bulbs in use, fluorescent lighting isn’t the choice you’re looking for during the later stages of growth. For the first stage of growth, though — rooting clones or seeds — it can’t be beat. Fluorescent lighting in this situation is excellent because it is a soft and gentle lighting source that generates very little heat.

Your clones will thrive under the light of a bank of gentle fluorescent bulbs. The rule of thumb for clones under fluorescents is that you put them under a more powerful light source when the roots take or when new growth sprouts from the top of the plant. You don’t want to keep them under for too long because the plant will go phototropic. A phototropic plant is one that has expended all of its energy in attaining height to reach a feeble light source. The space between the leaf branches (the internodal space) will be large, and the plant will be weak and feeble.

Mercury Vapor

Mercury vapor is stronger than fluorescents, but you still don’t want to use them for the later growth stages. The light isn’t strong enough for the amount of heat the bulb puts out. You’ll end up baking the topmost branches of your plant.

Even for rooting, mercury vapor isn’t the way to go. There’s too much heat and the light is a very harsh shade of blue-white. Although this bulb is cheaper per lumen than any of the following lighting methods, because of its heat it is not used much in a practical growing environment.

Metal Halide

Mercury vapor is stronger than fluorescents, but you still don’t want to use them for the later growth stages. The light isn’t strong enough for the amount of heat the bulb puts out. You’ll end up baking the topmost branches of your plant.

Even for rooting, mercury vapor isn’t the way to go. There’s too much heat and the light is a very harsh shade of blue-white. Although this bulb is cheaper per lumen than any of the following lighting methods, because of its heat it is not used much in a practical growing environment.

Metal Halide

Metal halide is the way to go after your clones have taken root. The light has a large amount of blue to it and the heat produced is acceptable. Fast growing (and smokable) green plants need a lot of blue light to grow thick and lush foliage. You won’t be disappointed in your purchase of a metal halide system because it is so effective.

This bulb puts out TONS of light, and it’s the absolutely perfect spectrum for your plants in the main foliage growth stage. You can hang your bulb to within a foot of the topmost branches and still they won’t scorch under the heat. Metal halide is the way to go for foliage growth, people.

High-pressure Sodium

The metal halide bulb is the way to go for the foliage growth stage of your plant, but only the foliage growth stage. When your plant starts flowering, what you want is a light source with more orange light to it. The high-pressure sodium bulb is exactly what you need for this because the light is very orange. The high-pressure sodium also puts out several thousand more lumens than the metal halide. 

So why not use high-pressure sodium for the entire life cycle of the plant? Because of the spectrum. The green growth period needs bluish light that simulates a summer sun and the budding / flowering stage needs light that is more orange, which simulates the waning light of the autumn sun.

Your buds will grow fat and healthy under the soothing light of a high-pressure sodium bulb. The resin sacs will be full and firm and the foliage will make killer oil or hash after it’s harvested. You just can’t go wrong with this system in the budding / flowering stage.

To recap: Use fluorescent tubes to root your clones or seeds. After they’ve seeded, use a metal halide bulb to produce thick and lush foliage. When the flowers begin to appear, switch to the high-pressure sodium. You’ll actually be able to see daily differences in growth when they’re under the high-pressure sodium.

What Intensity Should I Use?

So you now know what the pros and cons of each type of lighting are. The next thing to consider is intensity. You might recall that I previously said you just couldn’t get enough light for your plant, and this is true. But if you’re just starting out, you don’t want to pay several hundred or even thousands of dollars for something that you just might not use much. To that end, I’ll present the recommended amount of light for each lighting system. The recommended amount is a compromise between what the plants need for normal, healthy growth and what the average person wants to pay in terms of cost of equipment.

The amount of space you’re lighting is measured in square feet. Makes sense. The intensity of lighting then, is measured in lumens per square foot. Some might measure this in watts per square foot, but the amount of actual light each lighting method puts out varies per watt, so this isn’t a very accurate way of ascertaining your lighting requirements.

Intensity – Fluorescents

As I’ve previously said, you want to initially root your clones or seeds under a bank of fluorescent bulbs. Depending on how much space you’re lighting, you would use a bank of two or a bank of four bulbs. Some would say that the newly rooting clones need the same light intensity as a plant in the green-growth stages. This just isn’t true. Put your rooting clones under the full radiance of a metal halide and they’ll die, slowly. The intensity of light and heat will dehydrate the leaves. Since a clone doesn’t have roots to suck up more moisture, you’ll wake up one morning to a dry, brittle, and ultimately dead crop of clones. This won’t become apparent at first because the foliage will actually grow pretty good. What doesn’t grow though, is the roots.

I did some research on this one, and can expand the reasons for subtle fluorescent lighting for clones. The rooting process is dependent on a specific pair of hormones. That is, the levels of each of these hormones determines the speed of root growth. While under intense light, the plant will want to take advantage of this intense level of light and speed up the growth of foliage. Since there’s only a finite amount of energy that can be put into growth in total, the hormones that determine the speed of root growth aren’t generated in previous, normal amounts. This translates into fairly slow root growth.

While the clone is under the intense metal halide light then, the roots don’t grow fast enough to support the plant that is sprouting so much green foliage. The roots can’t keep up, and the plant eventually starves itself under the nutritional needs of the foliage — nutritional needs that the underdeveloped roots can’t keep up with. Foliar feeding can slow this, but the outcome will ultimately be the same dead crop.

Because of the changes in biology that the plant goes through after it is matured and has an adequate root system, this process of slowing root growth in favor of foliage growth isn’t as prevalent in older plants. I’ve been unable to find the specific reason why. I’ll keep searching.

So that’s the theory, let’s look at some numbers now. Since a fluorescent bulb puts out about 3,000 lumens and usually comes in pairs of two bulbs, we’ll work first with that situation. Each bulb for our math is about 4 feet in length and puts out 3,000 lumens each for a total of 6,000 lumens. Assume the normal width of the growing area under the effective reach of the light to be at 1 foot, and you have a growing area of 4 square feet for two bulbs. Divide the value of 6,000 lumens given out in total by the number of square feet, and you have a value of 1,500 lumens per square foot. This is a good level of lighting for rooting clones or seedlings, so try and shoot for that.

If you have more clones than would fit in 4 square feet, use a pair of double-bulbed light fixtures to double your clone rooting area or increase as needed.

Intensity – Metal Halide

Metal halide is the first step in the foliage growing cycle of your plant, so it’s a good idea to get these numbers straight. This stage of your plant’s life cycle is a no-brainer. All you have to do is feed them regularly and watch for critters and nutrient deficiencies. Be patient, they’ll flourish.

Through years of trial and error and research, I’ve come to the conclusion that one 1,000-watt metal halide bulb in good working order can service an area of 25 square feet (5 feet square), with a total average light output at 90,000 lumens for the 1,000-watt bulb. This, I’ve found, is the optimum level of lighting for your plants during the long foliage growth period.

Your growing area then, would be a square area that would be about 5 feet on each side. Each square foot of growing space would receive a very respectable 3,500 lumens. This will produce huge amounts of healthy green foliage, paving the way for some really killer couch lock buds later on.

Now you can always go gung ho and add a second bulb, but I don’t recommend this, for a few reasons. The first is cost. You have to buy the bulb, the fixture, the ballast, and pay for electricity. The second is simple heat. There are two sources of unwanted heat now. If you don’t mind either of these, the power company will. A regular on and off increase and decrease of exactly 2,000 watts for months at a time, even in a larger house, won’t go unnoticed. Expect a team of rude visitors.

Intensity – High-pressure Sodium

Once the flowering cycle starts, switch to that big 1,000-watt high-pressure sodium bulb. This bulb actually puts out more light than the metal halide, but in a spectrum that isn’t conducive to foliage growth. High-pressure sodium is all about the buds. This is the reason why we’re using it here — to increase the density and potency of your future buds.

A 1,000-watt high-pressure sodium bulb will put out 112,500 lumens. This amount of light in 25 square feet will work out to 4,500 lumens per square foot! Just think of those fat, juicy buds. During the flowering / budding stage, overkill is the way to go. More light translates into more potency, and that’s what you’re going for. The previous months of growth are all for naught if your buds don’t turn out very well. You’ll be getting about the same amount of heat from the high-pressure sodium as you would from the metal halide, so that really isn’t an issue.

Tailor To Your Situation

While having the 1,000-watt bulb option is great, not everyone wants to go that far. Perhaps the reason is money, perhaps space, or perhaps you just don’t want that many plants. Whatever the reason, feel free to tailor the numbers to fit your situation. This is the primary reason why I presented the smaller wattage bulb to you in the bullet list at the beginning of this article. There are smaller bulbs such as the 250-watt which are good for single or double plant growing, and by all means use them if you have the need. The common bulbs, though, start at 400 watts.

That’s lighting in a nutshell. Happy growing.