Marijuana Growing Help Chapter 8

      Plants use light as energy to fuel photosynthesis, a process in which water and carbon dioxide (CO2) are the raw materials used to make sugar. Sugar is the basic building block of all plants. By twisting the sugar molecule, plants form carbohydrates, which are more complex molecules. Plants use carbohydrates to build tissue. When nitrogen atoms are integrated into the molecules, amino acids are formed. These are eventually grouped together to form proteins.
      Light is also used to regulate many varieties of cannabis' reproductive cycle. Scientists speculate that the plant produces a hormone during the dark period (night in nature) which induces the start of the reproductive (flowering) cycle. When the hormone builds up to a critical level, flower growth commences. The number of hours of darkness required to induce flowering differs for each variety.
      Gardeners have a choice of lamps to illuminate their garden. Incandescents, tungsten-halogen lamps and screw in "grow bulbs" are inefficient sources of light. Although they are inexpensive to purchase, their cost of operation makes them the costliest source of light.


      Until the early 1980’s most indoor growers used fluorescent lights to illuminate the garden. These tubes have tremendous advantages over screw-in incandescent lights. A fluorescent tube emits about 3 times as much light as an incandescent of the same wattage and has a light spectrum that plants can use more efficiently.
      Fluorescents have their limitations. Light is emitted over a large area, the entire surface of the tube, so it is not concentrated. Because the tubes are bulky only a limited amount of light can be delivered to a given area. The fixtures are usually placed within inches of the plants so that the light does not spread and become less intense. When the light fixtures are hung they are hard to manipulate and make it more difficult to tend the garden.
      Standard fluorescents have an efficiency of about 30%. Seventy percent of the electricity is not turned into light but into heat. There are newer types which are a little more efficient, but the increase in light is of only marginal help.
      VHO (very high output) FLUORESCENTS are also available. They use about 3 times the electricity of standard fluorescents and emit about 2½ times the light. While they are not as efficient as regular fluorescents, each tube delivers 2½ times more light to the garden.
      The inner surface of each fluorescent tube is covered with a phosphor which glows when tickled by the flow of electrons through it. Fluorescent tubes are named for the spectrum of light which they emit. Some of the spectrums are more conducive to plant growth than others. Deluxe warm white, warm white, and deluxe cool white are three types which promote fast growth. Special grow bulbs concentrate light in areas used most efficiently by the plant. However, they are fairly dim and plant growth is slowed when they are used.


      High intensity discharge lamps (HIDs) are easier to use and more efficient. Low wattage HIDs are sometimes sold for household outdoor use. Large wattage lamps are used to light yards, streets, parking lots, stadiums and other large areas. They come in two versions:
      METAL HALIDES or MH lamps emit a white light that looks slightly bluish. They are used to light stadiums, convention centers and other large areas where a natural looking light is desired.
      HIGH PRESSURE SODIUM or HPS lamps emit a pink or amber light. They are used to illuminate parking lots and other areas where the color of the light is not important. HPS units are more efficient than MH lamps. They are often used alone with no detrimental effect to the plants, and will promote faster plant growth than MH bulbs during both vegetative growth and flowering. Combinations of bulbs are not required, as the HPS lamp has all the light spectrums necessary for healthy growth.
      MH lamps come in 175, 250, 400 and 1000 watt sizes. HPS lamps come in 150, 400 and 1000 watt sizes. Each lamp has its own ballast.
      HID lighting systems are much more convenient to use than fluorescents because the lamps have a higher wattage and are more efficient at producing light than fluorescents. Large wattage systems are more efficient than smaller ones. MH lamps have an efficiency of 35-50% depending on the wattage. HPS lamps have an efficiency of 50-55%. Moving the lamp and reflector is fairly easy since they are fairly light. The light is powered by a heavy ballast; but it is connected only by a long electrical wire. Some 400 watt HID systems are manufactured with the ballast built into the same housing as the reflector. These lamps are harder to move around and are usually considered for lighting only if they are to be permanently mounted.
      This chart shows how much light each lamp emits, its lumen output per 100 watts and the area it covers adequately.

Watts # Of Lumens
# Of Lumens
Per 100 Watts
Square Feet

100W Incandescent 1,750 1,750 N/Applicable
4’ FL (CW-40W) 2,960 7,400 1-2
8’ FL (CW-75W) 5,800 7,733 2-4
MH 175W 14,000 8,000 5-10
MH 400W 40,000 10,000 12-20
MH 1000W 125,000 12,500 35-70
HPS 100W 9,500 9,500 3-6
HPS 15OW 16,000 10,600 5-10
HPS 400W 50,000 12,500 15-30
HPS 1000W 140,000 14,000 40-80

      Because of the ease and convenience of operating a HID lamp and their increased efficiency they are recommended for lighting indoor gardens.
      Gardens should receive between 1000-3000 lumens per square foot. Of course, plants in a 3000 lumen garden will grow faster and flower more profusely than those under dimmer lights. Successful gardens usually are lit at between 1500-2500 lumens per square foot. During the vegetative stage, plants stretch out when they receive low levels of light. During flowering the flowers are looser and sparse.
      This chart shows the approximate amount of light received by gardens of various sizes with a very efficient reflector. Twenty percent of the light emitted has been deducted from the total to correct for reflector inefficiency and light which never reaches the garden. Light is never distributed evenly so some parts of the garden will get more light than others.

Garden # of # Of Lumens Per Square Foot
Size Sq. Feet MH 400 MH 1000 HPS 400 HPS 1000

3’ x 3’ 9 3,500 11,100 4,450 12,450
4’ x 4’ 16 2,000 6,250 2,500 7,000
5’ x 5’ 25 1,300 4,000 2,000 4,500
6’ x 6’ 36 900 2,800 900 3,100
7’ x 7’ 49 650 2,050 650 2,300
8’ x 8’ 64 500 1,560 390 1,750
9’ x 9’ 81 400 1,250 500 1,400
10’ x 10’ 100 300 1,000 400 1,100



      Sunlight comes from a distant source, so that the light rays hitting a small portion of planet Earth (say a garden 12 feet wide) are virtually parallel. Their intensity does not diminish over the length of a plant 6 feet tall.
      Light emitted from tubes and lamps travels in all directions. As the distance from the lamp increases, the intensity of the light decreases. It is not that any light is lost, just that the same amount of light is spread over larger area.
      HID lamps and reflectors come in two configurations. Either the lamps are held vertically or horizontally.
      Horizontally held lamps direct most of the light downward because the light is emitted along the length of the lamp. Only a small reflector is required to beam the rest of the light downward.
      Vertical lights emit most of their light horizontally. In order to reach a garden, the light must be reflected downward using a large, bulky reflector. Manufacturers have developed elaborate and innovative hoods, still they cannot reach the light delivery efficiency of a horizontal lamp.
      Horizontally held lamps have several other advantages over verticals. They take less vertical space, which is crucial for short gardens, and the reflectors are much less bulky. All in all, horizontally held lamps are considered the best configuration for the closet garden.
      Aluminum reflectors deliver the most light, more than white ones. Stainless steel reflectors absorb some spectrums of light and should not be used.

A small horizontal reflector actually delivers more light to the garden below than this large horizontal reflector. The small vertical reflector allows much of the light to escape to the sides.


      A garden lit by two tubes per foot of width with a high quality reflector receives about 1,100 lumens per square foot. A garden lit by three tubes per foot of width receives about 1700 lumens per square foot.
      Fluorescents come in many lengths, but the two most commonly used by indoor gardeners are 4 and 8 ft lengths. They are convenient to use and are more efficient than other sizes.
      Poorly designed fluorescent fixtures, with no baffles between the tubes to reflect light downward may lose up to 40% of the light. Instead, tubes are mounted onto a reflector with individual baffles between the tubes so that light is directed downward to the garden. A good reflector may keep losses down to 20%. An alternative is to use tubes with reflective surfaces. These are made several manufacturers. Often stores do not carry them but will special order them.

Reflectors without baffles are very inefficient so light is lost. Baffles direct the light downward.

      New fluorescent configurations have made it easier to build a garden. Circle tubes and thin tubed 8" doubles screw into incandescent sockets. Although these bulbs are not very efficient they are step up from incandescents. Combinations of circle lights and tubes can illuminate a garden very brightly. They can be used in extremely small spaces. These lamps always seem to be on sale. When electrical costs are not a factor they are a inexpensive way of setting up a garden.

These units easily provide over 2000 lumens per square foot.

      As tubes age they become less efficient. On the average, they lose 25% of light they were rated for after about a year of use. Lights which are turned on and off a lot wear out faster. Three to six inch sections on both sides of the tube dull out from deposits after a short term of use. Growers figure the effective length of a 4 ft tube as 3 feet 4 inches and of an 8 ft tube as 7 feet.

Light Spectrums and Photosynthesis

      Each source of light has a characteristic spectrum, which is caused by the varying wave lengths of light therein. Fluorescents and other electric lights emit different shades of light. To our eyes midday summer sunlight looks neutral, incandescent lights have a reddish tint, fluorescents vary in spectrum according to their type, MH lamps a have a blue coolness to them, and HPS lamps look pink-amber.
      To produce chlorophyll, plants need light from specific spectrums, (TABLE 1) mainly red and blue. This is called the chloroplast light spectrum. Once the chlorophyll is produced, a slightly different spectrum of light (TABLE 1) is used by the plant for photosynthesis, the process which results in the production of sugars. Plants use red and blue light most efficiently but they also use orange and yellow light. Plants are continually growing, producing new chloroplasts and chlorophyll so both spectrums of light are being used by the plant continually. Plants reflect green light rather than using it.

Action Spectrum of:  (A) Photosynthetic Response
(B) Chlorophyll Synthesis     

      Although the MH and HPS lamps emit different color light both lamps emit high levels of light in the critical red and blue wavelengths. Either lamp can be used for cultivation. HPS lamps produce faster growth because they emit more total light useable by the plant.
      Many shop owners maintain that combinations of MH and HPS lights produce the fastest growth, or alternatively, that MH units should be used for growth and HPS units for flowering. There is no indication that either of these theories holds up. HPS lamps produce faster growth than a combination of HPS and MH lamps. There is absolutely no need to or advantage to buying a MH unit. Plants grown under HPS show some stem etoliation (stretching) and ripen about a week later. This is more than compensated with a considerably larger crop.
      Some fluorescent tube manufacturers produce grow tubes which are especially formulated to provide a spectrum of light similar to the chlorophyll synthesis or photosynthesis spectrum or a compromise between them. The idea is sound, but grow tubes produce only 35-60% of the light of a cool white fluorescent, and less light useable by the plant. One manufacturer advertises Vita-Lite® and Optima® fluorescent tubes which emit a light spectrum color balanced close to the sun’s spectrum. However, they emit only 75% of the light of a warm white fluorescent.


      HPS systems are the most expensive to purchase of all of the lighting units. MH units are a little cheaper and fluorescents are the cheapest of all. However, this is figuring only the initial outlay. Factoring in the cost per unit of light produced, the positions are reversed. HPS lamps are the cheapest, followed by MH lamps and far behind come the fluorescents. In addition HID lamps are considered easier to work with in the garden and produce a better crop than fluorescents.

Cost in cents per 1000 lumens of various lamps.
(Expressed in cents per kilowatt)

Cost Per Kilowatt Hour Of Electricity
Lamp Output 10¢ 12¢ 16¢

100W Incandescent 1,750 .46 .57 .68 .91
4’ Fluorescent (CW-40W) 2,960 .11 .13 .16 .22
175W MH 14,000 .10 .12 .15 .20
400W MH 40,000 .08 .10 .12 .16
1000W MH 125,000 .06 .08 .10 .13
100W HPS 9,500 .08 .10 .13 .17
400W HPS 50,000 .06 .08 .10 .13
1000W HPS 140,000 .06 .07 .08 .11

Note about the chart: These figures denote the part of a cent used to produce 1000 lumens. In dollar terms the figures for a 1000W HPS are $.0006, $.0007, $.0008, $.0011.


Step By Step

  1. The successful gardens I have observed use a minimum of 1000 lumens per sq. ft during vegetative growth and 1500 lumens during flowering. These figures are bare minimums, the more light the better. Gardens with 1500-2500 lumens during vegetative growth and 2000-3500 during flowering seem to do best.

  2. The most efficient light source is a HPS lamp in a horizontal reflector. No other light source is needed. An HPS lamp supplies all the spectrums of light needed by the plant for normal growth.


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