|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 1980s 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.
HIGH INTENSITY DISCHARGE LAMPS
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:
|Watts||# Of Lumens
|# Of Lumens
Per 100 Watts
|4 FL (CW-40W)||2,960||7,400||1-2|
|8 FL (CW-75W)||5,800||7,733||2-4|
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|
LIGHTS AND REFLECTORS
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.
|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.|
FLUORESCENT LIGHT REFLECTORS
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.
|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.
|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 suns 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.
|Cost Per Kilowatt Hour Of Electricity|
|4 Fluorescent (CW-40W)||2,960||.11||.13||.16||.22|
|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
|Marijuana Growing Contents||Next Chapter|
CLICK HERE TO BUY CANNABIS SEEDS