Growing Marijuana - an introduction
Location, heating & ventilation
Germination & Vegetative Stage
Flowering stage, harvesting & curing
The type of lighting used depends on the size of room and the type of growing method. Indoor growing requires the light intensity, ideally, to be between 2000 and 3000 lumens/sq. ft.
Tungsten filament bulbs are very inefficient and produce more heat than light; some sort of gas discharge lamp is required. There is much debate about the relative merits of the various types: fluorescent tubes, mercury vapour, metal halide and high pressure sodium. Mercury vapour and metal halide are useful during the vegetative growth stage; HPS are perfect for flowering and acceptable for vegetative growth; fluorescent tubes are general purpose. In the writers opinion, only fluorescent tubes and HPS need be considered as other types involve a change of lamp when the flowering stage is induced.
Fluorescent tube systems
Fluorescent tubes and their fittings are available in a range of lengths. Longer tubes obviously emit more light than short ones. Standard 1in diameter halophosphate tubes are rated at about 30 watts per metre and output about 2,500 lumens per metre (and 85 lumens per watt). Tubes are available in a range of colour temperatures: 6000K (daylight), 4000K (cool white), 3500K (white) and 3000K (warm white). Daylight tubes have more blue in their spectrum and warm white more red. Results are not much different whatever tubes you use but, if you have the choice, either use a mix of warm white and cool white or change from cool to warm white when the flowering stage starts. A quick calculation reveals that to achieve 2000 lumens per sq. ft you have to virtually cover the entire area of the room's ceiling with fittings. The fittings can get quite hot, so allow a space between them for air to circulate.
The light intensity drops quite dramatically as you move away from the tube; in fact, intensity falls with the square of the distance such that the intensity at two feet distance is one quarter the intensity at one foot. This means that plant growth is only really luxuriant up to a few feet from the tube. Fix the tube fittings to a board which is, itself, attached by cords or chains to the ceiling; this way the tubes can be kept as close as possible (a few inches) to the top of the plants. The board will have to be moved daily when the plants are growing well.
Fluorescents are most useful in situations where you are not growing the plants very tall. For instance, as a light source for cuttings (clones) or for the "sea of green" method.
HPS lamps are available in 150W, 250W and 400W sizes. The lumen output is, respectively, 15,000, 28,000 and 50,000. This amounts to between 100 and 125 lumens per watt, according to the size of lamp. Their colour temperature is quite low at between 2000K and 3000K; they emit a pinkish, golden white light which is similar to autumn daylight. Note that recently, a new type of HPS lamp has been introduced, the SON-T Agro, which has an extra 8% blue light added to the spectrum making it more acceptable for vegetative growth than the standard SON-T.
There are several advantages to HPS lamps:
The efficiency, in terms of lumens per watt, is high.
There is usable light at a greater distance from the bulb, compared with fluorescent tubes.
In most domestic installations, there are only one or two lamps to change as opposed to maybe twelve or more fluorescent tubes.
Lamps have a long lifetime of 20,000 hours or more (although the output drops, gradually, during the life of the lamp - it's best to change them after one year's use).
The disadvantages are:
The cost of an entire system comprising control gear, lamp and reflector can be several hundred pounds.
The colour temperature is not ideal for vegetative growth although most growers report acceptable performance.
HPS lamps should be kept at least 18" from the plants to avoid leaf scorch or even fire! Water should be kept well away from the lamps as they can explode if splashed.
The lighting system needs to be switched automatically so that the plants receive 18 hours of light per day during vegetative growth and 12 hours per day when flowering. Stores such as Currys and Argos sell timers, which plug in to an ordinary 13A socket, for less than £20. They claim to be able to switch 13A but inductive loads such as HPS lamp ballasts can burn the contacts quite quickly. Use the timer to operate a mains relay with a higher current rating, e.g. 30A.
For larger or more permanent installations use a surface mounted timer such as those used in central heating systems. These may have more than one timed outlet which may be useful if you are operating a two shelf "sea of green" setup.
Keep electrical circuits well away from water. Make sure that electrical wiring is installed following all the conventions used by the professionals, i.e. use cable of the correct current rating, use the correct rating fuses and earth all metal components (like control gear boxes) that could be touched.
Keep anything flammable away from hot lamps. If you can arrange it, fix a switch near the front door which acts as a master control switch for the whole grow room's systems. If you have a visit from the landlord or the boys in blue then you can shut down the lights and fans easily before anyone hears or sees anything suspicious.
Much of the information contained on this website would be illegal were it to be practiced in most countries around the world - including the UK. Our intent is to educate and inform NOT to encourage anyone to break the law.
Copyright © 2002 SKD