When grown indoors, marijuana plants often have a limited amount of natural sunlight available to them. It is almost always necessary to supply the plants with artificial light to make up for the lack of natural light in a grow room.
Lights for growing plants are referred to as grow lights. The most common grow lights for marijuana plants are
--- metal halide (mh)
--- high pressure sodium (hps)
--- light emitting diode (led)
In the first stages of life, marijuana plants do not need a large amount of strong light. As they mature, especially for flowering, their light requirement increases.
Fluorescent Lighting: For germination, clones, seedlings, vegetative stages the amount of light required to grow them is minimal compared to flowering. Fluorescent light works very well for this purpose and can be fairly inexpensive to purchase.
Because the light needed to grow younger plants is not as great, running a low wattage fluorescent light will cost less to operate than stronger light systems.
Compared to other sizes of fluorescent light fixtures, the 48 inch size is most common and produced in larger quantities. Because of this, purchasing and getting replacement bulbs for them is usually cheaper.
A 48 inch fluorescent light fixture that is suitable for growing marijuana plants, hydroponically or with soil, can be acquired for a reasonable price.
There are fluorescent light fixtures designed strictly to be grow lights. They are usually more expensive than standard fluorescent light fixtures.
Under most circumstances fluorescent grow lights don't do a better job than standard fluorescent light fixtures. One exception is the light reflection characteristics.
Fluorescent grow lights should be designed to reflect light down, to the plants under it. A standard fluorescent light fixture may or may not reflect light down.
They may have nothing to reflect the light, so some of the light goes to the side as well as down. Keep this in mind when purchasing a fluorescent light for growing marijuana.
Fluorescent light fixtures are usually two to four feet long, but other sizes might be available depending on where you shop.
Look for a model that holds at least 2 bulbs, rated at 30-60 watts each. A minimum of 60 watts is recommended to grow at least 12 clones or seedlings in a small soil garden or hydroponic system like a cloner.
When clones or seedlings get large enough, they can be moved from a small soil garden or hydroponic system like a cloner, to a larger soil garden or hydroponic system that will give them more room to grow.
With fluorescent light the plants have to be directly under the bulb. A large number of plants might necessitate multiple fixtures in order to provide adequate illumination for each of the plants.
Fluorescent lights come in 3 bulb diameter sizes T5, T8, T12. You have to get the correct bulb to run in the fluorescent light fixture you purchase.
T5 bulbs are the most efficient.
T8 bulbs are the second most efficient.
T12 bulbs are the least efficient.
Most fixtures can only run one type, T5 or T8 or T12, but some can run more than one type of bulb. Get a basic fixture with no light cover.
If you buy a fluorescent light fixture with a light cover, remove the cover prior to installing it. This is so a minimum of light energy gets wasted.
Fluorescent light bulbs colored between 4100k-6500k (or slightly higher) are recommended for germination, clones, seedlings, during the vegetative stage. A higher k rating will limit the height of a plant compared to a lower k color rating.
Remember to get bulbs that are the right color, wattage, length, bulb type (T5, T8, T12) to work with the fluorescent light fixture you choose.
Try to place the plants under the center of the fluorescent bulbs. The strongest light will fall under the center of the bulbs, so it is best for plants to be under that region.
Depending on the wattage, fluorescent lighting should be hung so it is about 3-12 inches/8-31 cm above the tops of the plants. Lower wattage lights can be closer to the plants than higher wattage lights.
As the wattage increases so does the amount of heat produced. That is the reason for hanging higher wattage lights further away from the top of the plants.
A fluorescent light fixture that holds 2 bulbs rated at 30 watts each, that's 60 watts total, should be hung no closer than about 3-6 inches/8-15 cm above the tops of plants.
For germination, clones, seedlings, a fixture with 2 bulbs rated at 30 watts each should be started a bit further away, about 8 inches/20 cm, until the plants get accustomed to the light and develop healthy leaves.
The larger the leaf system, the closer the bulb can be without harming the plants. If the tops of the plants that are closest to the light start to dry out an turn brown while leaves further away from the light look ok, it is most likely that the light is too close.
Fluorescent lighting is not very strong, so it is not recommended for flowering when marijuana plants require more intense lighting.
If you are forced to run fluorescent light for flowering, as a minimum try to get a fixture that holds 4 high output T5 bulbs that are 20 watts, or more, each.
4 bulbs will produce stronger light, when compared with a fixture that only has 2 bulbs of the same wattage. This will increase the harvest size.
Metal Halide: You can run metal halide lights during the first few weeks of life, but fluorescent light will provide enough light for germination, clones, seedlings.
After the plants have developed a strong root system and several sets of leaves, the vegetative stage, you can continue supplying fluorescent light.
If you want the plants to grow at a faster rate, you can increase the amount of fluorescent light or switch from fluorescent to a metal halide or high pressure sodium light source at any time.
Metal halide is also a good source of lighting for plants that are flowering. Most marijuana growers choose high pressure sodium over metal halide for flowering because hps bulbs produce more light pew watt.
With metal halide for flowering, the yield might be slightly smaller but it will probably be a bit more potent.
You can use mh for germination, clones, seedlings but it is best to avoid hps.
An hps light will tend to make the plants grow tall and straggly if supplied for germination, clones, seedlings or plants during vegetative growth.
Running a 250 watt or higher metal halide or high pressure sodium light when you could be running a fluorescent light that draws under 100 watts is not cost effective, unless the plants cover a large area or you need them to grow faster.
Over the course of time you might waste more money on electricity, by metal halide or high pressure sodium lights, than what it costs to buy a fluorescent fixture.
Assuming the cost of electricity is 10 cents per kilowatt hour, running a 100 watt fluorescent fixture will cost 20 cents per twenty hours. Running a 250 watt mh light will cost 50 cents per twenty hours.
If the lights were on 20 hours a day, that would work out to $6.00 for a 100 watt fluorescent vs $15.00 for a 250 watt mh light, per 30 day period.
When mh or hps is run for germination, clones, seedling, vegetative stages, it will have to be hung further away from the plants than fluorescent light or it will burn the plants.
High Pressure Sodium: Most growers will switch to high pressure sodium at some point when the plants enter the flowering stage.
High pressure sodium bulbs are more efficient, more units of light per watt, than metal halide bulbs of the same wattage.
This means a high pressure sodium light should produce a slightly larger harvest when compared to flowering with a metal halide light of the same wattage.
High pressure sodium is not recommended for lighting with plants in the germination, clones, seedling, vegetative stages because plants will tend to stretch and grow too tall, with more space in between branches.
LED Grow Lights: The author of this page has tried growing with various led grow lights for flowering marijuana plants since 2008, with dismal results.
Personally, the only led lights that have been worth purchasing have been low wattage panels. They are placed at the dark edges of a grow area that receive less light.
Choose the color of the supplemental light based on the color of the main light. If the main light is metal halide, choose all red or red/blue led supplemental lighting.
Since metal halide light falls mainly in the blue end of the light spectrum, additional red light would be most beneficial to balance things.
If the main light is high pressure sodium, choose all blue or red/blue led supplemental lighting. Since high pressure sodium light falls mainly in the red end of the light spectrum, additional blue might be best.
An led light is a good alternative if you can't use metal halide or high pressure sodium because they produce too much heat. Try talking to a grower who actually gets good results flowering with led lighting.
To initiate flowering in marijuana plants, reduce the photoperiod (the time the light is on/off) to 12 hours on and 12 hours off, per 24 hour period.
With plants that were getting 18 hours or more of light per 24 hour period, you will be able to see them start to flower 10-14 days after the light is reduced to 12/12.
Marijuana plants should be forced to flower when they look healthy, have established a strong root system, have developed several sets of leaves, are between 6-12 inches/15-30 cm tall.
If they are allowed to get any taller before being forced to flower, they have a tendency of getting too big. They only stop growing several weeks after they have started to flower.
Amount of time light should be on:
Germination, clones, seedlings 16-18 hours a day.
Vegetative growth 18-24 hours a day.
Flowering 12 hours a day.
Marijuana clones that have been taken from flowering plants should get 18 hours of light per day, or they may stay in the flowering stage.
Plants started under fluorescent light can be changed to metal halide or high pressure sodium during the early stages of life. But do not wait any longer to switch, from fluorescent to mh or hps, than when you start the 12/12 light cycle.
If possible start the 12/12 light cycle with metal halide and don't switch to high pressure sodium until a few weeks after the plants have started to flower, and the growth rate slows down. This will help limit the height of the plants.
Minimum requirement for germination, clones, seedlings, vegetative stages is about 60 watts of fluorescent light.
Minimum requirement for start of 12/12 light cycle and flowering is 250 watts of metal halide or high pressure sodium.
Stronger light and longer days during the vegetative stage will result in quicker growth rates, so the plant will be able to flower and harvest earlier. Stronger light for the flowering stage will result in a larger harvest.
The marijuana strain you grow will have to be compatible with the amount of light you are able to provide. A pure indica or mainly indica strain will be capable of producing high quality potent marijuana even at light levels as low as 250-400 watts.
An indica/sativa mix, mainly sativa, or pure sativa strain will need at least 600 watts, and depending on the particular variety more light might be required. The more sativa heritage, the greater the intensity of light it will need.
The harvest amount and area covered described below is based on experience flowering mainly indica strains that have some sativa heritage, all were 60% indica or higher. You could get a larger harvest than what is stated below with high yield strains like big bud.
A 250 watt grow light will supply enough light to cover a 6.25 square foot area, 2.5 foot by 2.5 foot grow area or about 3.0 foot by 2.0 foot grow area, etcetera.
Enough to produce about 2-5 ounces of marijuana per harvest.
A 400 watt grow light will supply enough light to cover a 10.5 square foot area, 3.25 foot by 3.25 foot grow area or about 4.0 foot by 2.6 foot grow area, etcetera.
Enough to produce about 4-10 ounces of marijuana per harvest.
A 600 watt grow light will supply enough light to cover a 16.0 square foot area, 4.0 foot by 4.0 foot grow area or about 5.3 foot by 3.0 foot grow area, etcetera.
Enough to produce about 6-16 ounces of marijuana per harvest.
A 1000 watt grow light will supply light to cover a 25.0 square foot area, 5.0 foot by 5.0 foot grow area or about 7.0 foot by 3.5 foot grow area, etcetera.
Enough to produce about 10-24 ounces of marijuana per harvest.
The area covered with the most intense light will be determined by the shape of the reflector the light is housed in. A 600 watt light in a square or circular hood reflector will cast maximum light over a square or circular area roughly 4 feet by 4 feet.
If the same 600 watt light were housed in a rectangular reflector, it might shine the strongest light over an area perhaps 5.3 foot by 3.0 foot in size. The exact area would depend on the dimensions of the reflector.
Keep this in mind when planning the shape of your soil garden or hydroponic system, and light you choose. Don't worry if your grow area is larger than the amount of light available.
It just means that some of the edges of the garden will get less light than plants closer to the light source. You can add supplemental low wattage led or fluorescent light to cover the dark edges.
The marijuana produced by a higher wattage mh or hps bulb will be more dense than that produced by a lower wattage light, growing the same strain, but if grown correctly the potency per weight will be the same.
The author has extensive experience growing one particular mainly indica strain with various wattages of lighting. The approximate size of the harvest is stated beside the light wattage:
200 watts - about 3 ounces.
250 watts - about 4 ounces.
400 watts - about 8 ounces.
600 watts - about 12-14 ounces.
Results will vary with different strains and growing conditions, but should give you an idea of how increasing the wattage will increase the harvest size. Lowest wattage was 200 watts (400 watt dimmable ballast at 50%). 250 watts is preferred for an extra ounce at harvest time.
The number of plants should be the minimum possible. For the light wattage and harvest size figures above, 2 plants were grown at a time for all light wattages listed. More plants could have grown in the same sized area, but the harvest would be the same size. More plants usually increase cost and work time.
A dimmable ballast allows adjustment of the amount of power being supplied to the light. Most will allow either metal halide or high pressure sodium bulbs to be used without the need to change any settings.
When an mh or hps bulb is turned off, wait a few minutes for it to cool before turning it on again.
With a dimmable ballast you can run a bulb at, or slightly below its rated wattage.
Try to use a bulb that is as close to the ballast wattage, without exceeding the wattage of the bulb, as possible.
If you are running a dimmable ballast at 200 watts, run a 250 watt bulb rather than a 400, 600, 1000 watt bulb. Underpowering the bulb too much might alter the light spectrum, and can shorten the life of the bulb.
Never run a bulb at a power level higher than the rated power of the bulb. If you do, there is a chance that the bulb will be damaged or ruined. It can also cause a bulb to shatter or explode.
Having said that, a trusted grower says he runs:
600 watt bulbs at 750 watts
400 watt bulbs at 500 watts
250 watt bulbs at 300 watts
and obtains good results. However, when 150 watt bulbs were run at 200 watts, they were destroyed.
It's recommended not running bulbs higher than their rated power. If you feel the need to do so, be aware of the possible negative consequences (bulb might be damaged, ruined, explode or the life of the bulb may be shortened), and don't increase power to over 25% of the bulb rating.
The dimmable ballast is capable of being able to switch power levels to suit the stage of life the plant is in. During early growth a plant requires less light than when flowering. So you have the option of turning up the power and installing a higher wattage bulb for flowering.
Lifespan Of Grow Light Bulbs
Fluorescent bulbs should be changed after about 1-2 years of use, more expensive bulbs may last longer. An led light will last several years.
But with most led grow lights the entire fixture, lights and housing, has to be replaced when the light dims to the point where the harvest size suffers.
With mh and hps, replace bulbs after 6 to 12 months of use. If the light is on 24 hours a day replace it after 6 months. If the light is on 18 hours a day replace it after 9 months. If the light is on 12 hours a day replace it after 12 months.
You can run the same light bulb till it burns out, but it will become less efficient, produce less light and a smaller harvest, the longer it's run. If you buy high-quality, usually expensive, grow light bulbs they usually remain more efficient for longer than cheaper bulbs.
One note about replacement mh and hps bulbs, they can be divided into three categories: vertical (base up), horizontal, universal. Vertical bulbs have to be installed in vertical position fixtures. Horizontal bulbs have to be installed in horizontal position fixtures.
Universal bulbs can be used in a horizontal or vertical position. If you can't locate a vertical or horizontal bulb for your particular lighting system, or if you are unsure, make sure you get a universal bulb. It will work in either type of fixture, but there is usually a slight loss of efficiency.
Grow Light Color
See the grow light color comparison chart here for a rough idea of how different lights look to the human eye.
Fluorescent light between 4100k-6500k (or slightly higher) is recommended for germination, clones, seedlings, vegetative stages of life. It can be employed if you are going to flower with fluorescent light. A higher k rating will limit the height of the plants more than a lower rating.
There are full spectrum fluorescent grow lights. They are usually more expensive and won't hurt, but don't make much of a difference for germination, seedling, vegetative stages.
If you are forced to flower under fluorescent light, which is not recommended because it takes longer for the plants to mature and the size of the harvest will be small and less potent, full spectrum lights would probably be the best choice.
The most common colors for metal halide grow lights range from about 4000k to 7500k, with the standard metal halide bulb being 4000k to 4200k. Less common mh bulbs are 3000k and bulbs higher than 7500k.
The color of light produced by a mh bulb rated at 4000k to 4200k is what most people see as white light. Metal halide bulbs with a rating below 4000k will produce light with more of a yellow tinge to it, the lower the rating is below 4000k, the more yellow the light will appear.
Metal halide lights with a rating over 4200k will produce light with more of a blue tinge to it, the higher the rating is above 4200k, the more blue the light will appear. 8000k-20000k lights are more blue/purple in color.
The most common colors for high pressure sodium grow lights range from 2000k to 2700k, with the standard high pressure sodium bulb being about 2000k to 2200k.
The color of light produced by a high pressure sodium bulb rated at 2000k to 2200k is what most people see as orange/pink/red light.
Bulbs with a rating below 2000k will produce light with a more of a red tinge to them, the lower the rating is below 2000k, the more red the light will appear.
High pressure sodium bulbs with a rating over 2200k will produce light with a more yellowish tinge to it, the higher the rating is above 2200k, the more yellowish the light will appear.
With led lighting, the color of the bulbs will be disclosed by the seller. The most common colors of led grow lights are blue, orange, red, white, or a combination of these colors.
If an led grow light is described as blue, the light will look blue to the human eye. If an led panel is described as red/blue, it will have some lights that are red and some that are blue.
An led grow light might also include infrared and/or ultraviolet bulbs. These are outside the color range most people can see. You might think the bulb is burned out or not working when looking at them.
Lumens are a rating of how strong the light output of a bulb is. The higher the lumen rating is, the stronger the light produced by the bulb is.
Unless you have a reason to do otherwise, novice marijuana growers should look at the lumen rating when comparing different bulbs of the same wattage and type.
For example, most 400 watt metal halide bulbs have a lumen rating between 30,000-40,000 lumens. If you were only going to purchase a single bulb, you should buy the 400 watt mh bulb with the highest lumen rating, that falls into the price range you can afford.
Based on personal observation, metal halide bulbs rated at 5000k to 7500k are best when metal halide is the chosen light at any stage of growth, be it germination, clones, seedlings, vegetative, or flowering stages..
Metal halide bulbs rated at 5000k to 7500k will keep plants growing a bit shorter than lower k metal halide bulbs during the early stages of life. A higher k rating will limit the height of the plants slightly more than a lower rating.
It should be noted that most growers choose hps bulbs for flowering because of the higher lumen rating, which equals a larger harvest, when compared to metal halide bulbs of the same wattage.
For example, most 400 watt metal halide grow lights have a lumen rating between 30,000-40,000 lumens, while most 400 watt high pressure sodium grow lights have a lumen rating between 50,000-55,000 lumens.
Ed Rosenthal, in the marijuana grower's handbook, recommends switching from high pressure sodium to a metal halide bulb for the last two weeks of flowering to increase potency.
There are other advanced cannabis growers that recommend going to the extreme of using only 7500k to 10000k metal halide bulbs for the final weeks of flowering to increase potency.
List Of Materials
light reflector hood
optional lux meter
optional light hanger
optional led grow light
optional air cooled lighting
*** bulbs: Depending on the kind of light there are various bulbs available. Get an appropriate light that will work with your light system.
Replace bulbs regularly as they dim over time. See the section of this lighting guide about lifespan of grow light bulbs located further up on this page.
*** fluorescent light: You might want to think about getting an inexpensive 48 inch fluorescent light fixture for the early stages of growth.
A minimum of 60 watts of fluorescent light is recommended for plants in a small soil garden or hydroponic system that covers a small area, like a cloner.
A larger soil garden or hydroponic system where the plants are spread out over a larger area might require a more wattage, or several fixtures.
*** dimmable ballast: A dimmable ballast can be run at different wattages, allowing adjustment of the strength of light being supplied to the plants.
Most, if not all, will allow either metal halide or high pressure sodium bulbs to be used without the need to change any settings.
See the sections of this guide about harvest size based on lighting and dimmable ballasts to help you make a decision on what is appropriate for your needs.
Do not plan or growing under a 1000 watt light unless you have an adequate ventilation or cooling system installed. A more realistic expectation for most people is 250-600 watts of light.
For flowering, 250 watts or more of metal halide or high pressure sodium is required. Even a 250 watt light can heat a small grow area to dangerously high temperatures.
*** light reflector hood: A metal halide or high pressure sodium bulb is put in a reflector hood to focus the light they produce on the plants growing under them.
The area covered with the most intense light will be determined by the shape and size of the reflector that the light is housed in.
A light in a square or circular shaped hood reflector will cast maximum light over a square or circular shaped area underneath it.
If the same light were placed in a rectangular reflector the maximum intensity light would be cast in a rectangular area under the light.
A larger reflector hood will spread the light more evenly over a larger area than a smaller reflector hood will. In contrast a smaller hood will concentrate the light in a smaller area.
Keep these points in mind when getting a grow light system and install a light reflector hood that is roughly the same shape as your soil garden or hydroponic system.
*** optional lux meter: You want plants to get close to the grow light without being harmed. Some people try to limit the amount of light to a certain level, saying too much light can actually slow growth rates.
An inexpensive lux meter can be utilized to measure light levels from metal halide or high pressure sodium bulbs. Position the meter at the spots where the plants are closest to the light.
At the places where the plants are closest to the light, most people that use a lux meter for this purpose say, the meter should read no more than about 50,000 lux for optimal lighting levels with marijuana plants.
These growers try to aim for about 50,000 lux in the belief that anything over 50,000 lux is not used by the plant. And at levels over 75,000-90,000 lux growth can be slowed.
To lower the lux reading, the light reflector hood can be raised. In addition, the lux meter can be positioned at different points to see how much light is getting to different parts of the grow area.
Sometimes moving the light or containers the plants are growing in, a small amount, will help make sure that the buds are positioned where they get as much light as possible, without being harmed.
*** optional light hanger: For hanging the light and adjusting the height, you can make a light hanger or buy a pre-made kit.
*** optional led grow light: Having tried since 2008 with several led grow lights that consumed 90-250 watts of power, it was found that a 250 watt led did not preform as good as a 250 watt mh or hps light
Some growers report good results with led as the primary light for flowering. It is suspected that this might be novice growers who have never flowered under anything but led.
However, led grow light panels under 60 watts, have been found to quickly pay for themselves when utilized as supplemental lighting. Two or more lights, around the dark edges of the growing area.
*** optional air cooled lighting: Grow lights can produce a lot of heat. Sometimes this heat raises temperatures above a safe limit for the plants.
An air cooled lighting system has an enclosed light that utilizes air to provide cooling. You can buy light systems that come with an air cooled enclosure or you can buy an enclosure to put your existing light into.
The enclosed light has an air intake supply and an air exhaust. When the air exhaust is fed to another room or outside, most of the heat produced by the light is expelled from the grow room.
See through plastic or glass is fitted to the bottom of the unit, or around the light, so light can pass through but most heat doesn't.
An inline fan is then hooked up to the duct one or both ends. Heat can be quickly expelled through the air exhaust and replaced with cooler air from the air intake.
1 --- When switching from fluorescent to metal halide or hps for flowering, do so no later than when you start the 12/12 light cycle to force flowering.
You could start mh or hps for germination, clones, seedlings, vegetative stages, but as was mentioned, don't wait any longer to introduce mh or hps than when the 12/12 light cycle is initiated.
2 --- To keep the height of the plant from increasing too much during flowering, if you are going to run high pressure sodium, instead of running hps before or at the start of the 12/12 light cycle, wait until plant growth has slowed.
Run metal halide before or to start the 12/12 light cycle, and then switch over to high pressure sodium a few weeks into flowering, when the height of the plant slows or stops increasing.
3 --- Never touch a metal halide or high pressure sodium bulb with your hands. Body oils will shorten the life of the bulb. If you do touch a bulb, wash it off with a soft clean cloth that has been dampened with water.
Wait till the bulb is completely dry before turning it on, water can cause the bulb to crack. A lit, or hot, bulb can crack or explode if it comes in contact with liquids like water.
4 --- The most common mh and hps bulb wattages for growing marijuana are 250, 400, 600, 1000 watts. Metal halide and high pressure sodium lights can be run at the same time for a more balanced light spectrum.
This is not always possible because the cost of multiple grow lights will be more that of a single fixture. Also, a multiple grow light system will also take up more space. But if possible, can be more beneficial to the plants..
One of the better ways to accomplish this is with 3 lights. A stronger high pressure sodium light is placed at the center of the garden, and 2 lower wattage metal halide lights are hung closer to the darker edges.
5 --- The author of this guide prefers metal halide grow lights when flowering marijuana plants for several reasons. They do not generate as much heat, so it is easier as well as cheaper, to cool and maintain favorable temperature conditions in the grow rooms.
The light that metal halide bulbs emit is close to sunlight in color, this makes it easier to see the visual clues a plant gives that relate to its health and overall well-being. HPS light makes it harder for him to see these signs.
For him, the potency of metal halide grown plants surpasses what high pressure sodium would allow him to produce under the same conditions. The trade off is the less intense light results in a smaller harvest.
He does not recommend this approach for those who are after the largest harvest weight possible, but thinks it could be worth a try for experienced growers who might prefer potency over quantity.
Metal halide bulbs that produce light between 4100k-7500k make visual inspection of plants similar to what it would be outdoors on a sunny summer day. 8000k-20000k metal halide light is more blue/purple in color, which is harder to view under.
6 --- If started under fluorescent light, when switching to mh or hps in order for the heat produced by the bulbs not to harm plants, a 250 watt light system should be started 18-30 inches/46-76 cm above the tops of the plants.
A 400 watt light system should be started 3-4 feet/91-122 cm above the tops of the plants, A 600 watt light system should be started 4-5 feet/122-152 cm above the tops of the plants, a 1000 watt light system should be started 5-6 feet/152-183 cm above the tops of the plants.
This may seem excessive but it is better to be cautious than harm the plants. Assuming the heat doesn't have a negative effect, the light should be lowered a bit everyday until it is as close as it can be, without harming the plant.
When mh and hps bulbs get within 12-24 inches/30-60 cm of the tops of plants, the heat produced by the bulb will probably burn the tops of plants that are closest to the light. The higher the wattage, the more heat a bulb will produce.
Air cooled grow lights will allow the plants to get closer to the light without getting burned. You will probably be able to get lights within 6-12 inches/30-60 cm of the bulb, or closer.
Wether you choose air cooled lighting or not, you will have to figure out the optimal distance yourself because it will vary depending on wattage, bulb efficiency, quality of reflector, and other factors. As the plant grows, you will have to raise the light to maintain a safe distance.
Marijuana Grower's Handbook
Very good book by Ed Rosenthal that encompasses indoor and outdoor marijuana cultivation with soil and hydroponics. A reference manual with over 500 pages and color images throughout.
Marijuana Grower's Handbook
The Cannabis Encyclopedia
Easy to read with comprehensive focus on growing and consuming for medical and recreational purposes. This will be of interest to growers, patients, caregivers, consumers, or anyone interested in consuming, growing, or producing cannabis products.
Over 2,000 color images on 596 pages. Loaded with recent information covering all aspects of marijuana. The grow section explains hydroponics and soil growing, growing indoors, outdoors, and in greenhouses.
The Cannabis Encyclopedia