Detailed information about indoor hydroponic lighting systems for growing marijuana.
Marijuana uses all light from the visible electromagnetic spectrum, and some light that is outside of the visual spectrum, including UV light.
Light bulbs do not emit a equal intensity of light across the entire wavelength of light. Some bulbs are mostly blue (cool) and some are mostly red (warm). There are also full spectrum bulbs that produce a balance of red and blue. The wavelength of light that a bulb favours is sometimes referred to its “colour temperature”.
Cool light is what you see in a sports stadium being used to light the field. It has a blue / white colour temperature. Warm coloured light is what you see coming from a street lamp. It has an orange colour temperature.
High pressure sodium lamps (HPS) bulbs are more intense in the red spectrum. Metal halide lamps are more intense in the blue spectrum. In the example above, the sports stadium uses metal halide and the street lamp is HPS.
Many growers believe that blue spectrum light promotes root growth during the vegetative growth phase, while red spectrum light promotes more flower growth during the bloom phase. It is common practice to switch from metal halide in vegetative growth to hps during flowering to provide the right wavelength to optimize the different growth phases. There is no conclusive scientific evidence that the light colour provides these benefits. The practice of switching the bulbs / wavelength for the growth phases is an optimization technique, not a necessity for growth. Cannabis will grow under exclusively metal halide or exclusively high pressure sodium.
Since high pressure sodium draws less power to create the same volume of light as metal halide, many growers favour high pressure sodium and use it for all or most of the growth cycle to save money on electricity.
A unique benefit to metal halide bulbs is they can emit more ultraviolet light. Ultraviolet light promotes THC production. If you are using HPS for your grow, a trick you can try is to switch to metal halides in the last 2 weeks to increase bud potency.
tip: If using metal halide lighting to expose plants to UV, remove protective glass on your light hood. The protective glass blocks the UV light.
safety note: In addition to UV, Metal halide bulbs can release other deadly forms of radiation if the outer bulb is broken. If you have a break in the outer glass bulb, turn it off right away and throw it out.
Photoperiodism refers to developmental responses in plants due to changes in light cycles. The chemistry of a marijuana plant changes according to the percentage of light vs. darkness it is exposed to in a 24 hour period. Specifically, marijuana transitions from a vegetative (grow) phase to a flowering (bloom) phase when kept in the dark for between 9-11.5 hours, for 5-7 days consecutively.
When evaluating light systems the important thing to consider is the total amount of light output, not the amount of power the bulb draws. The wattage of a bulb is not useful information because does not tell you the amount of light the bulb will produce. A helpful measurement of light output is Lumens.
A 400 watt HPS lamp gives off the same amount of Lumens as a 200 watt LED light. Watts is an irrelevant measurement. Light output is what matters.
More light output results in stronger growth.
The more light that reaches the plants, the more they will grow.
Light output diminishes the more it spreads. Good garden design, including use reflectors, positioning of plants, and use of reflective material for the walls, help optimize the actual amount of light the plants get.
Pruning techniques also help optimize the available light.
Low light will cause stems to stretch and buds to not be tight.
Fluorescent bulbs are available with different light spectrums. They are labelled as warm white or cool white. There are also bulbs that are labelled as being specifically for plant growth. Cool white emits more light at the blue end of the spectrum. Warm white emits more from the red end of the spectrum. Bulbs that are labelled specifically for plant growth emit light from both the red and blue end of the spectrum.
T-5s are the best fluorescent lighting system because they output more light and are more compact than T-8 and T-12s.
Comparing fluorescent with high pressure sodium
8, 4 foot T5 HO tubes produce 40,000 lumens and draws approximately 435 watts of power. A 400 watt HPS light uses 440 watts and emits 50,000 lumens. Although 400 watt HPS is still slightly better, fluorescents do nearly the same job for the same amount of power draw. Since they distribute heat over more bulb area they can be positioned closer to the plants for less lost light.
Compact fluorescents (CFLs)
Compact fluorescents are great for rooting clones or growing seedlings.
An advantage they have is they have is a small footprint. They can be positioned close to the top of the canopy and direct light right down on to the plants.
To use them efficiently, create an enclosure using mylar so that light is bounced back onto plants, or install them in a reflector that bounces light down. Without reflection, these bulbs will scatter light in all directions and lose efficiency. They should also point down onto the plants because most light from these bulbs is emitted from the top of the bulb.
Metal halide lamps emit light that is less intense in the red spectrum and more intense in the blue spectrum.
Metal halide is often used for vegetative growth because it reportedly promotes promotes stronger root growth compared to high pressure sodium (HPS). However, there is no conclusive evidence that this is true. If it is a myth, high pressure sodium would actually be a better choice for vegetative growth because it is more economical.
Metal halide used in the last 2 weeks of flowering will boost the THC content and make buds more potent.
Metal halide light systems are available in different strengths (400 watt, 600 watt and 1000 watt). They deliver a lot of lumens and are a top choice for lighting grow rooms.
High pressure sodium bulbs emit light that is higher in the red spectrum and lower in the blue spectrum.
HPS is often used during the flowering phase because red and orange light seems to promote more flower growth.
High pressure sodium can also be used for the vegetative growth phase. It emits 15% more light than metal halide and therefore is more economical.
If using high pressure sodium for vegetative growth, a wide spectrum bulb is ideal because it includes some blue light.
High pressure sodium light systems are available in different strengths (400 watt, 600 watt and 1000 watt). They deliver a lot of lumens and are a top choice for lighting grow rooms.
|Floor space||HPS light||Watts per square foot|
|2′ x 2′ (60x60cm)||250 watt||62.5|
|2.5′ x 2.5′ (76x76cm)||400 watt||64|
|3.25′ x 3.25′ (100x100cm)||600 watt||56.8|
|4′ x 4′ (120x120cm)||1000 watt||62.5|
Convertible digital ballasts power both metal halide and high pressure sodium bulbs and are vastly superior to magnetic ballasts. They are more efficient, make no noise, weigh less, are easier on the bulb, are safer, give off less heat, and although they are slightly more expensive at first, are cheaper in the long run in energy savings.
LED lights have a several advantages over other lighting systems. They create little heat, have a narrow profile, and are energy efficient.
Although they cost more than HPS lighting to buy, there are several ways LED saves money over time. LEDs use less than half the electricity of HPS lighting to generate the same of amount of lumens. A 300 watt LED light gives off the same amount of light as a 600 watt HPS bulb. The LEDs also last longer than HPS bulbs. If heat is a problem in your grow room, LEDs can prevent a need for having to purchase extra cooling or ventilation equipment.
LED light banks contain a combination of red, blue, and sometimes green and amber LEDs to provide a wide spectrum of light for your plants.
The design of your reflectors has a big impact on how much light that your plants actually receive vs. how much is wasted. A bad reflector can cut the amount of light that hits your garden by half.
Most fluorescent tube ballasts are not efficient because they place the tubes too close together and most of the the light gets trapped. Only 60% of the light even makes it out of the reflector. An efficient fluorescent tube fixture has reflectors between the individual tubes.
For CFLs, a bowl reflector works well.
Metal halide and high pressure sodium reflectors
MH and HPS bulbs emit most lights from their sides, so a horizontal orientation of the bulb is best for horizontal gardens. Vertical orientation is not as good unless the garden is a vertical style design.
The style of horizontal reflector you should choose depends on the size of your garden. Some reflectors are designed to broadcast the light over a wide area and others focus the light more tightly. Choose a reflector that best suits the design of your garden.
Heat build up can be a problem in the garden and MH and HPS lights produce a lot of it. An air cooled reflector has ports on either side to attach venting hoses and the front is enclosed in glass. Fans blow air through the reflector moving 60-90% of the lamp heat out of the room.
For horizontal gardens, create a wall around your garden using mylar or white reflective sheeting. This will bounce light back onto the plants and help you get more usable light from your lamps.