Plants are a paradigm of efficiency. They are unique in the biological world in that they have a vertical monopoly on food production and consumption- they produce their own food. It is amazing enough that this is unique in thebiological world, but it is even more impressive that the entire scope of living life on Earth relies on fundamental plant processes- the production of oxygen and the source for basic nutrition. For a detailed explanation of photosynthesis and other plant processes go to the

Photosythesis : The process by which carbon dioxide and water are combined in the presence of light energy and chlorophyll to form carbohydrates. Photosynthesis takes place in the plant cell's chloroplasts. Inside the chloroplasts, chlorophyll absorb light energy from the sun. The chloroplasts then use that energy to jumpstart the process of photosynthesis. The carbohydrates/ sugars are the plant's internal energy storehouse; they are used to build and maintain plant tissue.
plant bio page of this website.

The energy for a plants food production engine comes from light, in the form of wavelength energy. In regards to indoorgrowing, think of light to a plant the same way you would the sun to the Earth. Without the sun's energy, life on Earth would not be sustainable; similarly, a plant cannot maintain plant processes without adequate influx of light energy. The sun is every spectrum of light, but a plant only uses certain aspects of this spectrum to accomplish food production. Bottom line, in an indoor scenario it is the goal of the gardener to replace the sun.

Generally, plants use blue light during the vegetative stage and red light during the flowering

stage. Think about this in outdoor terms. In North Carolina we experience seasons- times of year when the climate is different, but predictable (at least for now). In the spring, at the beginning of the outdoor growing season plants are vegetatively growing, there is an 18-hour day and a bright blue sun. In the fall, when a plant is flowering, there is a 12-hour day with the sun lower in the sky, resulting in red light from the sun. Plants have evolved to use this variance to induce specific plant processes. It is standard procedure to use blue light to "veg" and red light to bloom. However, photoperiod is very general in nature. You can imagine a plant endemic to the higher latitudes would be more prone to this phenomenon than a plant on the equator that experiences relatively consistent weather year round. The best way to determine if changing your photoperiod has a beneficial effect on your plant growth is to test it yourself. The proof is in the pudding!

There are many different forms of light. Here we will focus solely on those suitable for growing plants.

Types of Indoor Lighting

High Intensity Discharge (HID)
There are several types of HID lighting, but only two generally accepted for plant growth: Metal Halide (MH) and High Pressure Sodium (HPS).

MH
Metal halide lamps are rich in the blue spectrum, which is very close to full summer sun. This promotes fast vegetative growth and compact, stocky plants with short internodal leaf spacing. If plants are not being grown to flower, a MH light is all that is needed.

HPS
High-pressure sodium lamps are high in the red and yellow spectrum, which imitates the fall sun. HPS lamps promote fruit and flower production. However, any premium full-spectrum HPS light can be used from seed to harvest with minimal sacrifice in production.

 

HID vs. Fluorescent's
The fundamental difference is that fluorescent's create light by passing electricity through a gas vapor under low pressure and HID's create light by passing electricity through a gas vapor under high pressure. The light emitted by fluorescents is gentler and more diffuse; it doesn't release a lot of heat, and doesn't make the plant work as hard. For this reason they can be kept closer to the plant. HID lights produce more heat than fluorescents, resulting in a potential liability in an unventilated growroom (PG always recommends ventilating your growroom. However, HID's have a more intense, and therefore further penetrating light spectrum. For this reason, HID's are preferred for larger plants, for plants that require high levels of light, or situations where large areas need to be covered.

Fluorescents
Until the introduction of compact fluorescents, fluorescent lighting was mainly used for propagation and early vegetative growth. Traditionally, the efficiency of fluorescents has not been comparable to HID's:

 

(42) of the average 40 watt 4' fluorescent tubes (1680 lumens/bulb, 31.5 lumens/watt)

=

(1) 400 watt HPS lamp (50,000 lumens/bulb, ~130 lumens/watt)

That's 1680 watts to accomplish the light out put of one 400-watt lamp!
(or ~ $9.00 vs ~$36.00 on your power bill every month)

You can see how the efficiency of the lamp is so important in choosing your light and for your power bill. It's the ongoing cost no the upfront cost that is so important when choosing a light.

With the developments of compact fluorescents and T5 technology, the light output of fluorescents rivals that of HID's. Each 54-watt bulb in the Tek-Light produces 5000 lumens:

5 Fluorescent (216 watts) (20,000 lumens, 92.59 lumens/watt)

=

250 MH (23,000 lumens, 92 lumens/watt)

=

250 HPS (28,500 lumens, 114 lumens/watt)

Choosing a Light

Type of plant

Crop's containing leafy plants, such as lettuce, herbs, spinach, etc., can be maintained using blue light only. Plants being grown for their fruit or flowers will benefit greatly from an influx of red light to optimally grow and flower. If one light must be chosen, metal halide light most resembles the sun and is adequate for any and all growth. However, progression in the light manufacturing industry now allows seed to harvest growth under any premium full-spectrum bulb provided there is sufficient overall light levels. The only way to definitively determine what works best for you is to try it out. Experimentation is a bastion of indoor gardening and is half the battle in creating an ideal plant environment.

How to determine light efficiency

Plants "see" light differently than human beings do. While they are a good general standard to measure light efficiency, lumens, lux, or footcandles, should not be taken as gospel for plant growth since they are measures used for human visibility. A more correct measure for plants is PAR

Photosythetically Active Radiation (PAR) : used to refer to the portion of the light spectrum optimal for plant growth, namely about 400 to 700 nanometers in wavelength.
watts. In addition to quantity of light, considerations of quality are important, since plants use energy in different parts of the spectrum for critical processes. For a detailed discussion of how to determine light efficiency for plants see: http://www.sunmastergrowlamps.com/SunmLightandPlants.html

               

How much light do I need?

Technology has advanced so much in the last 15 years that we are constantly refining the process and updating what we know works best for growing. Current theory holds that the minimum amount of lighting needed to sustain a good rate of growth is around 20-25 watts per square foot. Mid range is around 30-45 watts per square foot. Optimal is 50-75 watts per square foot, but 75-100 watts/square foot is not unheard of. There is really no such thing as too much light, but using an HID light in a small space may result in high temperatures that are hard to control. To determine adequate light for your space use the Calibration and Conversion tables.

Which type of lamp is best for me?

As you will notice, there are many different kinds of HPS lamps and many kinds of MH lamps. Some are definitely better than others. For example, notice the increase in light between the Hortilux Blue MH (left) and standard MH (right) below:

                         

Standard HID lamps were originally designed for streetlights and stadiums, not for growing plants. A hortilux or "full spectrum" lamp was designed specifically for horticulture and is recommended by PG for all stages of growth, even switchable systems. However, standard MH lamps can be used with success with a red supplement (such as a low K fluorescent, or standard HPS) for flowering plants. Standard HPS lamps utilized from seed will result in leggy plants that are stretching looking for the blue light absent in standard HPS lamps. If using one lamp from seed to harvest using HPS lighting always use a Hortilux or "full spectrum" lamp.

When using florsecents pay attention to the Kelvin (K) rating. The lower the K value the more red the spectrum and vice versa. Aim for ~6,000 - 6,500 K for a blue spectrum and 2,700 - 2,500 K for red spectrum lamps.

 
    

Using your Light  

Light Movers

The most efficient way to use HID lights is to have them moving within the growroom. Light movers can increase the coverage area of your light and significantly improve the dispersal of light throughout your garden. There are many advantages to this, and a number of different ways it can be done. Moving the lights will eliminate plants tendency to grow toward the light source and provide light to areas which otherwise may be shaded. Since the light is moving, it can pass quite close to the plants without burning the leaves. The size and shape of your room will determine the type of light mover that will best suite your needs.

Linear movers

Lineal movers carry the light fixture slowly along a track and back again during the light cycle. Most are six feet long, support a single lamp, and are recommended when the growing area is long and narrow.

Circular movers

Circular movers are best when the length and width of the room are similar. They are designed to carry one, two, or three lights, in a 360-degree circle, ideally lighting a ten by ten foot area. This diameter can be reduced but rarely extended. Two arm and three arm movers are most popular, with the latter supplying much more light per square foot.

Electricity Cost

The average garden light will increase your power bill $5-10/month depending on the exact size of the system and photoperiod of the light. To calculate the exact cost of using your light refer to your power bill and the Calibration and Conversion tables .

Voltage

Most high intensity lights can be run on either 120 volt (standard house current), or 240 volt (e.g. used for electric dryers, stoves, etc.). Electricity cost would be the same but the latter would draw half the amps allowing the grower to run twice as many lamps on the same electrical circuit.

Timers

Timers are essential for a properly operating growroom. By nature, a plant requires darkness for at least six hours daily to incorporate the food that it makes for itself during photosynthesis. It is a common misconception that 24 hours of light on a plant benefits its growth. While some plants can be more receptive to this than others, it is a good idea to allow the dark period so as not to overexert your plants. A light timer is the bottom rung in automating your grow room. Many growers do not realize the extent that a grow room can be automated. For details pertaining to grow room automation go here . Automation can not only reduce maintenance and electricity costs, but significantly increase yields by way of a more idealized growing environment. Light timers are available for either 120 or 240 voltage, but always check to see that the amperage rating on the timer exceeds that of the light or lights.

Lamp and Ballast Maintenance and Troubleshooting

A lighting system consists of a lamp, ballast

Ballast : a device used to regulate flow of electricity to match the needs of a specific bulb.
, and reflector. Most lighting systems come with some sort of warranty. The extended warranty usually comes with the ballast since it is the most technical component of your lighting system. PG offers 5-year warranties on all HID system ballasts and 1-year warranties on all HID lamps. Your reflector does nothing but reflect so there is no need for a warranty.

In putting together a lighting system it is crucial to match the wattage on the lamp with the wattage on the ballast, and also to match Halide bulb with Halide ballast and Sodium lamp with Sodium ballast (except switchable ballasts and conversion lamps). The ballast acts as a current control for the electricity coming out of your socket. Because of this, the wattage of the lamp must match up with the wattage of the ballast or the current will either be too strong or too weak for the lamp, resulting in a shorter life span or non-start situation. In the case of matching MH with MH and HPS with HPS, the only
difference between a MH and HPS ballast is the presence of an ignitor in the HPS ballast. If a MH lamp is put into a HPS ballast the MH lamp will receive an ignition charge it is not manufactured to withstand resulting in a decrease in lamp life. Conversely, if a HPS lamp is used in a MH ballast the HPS lamp will not receive the ignition charge necessary to "fire" the lamp, resulting in a non-start. Reflectors are universal; they have nothing to do with wattage or electricity. They simply direct light down onto your garden. It is a good idea to change your lamp once a year. HID lamps act off of excited gases. The gases get used up over a period of time by undergoing chemical reactions with the glass tube encasing
them and after about a year up to half of the usable light will be gone. You may not be able to see this with your eyes, but your plants will show you the difference. It may take up to 30 seconds for the lamp to ignite and up to five minutes to reach full brightness. As a lamp ignites, it tends to flicker and change color for several minutes. This is quite normal, especially with halide lamps, which may appear to change color slightly during normal use. If the lamp does not ignite after 30 or 40 seconds, unplug it. After the power has been disconnected, check:

•  that the lamp is screwed in all the way

•  that the timer is set on the "on" position

•  that all plugs or electrical connections are O.K.

NOTE: Do Not Open The Ballast Enclosure To Check Wiring Yourself!

Heat kills ballasts. It has been said that a ballast operating with an internal temperature of 150 degrees or less can last for 20 years. Every degree above 150 decreases the life of your ballast by 2 years! So it is important to maintain a temperature of less than 150 degrees inside your ballast. This does not take special attention for normal growing temperatures will not allow this threshold to be reached. With a remote ballast you can simply keep your ballast outside of a warm grow room if need be.

Recycle your lamps: http://www.nema.org/lamprecycle (or lamprecycle.org)

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