Plant life is essential to the existence of mankind.  The entire human species as well as all animal life are dependant on plants for survival. Plants provide a life sustaining food source through the chemical energy they produce which has its source in photosynthesis.  Plants, through photosynthesis, provide directly or indirectly all the essential amino acids and most of the vitamins necessary for health.

Photosynthesis is the production of sugar from carbon dioxide and water in the presence of chlorophyll, using light energy and releasing oxygen. 

One of the major external factors effecting plant growth and photosynthesis is light.  The intensity and duration of the light supplied has a tremendous influence on plant growth.  Plants grown with insufficient light will exhibit tall, spindly, elongated stems.  Leaves, if they develop at all, will not properly expand. Without sufficient light a plant's ability to produce chlorophyll and utilize nutrients efficiently is impaired causing leaves and stems to turn pale yellow.  It is impossible to bring fruit or flowers to their full potential without adequate light.

Photosynthesis is directly proportional to the intensity of the light source, meaning that the number of photons absorbed is the most important factor for photosynthesis.  This means that as the intensity increases, the rate of photosynthesis in a plant also increases.  Photosynthesis will increase until it reaches the plants light saturation point.  The light saturation point is the point above which an increase in light intensity does not result in an increase in photosynthetic rate.  The light saturation point of a plant determines the relative light requirements of a plant.

We have not been able to duplicate the life supporting energy that is produced through the process of photosynthesis in any synthetic form. However, through years of research we have been able to adequately replicate the light source responsible for making photosynthesis possible.  Scientists have harnessed the blue and red light spectrums produced by the sun during the vegetative and flowering stages of plant life.

In the past, crop production has been limited to environmental conditions and seasonal changes.  Even in glass house cultivation the amount of natural light delivered in short daylight seasons was not sufficient to adequately flower and fruit crops.  Supplemental light was necessary to simulate a longer day period to enhance vegetative growth or trigger flowering.

Sophisticated technology has gone into the development of artificial lighting for the horticulture industry.  Indoor crop production can now continue cost effectively, throughout the year, regardless of external conditions.  There are lamps designed specifically to simulate the light required in the early stages of growth and others that will mimic the late season hues of harvest time.  Anything cultivated outdoors in any climate or season can be reproduced indoors with the new artificial light sources. 

High Intensity Discharge (HID) lamps are a type of light that uses electric arc to produce light. Metal halide and high-pressure sodium lamps are part of this group of lamps.  Used in a combination these two types of HID lamps can be a complete substitute for natural sunlight.  Mercury vapour lamps were a popular member of the HID family but proved inferior to the more efficient metal halide and high pressure sodium lamps.  They are virtually obsolete in the plant growth industry and not recommended by professional growers.

An HID lamp or bulb must be run by a remote ballast and requires a special light socket called a mogul socket.  The ballast consists of a transformer and a capacitor and in the case of high pressure sodium lamps an igniter.  All these components are housed in a vented, galvanized metal box which is wired with a power cord and plug.  There is another cord that reaches from the ballast box to the mogul socket and assembly.  The mogul socket is a large ceramic socket designed to accommodate the oversized base of an HID lamp and conduct the high voltage.

A light reflector is attached to the mogul socket assembly and shields the lamp in an HID setup.  The reflector is essential to ensure that all the light is reflected toward the plants and not wasted throughout the room.  The style and size of the reflector will depend on the size of lamp, desired light intensity and area of coverage.  Reflectors are designed to accommodate lamps that burn horizontally or vertically and are available in a range of reflective materials.  The most popular being white powder coated finish made from cold rolled steel.  Some reflectors are enclosed and vented with fans to exhaust built up heat that occurs with the high wattage HID lamps.  Reflectors range in shape from rectangular, cylinder, parabolic and 'M' designs.

HID's decline in lumen output slowly and gradually over the life of the bulb.  Technically a bulb can last 12,000 hours but the efficacy will be substantially diminished after about 1 year of continuous use at 18 hours a day.

Metal Halide is the technology that brought the sunshine indoors. By carefully blending specific halides and metals in an arc tube scientists were able to develop a line of metal halide lamps that are designed specifically for plant growth. The new metal halide lamps deliver the desired spectral output of colour and the necessary intensity to generate photosynthesis in plant life. An abundance of blue light emitted by a regular metal halide makes them the best light for propagation and vegetative growth, promoting short internodal length. 

High Pressure Sodium lights do not emit as broad a spectrum as metal halides lights, but have some advantages, especially when used in conjunction with halide. Sodiums last longer, and burn brighter, but are significantly more expensive. High pressure sodium lamps are used primarily in the flowering cycle of a plant's life as they emit a red/orange light that is typical of autumn's harvest sunlight.  Glass house production incorporates high pressure sodium lamps to supplement daylight.

Traditionally a metal halide lamp and ballast were required for the vegetative growth cycle. The grower then had to switch to a high pressure sodium lamp and ballast to utilize the red light required for flowering.

Metal halides have advanced to the degree that there are lamps designated for very specific stages of growth. The new lamps provide richer, healthier, more nutritious light by blending the halides of different metals in the arc tube design. Lamps designated as 'cool' are for spectacular vegetative growth! Their deep cool blue colour promotes vegetative growth, leafing & rooting.  'Warm' rated lamps encourage dramatic flowering! These lamps sheds a very warm red/yellow spectrum promoting lush, compact flowering.  The is also a 'neutral' lamp that is suitable for all phases of plant growth! The balanced light carries a wide spectrum with characteristics of both 'warm' and 'cool'. When general growth is desired, for a variety of plants use this throughout the life of the crop.

As you can see, it may not be necessary to switch to a high pressure sodium lamp and ballast when flowering is desired.  There are several choices of metal halides on the market that have a full spectrum.  It is simply a matter of changing the bulb from the predominately blue (cool) spectrum metal halide to the red/yellow (warm) spectrum metal halide. There are also metal halide conversion bulbs that emulate the high pressure sodium spectral output but use of these bulbs is not encouraged because of their high cost and inefficiency.

Both metal halide and high pressure sodium bulbs go through a stage of 'seasoning'.  Lamps will dim and brighten and change colour slightly due to the gasses burning in the bulb. Colour and light intensity will stabilize after about 100 hours.

The popularity of indoor plant propagation has risen significantly over the past two decades.  HID lamps are readily available to the retail consumer at reasonable prices. Systems are CSA approved and merely need to be plugged in to operate.   As with any electrical apparatus caution must be exercised during the operation of an HID lamp.  Always make sure that the ballast is elevated and not exposed to water.  Ensure the environment is safe and secure and wires are up and out of the way.  Remember that HID lamps are hot.  Take extra caution not to hit the bulb with overspray when you mist your garden as bulbs could crack.

HID lamps can be used to keep tropical flowers in bloom throughout the gloomy days of winter and add indoor sunshine to your life. Light up a garden and enjoy fresh herbs, berries, vegetables and salad greens year round.  Get growing!