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Lighting is one of the five major factors to consider
when creating a support system for your plant life, (water, nutrients, oxygen
and CO2 are the others). Thus if you plan to grow inside, out of season,
you must supplement the sunlight normally required for proper growth.
This can be done with three different kinds of lighting, incandescent,
fluorescent, and H.I.D. (High Intensity Discharge). These are the most
popular types of lighting for gardening purposes. There are many other
kinds of lighting, but they don't produce proper color (kelvins) that is
suitable for growing.
Incandescent lighting- These are the least efficient and least effective method of lighting, for plant
growth. Incandescent lighting is the same as what is used in the average
medium base household fixtures. This type of lighting is a very poor
choice of lighting for your garden because of their inefficiency.
Fluorescent lights- These are a little better choice than incandescent lights due to the fact that
they are about twice as efficient (lumens of light output per watt of
electricity used). However these lights are not very effective in comparison to
an HID (High Intensity Discharge) system and these lights must be less than 12
inches away from their subjects to ensure successful growth. However,
some find that our T-5 fluorescent lights are perfect for use with seedlings
and cuttings as well as for use with other plants that do not require the high
intensity of H.I.D. (High Intensity Discharge) lighting.
H.I.D.
(High Intensity Discharge)- These lights are by far the most efficient
and effective lights being used by growers today. There are two types of
H.I.D. (High Intensity Discharge) lamps that have suitable light spectrums
(color range) and intensity for plant growth. MH (Metal Halide) lamps are
rich in the blue light spectrum, similar to full summer sun. HPS (High
Pressure Sodium) lamps are rich in the red/yellow spectrums, similar to the
color of the fall sun.
METAL HALIDE
(MH)
MH (Metal Halide) bulbs are very efficient and produce
between 70 and 115 lumens of light output per watt of electricity used. MH
(Metal Halide) bulbs produce a light that is very close to full summer sun,
with a spectrum rich in the blue end. This promotes fast vegetative growth and
compact, stocky plants with short internodal leaf spacing.
MH (Metal Halide) bulbs create light by passing
electricity through an clear inner arc tube that is enclosed in the vacuum of
an outer clear glass tube. This inner arc tube contains mercury and other
metals in iodide form. When electricity is applied to these metal iodides
they give off very intense light and heat. The outer casing can also be
phosphorus coated. Most gardeners prefer the clear bulb, as it produces
the brightest white light available.
MH (Metal Halide) bulbs come in sizes from 70 to 1500
watts with the 250 w, 400 w and the 1000 w being the most popular sizes for
gardening. All MH (Metal Halide) bulbs need to run with a ballast (a ballast is
a transformer that steps up the voltage to the proper amount needed to ignite
the iodides), that is designed to run that specific bulb size.
The bulbs themselves need to be burned in a specific
position. They come in three types: Vertical (marked BU or BD), Horizontal
(marked HOR) and Universal (marked U). The universal bulbs can be burned in any
position, but they still are more efficient when burned vertically.
MH (Medal Halide) bulbs should be replaced about every
10,000 hours of use or approximately 18 months (as per an 18 hour / day on
cycle).
HIGH PRESSURE
SODIUM
 HPS (High Pressure Sodium) bulbs are the most efficient
bulbs that are available for grow lights. They are high in the red and
yellow parts of the light spectrum and low in the blue, this imitates the fall
sun. Because of this spectrum some plants that are grown with HPS (High
Pressure Sodium) lights will grow elongated and rather leggy, while many other
plants are not affected by the limited light spectrum. For plants that
normally bud and flower in the fall an HPS (High Pressure Sodium) is usually
the light of choice because it's light spectrum promotes flower
production.
There are color corrected HPS (High Pressure Sodium)
bulbs (such as the Son Agro) available for improved growing with HPS (High
Pressure Sodium) efficiency. These are designed specifically for indoor
horticulture, and have a more balanced color spectrum.
HPS (High Pressure Sodium) bulbs are made out of a
translucent ceramic arc tube containing a mixture of sodium, mercury and xenon
gas. This arc tube is suspended in an outer glass shield (bulb). HPS (High
Pressure Sodium) bulbs range in power from 35w to 1000w, with the 250w, 400w,
600w and 1000w being the most popular for horticulture use.
HPS (High Pressure Sodium) bulbs should be changed no
later than 24 months from initial use. These bulbs (like the Metal Halides)
also slowly lose their brightness over time, so to maintain proper light
intensity the bulbs must be changed at the end of their rated life.
Selecting a Grow Light
Plants have the unique ability to
manufacture their own food. The food
manufacturing process is called photosynthesis. Photosynthesis is carried
out by many different organisms, ranging from plants to bacteria (Figure 1).
The best known form of photosynthesis is the one carried out by higher plants
and algae, as well as by cyanobacteria and their relatives, which are
responsible for a major part of photosynthesis in oceans. All these organisms
convert CO2 (carbon dioxide) to organic material by reducing this
gas to carbohydrates in a rather complex set of reactions. Electrons for this
reduction reaction ultimately come from water, which is then converted to
oxygen and protons. Energy for this process is provided by light, which is
absorbed by pigments (primarily chlorophylls and carotenoids). Chlorophylls
absorb blue and red light and carotenoids absorb blue-green light (Figure 2),
but green and yellow light are not effectively absorbed by photosynthetic
pigments in plants; therefore, light of these colors is either reflected by
leaves or passes through the leaves. This is why plants are green.
The next chart shows the relationship
between chlorophyll activity and color of light. Common electric light sources
are indicated below the chart. You will notice that Metal Halide (MH) and High
Pressure Sodium (HPS) produce light in the most photo synthetically active
spectrums (colors).
As the chart above indicates, cool (blue) and warm
(orange) colors in the spectrum enhance
chlorophyll activity and food production. Cool light is most pronounced during
the summer months when the sun is highest in the sky. It is responsible for
keeping plants growth compact and shapely. Warm light, such as when the sun is
lower in the sky during the fall harvest months, is responsible for triggering
reproduction in plants in the form of flowers and fruits.
After you select the type of lamp you want then you
must decide how many systems will be required to cover the planted area.
The following chart shows the coverage area of the available lamp sizes.
HPS (High Pressure Sodium) lighting can cost up to 15%
more than comparable MH (Metal Halide) systems, however, the bulbs have a
longer life span and also have a higher lumen output than any other H.I.D.
(High Intensity Discharge) lighting. MH (Metal Halide) bulbs lose about
15% to 20% of their light intensity after one year where as the HPS (High
Pressure Sodium) bulbs lose only about 5% of their intensity in the same time
frame. MH (Metal Halide) systems produce between 70 and 115 lumens per watt of
electricity consumed, HPS (High Pressure Sodium) systems produce between 97 and
150 lumens per watt.
Growers continue to debate over which of these types of
H.I.D. (High Intensity Discharge) lighting is the preference for generic
use. But, the rule of thumb is that metal halide light spectrum is best
for dense vegetative growth and the high pressure sodium is best suited for
blooming growth. If you are starting to wonder if this article has a
specific answer to the question of which is better, well, I'll leave it up to
personal preference. Some people have been known to have a MH (Metal
Halide) and a HPS (High Pressure Sodium) lighting system, using the MH (Metal
Halide) for the vegetative cycle and then switching over to a HPS (High
Pressure Sodium) system when it's time for blooming. Another option is to
use a switchable ballast (available in 1000w MH/HPS and 400w MH/HPS).
These are ballasts that allow the grower the option of using an MH (Metal
Halides) or an HPS lamp (High Pressure Sodium), or they can use both (one bulb
for a period of time and then switch halfway through the grow period to the
other bulb, to simulate the grow seasons). These switchable systems are
slightly less efficient than the normal ballasts, but they are much cheaper
than buying an entirely separate system.
Regardless of which system you choose, you should
change out your bulbs no later than the end of their rated life; this is due to
the loss of intensity and therefore rendering the system less efficient.
MH (Metal Halide) bulbs should be changed after 12 to 18 months and HPS (High
Pressure Sodium) bulbs after about 24 months. This will ensure proper
light intensity and therefore proper growth.
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