Light bulbs

Leo Martin
Sun, 05 Mar 2017 10:45:32 PST
Norton asked about supplemental lighting for sprouting bulb seedlings

Technology is changing rapidly. In 5-10 years light growers will probably
be using LED lights, with only a few holdouts. LED lights are more
expensive to buy today, but less expensive over their lifespan when
considering the lower electricity usage. I have read analyses suggesting
the break-even point is 6 months to 2 years.

Light gardeners have abandoned incandescent bulbs. They are inexpensive,
but most of the electricity is wasted producing heat. The bulbs do not last

High-pressure sodium lighting (HPS) is very expensive to buy, uses a lot of
electricity, and produces the most heat of any lighting used for
horticulture. The lamps are so hot a person would get a third-degree burn
from a single touch lasting less than a second. Water on an illuminated
lamp - like from a spray bottle - causes an explosion, showering
near-molten glass all over the growing area. I would not recommend HPS to
any home grower, under any circumstances, because of the heat danger. Much
safer and equally effective alternatives exist. This kind of lamp was used
for commercial indoor herb growing for years, but these growers have
switched to other lighting.

Different kinds of fluorescent lighting work well for growing plants, and
have been standards for some years. Fluorescent grow lights will be
replaced by LEDs over the next few years for reasons of efficiency and
lifetime cost. Another problem with fluorescent lights is that they contain
mercury, and LEDs contain little or none.

Hobbyists used standard fluorescent tubes for years; compact fluorescent
lights (CFL) also work well, and use less electricity. Most indoor
fluorescent light growers choose long tubes because the fixtures fit better
under shelves or over windowsills. Spiral fluorescent lights also work
well. When a spiral lamp is used, the long axis of the spiral should point
down at the plants, since spiral lamps are designed to emit more light
along the axis rather than from the sides. All fluorescent lamps need to be
replaced at least every 12 months. Their light output begins decreasing
within 6 months, and most are down to 75% or less of the initial output by
a year. The human eye does not see this, but the plants do. If you choose
to use fluorescent lamps, write the date placed into service on the tube
with an indelible marker, and replace every 9-12 months. Standard
fluorescent lights are hot, but not as hot as an incandescent bulb. They
will cause a burn but most people would remove their hand from the hot tube
before serious injury.

Long fluorescent tubes are labeled as to the size of the tube. Standard
tubes are T8. They can work well for growing plants if wattage and color
are chosen correctly. Higher-wattage tubes produce more light, and most
people choose 40 Watt T8 tubes rather than 20 W. A very common fixture for
light growers is sold as a shop light; it holds two, 4 foot / 122cm long T8
tubes. They are easy to suspend underneath shelves. Several can be plugged
into a power strip, which can be plugged into a heavy-duty electric timer.

Aquarium, terrarium and some plant growers often use high-output T5 tubes
(T5HO), which produce more light for the electricity used than standard
tubes. T5s are smaller in diameter than T8s and require different fixtures.
High-output lamps require different fixtures than standard output. Many
indoor growers buy T5HO aquarium light fixtures for their plant stands. The
fixtures and tubes are more expensive than standard T8 fixtures and tubes,
but when considering the cost of electricity over the life of the fixture
they cost less to run than T8s. They are readily available at aquarium
shops and online. T5HO fixtures come in different lengths, for different
sizes of aquarium, so it may be easier to find a T5HO fixture to fit a
small space as compared to T8 fixtures. T5HO tubes are hotter than standard
T8 tubes.

Fluorescent lights are available in different light colors, which is
measured in degrees Kelvin (K.) This is a measure of how the light appears
to the human eye, and has nothing to do with the actual Kelvin temperature
scale. It also says nothing about the actual frequencies of photons making
up the light. It turns out that fluorescent lights with a light color of
6,500 K are good for growing plants, so this is what most people buy. Some
manufacturers refer to 6,500 K color temperature as "daylight", but others
use "daylight" to refer to other color temperatures. Look for the number on
the box or label. Other light colors alone are not as good for growing
plants as are 6,500 K. Generally, there is no advantage to adding lamps
with other light colors to lamps of 6,500 K. Aquarists can buy 10,000 K
high-output T5s, which provide brilliant white light. But, plants don't
grow very well under this; it is designed for corals. Many T5HO fixtures
are sold with 10,000 K and actinic blue tubes, neither of which works well
for plants, so it is better to buy a fixture and tubes separately.

LED lights of color temperature 6,500 K are not necessarily the best for
growing plants, The usefulness of 6,500 K color temperature applies only to
fluorescent lights.

Compact fluorescent lamps use less electricity for the light output
compared to standard fluorescents, so many light growers are switching. 4
foot long CFL tubes fit into some standard 4-foot, 2-tube fixtures, but not
all. Some CFL tubes are made to fit into older fixtures, and other need
fixtures specifically for CFL tubes. Most spiral CFL lamps fit into older
standard socket fixtures. The least-expensive solution if CFL is chosen
would be standard 4 foot long shop fixtures, each holding two CFL tubes.
Choose 6,500 K color temperature, and the highest Wattage available. Until
recently indoor commercial growers used CFL lamps of 6,500 K color
temperature at the highest Wattage they could get.

LED (Light-Emitting Diode) lighting is still being developed and perfected.
Until recently attention was placed on light for human use, and not much on
plant growth. So, most knowledge of how LED lighting works for plants has
been trial-and-error. LEDs last perhaps 3-5 years when used 12 hours per
day. They are more expensive to buy, but use much less electricity than
other lamps. Over their life they are less expensive to run.

Individual LEDs emit only certain wavelengths of light, and multiple
wavelength LEDs are combined to give the desired light color as seen by the
eye. Individual LEDs are rectangles about 2 millimeters square. Multiple
LEDs are wired into strips or rectangular arrays, and a protective covering
usually put over them, to form lamps. LED lamps are available as flexible
linear strips, lamps resembling fluorescent tubes, and lamps resembling
incandescent bulbs. The tube-like LED lamps require their own fixtures and
generally cannot be used in older fluorescent fixtures. The standard-socket
LED light bulbs can be fit into older sockets.

At first growers tried to match LED wavelength emitted to the known maximum
absorption peaks of chlorophyll, which produced ghastly-looking pink and
blue light. It has since been realized chlorophyll absorbs and uses other
wavelengths quite well, so these pink and blue LEDs are now considered
unnecessary. The idea of a "good" LED lamp for plants includes whether
proper wavelength photos are emitted. You will read of`a measurement called
PAR, photosynthetically active radiation. This measures what proportion of
the photons emitted by the LEDs are used by the plants. Recall color
temperature measures how the light looks to the human eye. Different
combinations of light wavelength can yield what we think is white light.
LED color temperature has not been found useful for deciding which lamp to
use on plants, so don't go looking for 6,500 K LED lamps. People are also
concerned with how many photons are emitted for the electricity input

Not many smaller LED lamps have been designed specifically for plants, so
it can be difficult to choose. However, by trial and error, people have
reported some lamps work well. In general, most bright white lamps produce
acceptable growth. An LED shop light Home Depot sells has been found to
work well:…
as have various LED light bulbs designed to be screwed into desk lamp

Commercial indoor growers are switching to LED arrays:…
and plasma lighting…
but these are for very valuable crops.

So, for Norton... Can you buy in Peru standard T8 fluorescent light tubes
and fixtures with 6,500 K color temperature ("daylight"), 20 or 40 Watt?
(40W is better.) We know those work well, and will not require importing.
This would be your simplest solution. Tubes must be changed once per year.

Or, can you buy in Peru T5HO intended for aquariums, with color temperature
6,500 K? Those work well and are more efficient than standard T8. The tubes
must be changed once a year.

It is hard to carry long, fragile objects on airplanes. Imported CFL tubes
only last a year. Imported LED lights will last 3-5 years.

People could bring you this light from the US, since it is know to work for
plants. It is not as fragile as are fluorescent tubes:…

They could also bring you a T5HO aquarium fixture from the US, perhaps with
some replacement 6,500 K bulbs. Again, these are long and fragile.

Of course, there is no way to tell what will be available in 3-5 years.

Leo Martin
Zone 9?
Phoenix Arizona USA

More information about the pbs mailing list