Growing under lights

Tim Eck
Wed, 19 Feb 2014 11:22:39 PST
I don't see how hydroponics can be certified organic since organic
fertilizers are real no-no's in hydroponics for reasons of sanitation.  You
don't pump sewage over food-plants.
Also, the last time I looked (3 months ago), sodium vapor lamps still give
the most bang for the buck, factoring in initial cost.  The sodium D lines
are a little more energetic than the short IR that chlorophyll likes but
they can shrug off the extra energy.  I think the blue is just a growth
regulator and green is a growth inhibitor since one says the sky is visible
and the other says you're being shaded.
The price of these LED arrays has plummeted this last year and should soon
overtake (undertake?) the plasma type lamps.  But who's going to do the
heavy lifting on artificial lighting now that everybody's legalizing wacky
weed and there's no incentive to grow it in the basement anymore?
Tim Eck

-----Original Message-----
From: []
On Behalf Of Leo A. Martin
Sent: Wednesday, February 19, 2014 1:14 PM
Subject: [pbs] Growing under lights

I've been doing some reading on indoor lights, for terrestrial plants and
planted aquaria. Here is an article I just submitted for the newsletter of
the Central Arizona Cactus and Succulent Society. While it understandably
focuses on succulents, the information is easily generalizable to bulbs.

I have been doing a lot of reading about lights for growing plants indoors.
It turns out white fluorescent lights of the proper color are at least as
good as specialty "grow-lights."

Fluorescent lights, including tubes and CFLs, are sold in different colors
of white.
They are labeled as to color temperature, which is measured in Kelvins,
abbreviated as K. The color temperature is almost always printed on the
tube, label or box, often in tiny print. Color temperature refers to the
relative proportion of various light wavelengths in the light. When we see
multiple wavelengths mixed together they form what we perceive as white

I discovered years ago that 40 Watt "cool white" fluorescent tubes are fine
for sprouting most cactus seedlings and growing them on for a few years,
before they are ready to be transplanted outside to individual pots. But
"cool white" is around 3,500K, and this doesn't contain enough of the proper
wavelengths for most other plants. Cactus seedlings do fine with cool white
because they generally don't need that much light to grow. Cool white
contains only small amounts of the wavelengths chlorophyll absorbs, but that
amount is sufficient for most cactus seedlings for a while. Cool white is
not adquate for Opuntia seedlings.

The best artificial light color temperature for almost all plants is 6,500K,
which is usually called something like "daylight." This has a lot of pink
and blue wavelengths, which are the ones absorbed by chlorophyll. Both
straight standard fluorescent tubes and CFLs of the proper light temperature
and wattage are great for growing plants indoors.

Straight fluorescent tubes are now sold in varying lengths and diameters.
New small-diameter tubes (T5, T8) are a lot more expensive than the old 1"
tubes. The old tubes work just fine at a much lower cost. 4 foot long tubes
of 6,500K and 40 Watts can be mounted in cheap 2-tube shop light fixtures
and suspended over your growing area.

Spiral CFL tubes are engineered to give the greatest amount of light down
the axis of the spiral. So, when using them for plant lights, they should
not be mounted sideways; they should be pointing down at the plants. Most
people who use them as plant lights use individual hanging light fixtures
with a conical reflector. A 45 Watt CFL provides as much light as a 200 Watt
incandescent light bulb, with much less heat, while using much less
electricity. It is possible to bloom high-light plants like Hibiscus indoors
in dim rooms under 45W CFL lamps.

Use a heavy-duty mechanical clock appliance timer with a 10 to 12 hour on
These are not always available at hardware stores, but they are much more
reliable than cheap electronic lamp timers. You may have to go online to
find one. Ace Hardware sells heavy-duty mechanical clock timers around
Christmas for use with outdoor lights. If you use multiple light fixtures,
plug the fixtures into a power strip, then plug the power strip into the
heavy-duty timer. Take care the total amperage of your array does not exceed
the amperage rating of the timer nor circuit breaker.

Many indoor growers have discovered most plants don't need long nights, so
plants can have multiple light-on periods per day. I have been told that
people who grow certain crops indoors now give their plants 11 hour on, 1
hour off cycles, and they get twice the growth rate - which means half the
time to harvest. I haven't tested this with cactus seedlings.

Many succulents only open their pores to absorb carbon dioxide at night,
storing it until the next day, when they use it to make sugar in the
sunlight. Succulents with this kind of metabolism probably shouldn't be on
1-hour night cycles. Cactus seedlings aren't like this; when immature they
open their pores during the day, so short nights shouldn't be a problem for
them. I have read that adult cacti are limited in the amount of carbon
dioxide they can absorb at night, since plant tissues become more and more
acidic as the carbon dioxide is stored, and there is a limit to what the
plant will tolerate. Cacti taste much more sour just before dawn than they
did just after dusk. This is one explanation of why cacti grow slower than
plants that can absorb carbon dioxide all day long. Cacti generally have
absorbed as much carbon dioxide as they can hold in the first few hours of
darkness. So I might guess adult succulents should have somewhere in the
range of 4-6 hours of darkness, but this is just a guess, and
experimentation would yield real information.

Crinkled mylar can be bought very cheaply at hydroponics shops to line
reflectors. This provides better light reflection than a pure white or
polished metal reflector.

Most big-box stores carry 6,500K CFLs, up to and including 45W. If not
available at the store they can be bought online. They also carry 6,500K,
40W standard tubes. Most fluorescent tube shop lights sold are 20W so,
again, read the label. You have to read the labels and do some searching,
because the sales people generally don't know anything about color
temperature, and they will try to sell you the much-more-expensive

Tubes and CFLs don't provide the same light output over time; they become
dimmer. Our eyes can't tell the difference, but the plants can. Replace
straight tubes once a year.
I write the date I put them in service on the tube with a marker. For CFLs,
read the manufacturer's information on hours of service. Replace the bulbs
when they are down to 75% of original brightness, unless you are still happy
with your results.

Commercial vegetable growers in southern California are now growing
organic-certified greens indoors in warehouses under arrays of pink and blue
LED lights. Remember, these are the wavelengths chlorophyll absorbs. The
photos are eerie because the light looks so strange. The greens are grown in
mats on shelves, stacked on wheeled carts, with the LED arrays underneath
each shelf, to shine on the plants on the shelf below. There may be 12 or
more shelves per cart. Dilute fertilizer solution is pumped to the top
shelves, and then flows down to the other shelves. Access is carefully
controlled so no insects enter the warehouses, and no pesticides are needed.

Leo Martin
Phoenix Arizona USA

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