Growing under lights

Leo A. Martin
Wed, 19 Feb 2014 10:14:26 PST
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

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 light.

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 schedule.
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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