Recently I published a post on growing seedlings under
fluorescent lights, http://gardenvariety-hoosier.blogspot.com/2013/02/lights-action.html. My intent was to share the information I had found
and show the new lighting setup I put together in the hopes that people would find this information useful. The old setup consisted of a shoplight with two 5000K fluorescent T8 bulbs (32
W). I was never satisfied with the setup since it did not provide strong enough light to grow robust plants. It seemed that every year the seedlings were more spindly and grew slower
as the lights aged.
This spring I modified the fixture to hold four
bulbs. The old 5000K bulbs were removed
and replaced with two GroLux bulbs and two 8000K bulbs, both Sylvania brand. (I originally intended to get 6500K bulbs but
happened to find the 8000K bulbs when I was at a farm supply store. The light output of either bulb is
practically identical by their spectral charts, both bulbs making a lot of blue light). In addition to doubling the light output I
believed that the new system would provide more light in the wavelengths that
plants use for photosynthesis, resulting in faster growth and stockier
seedlings.
Comparing the old and new lighting systems can’t provide a
lot of definitive information, because I changed several variables – from two bulbs
to four, with a different color
temperature. With the additional lights
I also reduced the daily light cycle from 15-16 hours to 12.5 hours. The objective was to create a light system
based on the best information that I could find that grew healthier seedlings. It should be noted that any time you do
research on lighting systems for plants you will inevitable link to a lot of
websites devoted to cannabis growing.
Those people really seem to know their stuff!
So
how did it work? The worst performing
plant under the old lights was always lettuce.
It would quickly go spindly and fall over. By the time the seedlings were large enough
to go out into the beds it was a real question whether they were healthy enough
to survive. This tray of lettuce
germinated on March 6. They are ready to
go outside except the weather won’t cooperate and we are supposed to get 6
inches of snow tonight. This lettuce is
not at all spindly, it's short and leafy, a great improvement.
I
start a new set of brassica seedlings every twelve days. This set was sown on March 1 and germinated
on March 3 and 4. The picture below was
taken on March 21. They were definitely
ready to go outside at this time if the weather would allow. (The temperature when I got up that morning
was 16 degrees). There’s two kohlrabi
plants at the bottom of the picture that were seeded about a week after the
rest of the plants in the picture. It
seems that I forgot to seed them when I sowed the other plants, although I did
label them. Must be pre-Alzheimer's.
This
is the same set of brassicas three days later.
They’ve been moved around a little to give those green solar panels
their own space. I can see that they’ve
grown quite a bit in three days, especially the two Gonzalez cabbage plants in
the back which look like they’re on steroids.
I’m hoping the seedlings can go out into the cold frame by midweek.
I’m very happy with the results. The four light fixture uses more electricity
than the old fixture but it’s also on fewer hours a day, 12.5 versus 16 for the
old fixture. If there’s more than two
trays the trays can be set under the lights crossways and the light raised up. Even when the light is raised up there’s
enough light to grow the seedlings. This
would have been impossible with the old setup.
If there’s one recommendation I can make for anyone who
needs to replace their fluorescent bulbs but wants a conventional bulb that you
can find at a big box store it’s this: get
the highest color temperature bulb that they have. A 6500K bulb will produce more of it’s light
at wavelengths that the plants can use than a 4000K bulb. That’s not to say that plants won’t grow
under a lower K bulb, they will, but they should grow better under a higher K bulb. (And I know that somebody will write me and
say that they’ve used a low K bulb for years and it’s worked fine). Take it all with a grain of salt. I’m convinced that the higher K bulbs are
more effective.
