Hi YouTubers! I'm AL Gracian from albopepper.com. You're growing plants indoors. You've picked out your lights, and selected
a good distance. But how LONG should the light cycle be? Should you run your lights twenty-four seven? Or do your plants need a "rest"? Also, is there any relationship between light
DURATION and light INTENSITY? In today's experiment, we're exploring these
ideas, specifically, with lettuce. It's important to note that different plants
respond differently to light cycle durations.
Some are classified as "long-day" plants. Some are "short-day". And others are "day-neutral". This gets into the concept of "photoperiodism". The period of uninterrupted darkness can trigger
whether or not a plant enters its flowering or reproductive phase. So, long-day plants flower when night duration
decreases below their critical photoperiod. But short-day plants require longer periods
of darkness, perhaps more than 12 hours of night to induce flowering.
Meanwhile, day neutral plants can flower regardless
of light and dark cycles. Well we're working with a bibb lettuce and
a quick web search tells you that lettuce is a long-day plant. So we need to keep the day cycles short, to
prevent bolting… right? Or… can lettuce be grown under long light
cycles? What about… 24 hours of light with NO dark
cycle at all!? Yeah, we're gonna test that. In today's experiment we're trying 3 different
light durations but we're also trying 3 different distances.
Hold up! You can't change more than 1 variable! Well we're actually trying to MATCH a variable. We're trying to create the same DLI for each
plant. DLI or Daily Light Integral is a sum of all
the photons reaching the plant canopy over a 24 hour period. Our target DLI for this test is 16. But we're using different light cycle durations. Plant 1 will get 12 hours on and 12 hours
off. Plant 2 will get 16 on and 8 off. And plant 3? Twenty-four hours on! Imagine getting 3 inches of rain in a single
day.
It might be a steady light rain, all day and
night. Or it might rain heavier for half of the day
then stop. Or there might be a torrential downpour within
a 2 hour window. The same number of rain drops in a single
day, but delivered in different ways. We're doing the same thing here with photons! PPFD tells us how many photons hit an area
in 1 SECOND. With a little math we can calculate the total
amount per DAY (the DLI). Since each light will run for a different
period of time, each setup will require a specific PPFD. Longer photoperiods call for the lights to
be moved further and further away, reducing how many photons hit the plant per hour. The objective? To try to deliver the same number of daily
photons… each in a different way. How will this affect plant growth? With similar daily photon dosages, will they
grow at the same rate? Or will that dark cycle alter plant growth? And what about lettuce number 3? Twenty-four hours of light… Will it bolt? What do you think? For our test, the lettuce seeds were pre-germinated
and then transferred to their rooting plugs.
Here we are at day one under our lights. On the left, is our 12/12 cycle, then the
16/8 in the middle and the 24 hour continuous light on the right. At day 9 all plants were growing but plant
3 was looking just a little larger. On day 16 all plants looked ok but plant 3
showed a little tip burn. Here we are on day 23. Plant 1 has its first signs of tip burn. Plant 2 has minimal signs of tip burn, but
the best looking growth.
And plant 3 has severe tip burn & deformity. Also the deepest color of green in its leaves. Even after day 29, lettuce number 3 has still
hung in there with no signs of bolting. Of course, it shows the highest deformity
& tip burn. Twenty-four hours of light with NO break at
all. But the plant grew quickly with no vertical
elongation. The light intensity was 185 PPFD. Compare that to our previous experiment where
175 PPFD produced MUCH less growth! Why? Because we had a long period of darkness each
day. With no break in light exposure, this received
60% more photons per day. Oddly, in an earlier test, we DID observe
serious bolting in ALL THREE plants. The cause appeared to relate to light QUALITY,
not duration. Those plants were getting a custom blend of
red and blue wavelengths.
The ratio of colors induced bolting. But switching to a broad spectrum white LED
has suppressed that response. So this particular lettuce variety responded
to light QUALITY more readily than light / dark DURATION. At day 30 we've pulled the plug. Here's how the plants turned out. Similar in size. But all have at least some tip burn. A reduction in DLI would help with this. Perhaps 14 or 15 would be better for the next
trial. All data has been collected and here are the
results of our test. The longer light cycle produced the most growth. Let's look closer at each plant weight. Obviously, this is a very small sample group. To get more meaningful data, a test could
be set up using several plants per photoperiod.
Or the exact same test could be run several
times to see if we get the same results. Although we have a step-wise increase, plant
3 was only 21% larger than plant 1. There is a boost, but certainly not enough
to justify the added electrical costs. How much did it cost to grow these plants? These SANSI LEDs offer some good efficiency. But the shorter the light cycle, the more
growth we got per watt. Plant 1 was smallest, but that 12 hour photoperiod
made it the cheapest to grow. So what's the take-away from today's test? Do your lights seem a little weak? Sure you can move them closer, but you can
also try increasing the light duration to compensate. Check to see if your plant is long-day, short-day
or day-neutral. Or try an experiment and see what happens! Be aware though that custom red/blue LEDs
can cause unexpected growth responses. We've seen that excess light can cause tip
burn in lettuce. But that can be resolved in a couple of ways. We can try raising our lights to reduce light
intensity. Or we can also try reducing the light duration. Reducing the day cycle brings the added benefit
of less power consumption.
Light intensity and light duration are both
key components that allow us to calculate the DLI. Low light plants, like lettuce, will fair
much better when we keep that DLI lower. So see how your plants are responding and
make adjustments as needed! Thanks for watching & thanks for your support
on my channel. Please subscribe if you haven’t already. And as always, Happy Gardening!.
