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.
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And as always, Happy Gardening!