Hi YouTubers!
I'm Al Gracian from Albopepper.com Have you tried growing lettuce indoors only to find
some major deformity in the leaves?
What causes tip burn?
Have you ever had lettuce grown under LED lights that stretched vertically, not growing
as expected?
Was the light too weak...
Or was there something else going on?
In today's video you get TWO experiments for the price of one, so if you skip to the
end, you're going to miss out!
Our initial premise is to tackle the issue of damaged leaf tips on indoor hydroponic
lettuce.
In my early experiments and light tests, I've occasionally had this brown, necrotic tissue
that would sometimes form on lettuce.
This physiological disorder is known as tip burn.
There are 2 basic types of tip burn in lettuce: Inner Tip Burn and Outer Tip Burn.
What causes the issue?
Is it a pH problem?
Are nutrients too low or too high?
Let's see what the researchers at Cornell have to say.
In an e-Gro Alert article by Neil S. Mattson, he points out that causes of OUTER tip burn
are not as clear as INNER tip burn.
Generally, outer tip burn seems to relate to insufficient water or high salt levels.
Low humidity and excessive air flow can contribute to this problem.
Today though, we're looking at inner tip burn.
The causes are easier to explain and to control.
Poor calcium supply in young, developing leaves causes the tissues to be brown and necrotic.
But often, it's not because the nutrient solution lacks calcium.
Instead, the cause is linked to insufficient evapotranspiration.
As the plants transpire, they pull nutrients up through their tissues.
Under high light levels, growth rate is boosted.
But if the plants can't transpire fast enough, they can't adequately supply their increasing
demand for calcium.
The solution?
In the "Hydroponic Lettuce Handbook" researchers have found that boosting air supply can increase
transpiration rates.
Simply using a fan to blow down on the plants can do the trick, well... up to a point.
Even without a fan, you can resolve inner tip burn by simply decreasing the growth rate.
And we're going to try that by decreasing the light intensity.
In today's test, we're looking at the impact that light intensity has on indoor hydroponic
lettuce and the formation of that inner tip burn.
How does lettuce respond as we move our lights closer?
Is there a sweet spot offering strong healthy growth, without inducing that inner tip burn?
Let’s find out!
I'm using 15w LEDs.
These small grow lights sold by SANSI are pretty cool and seemed perfect for tests like
this.
They have red / blue LEDs, tuned for vegetative growth.
I'm using a small compact lettuce.
This Tom Thumb is a butterhead that doesn't grow very tall.
So light levels should remain consistent throughout our test.
Using my Apogee Quantum Light Sensor, I was able to identify a target PPFD level of 278
with the light at a height of 13 inches.
To demonstrate a plant that is starving for light, I placed a second light at 21.5 inches,
for a PPFD of 175.
And now with light number three, we're going to blast this plant, trying to induce tipburn!
At 9 inches our PPFD rises to 575 which is well above the recommended threshold.
Our daily light cycle is 16 hours ON and 8 hours OFF.
These are the corresponding DLI values that result.
So let's sprint through this initial test.
As our plants grew, the leaves seemed to be vertical, rather than flat and compact.
This might be expected with a plant that's starved for light.
But even our brightest light was producing elongated growth.
As the plants grew, it just got worse.
Not what I expected from this lettuce cultivar.
It appeared that these custom colors, were triggering a shade avoidance response.
Even under super intense light, the plant just grew right up into the LEDs as though
they were searching for light.
My test was scrapped!
I contacted SANSI, sharing my results.
And they saved the day, sending me white, full spectrum LEDs.
They were nearly identical in PPFD readings.
Just a couple small tweaks and we were back in business.
Note the wide leaves and low compact growth.
That's what I was expecting initially.
But after two weeks, there was NO tip burn, not even at the highest light intensity.
Just a bigger plant with faster growth!
By week three though, things started to change...
Those young, inner leaves, showed that classic sign of tip burn.
And it just got worse.
Meanwhile, the ideal PPFD was yielding good, healthy, compact growth!
In theory, I thought I selected a good light intensity and light cycle duration.
And finally, I had some test results to confirm that.
Something to remember is that different cultivars of lettuce, may respond quite differently
to the same light source.
And custom LED light spectrums can further complicate things.
A Red/Blue LED may work great for some plants, while other plants may respond unexpectedly.
That's why, I've been moving to broad spectrum, white LEDs.
Having tested these units from SANSI, the full spectrum model is what I would recommend
for a small form factor like this.
So to sum up: what can you do to avoid inner tip burn on your lettuce plants?
Try a fan for improved plant transpiration.
Also you could try raising those lights.
That may allow you to grow more plants over a larger area.
Or select lower wattage lights.
Overpowering your grow area, simply results in excess heat, wasted electricity and inner
tip burn.
Thanks for watching.
I hope this info was able to help you out.
I appreciate all of your support on my channel.
Please subscribe if you haven't already.
And as always, Happy Gardening!