Preventing Tip Burn: Hydroponic Lettuce  (Indoor LED Grow Light) PPFD Experiment

Preventing Tip Burn: Hydroponic Lettuce (Indoor LED Grow Light) PPFD Experiment

September 13, 2019 63 By Ewald Bahringer

Hi YouTubers! I’m Al Gracian from 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!