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



Growing lettuce indoors under artificial lights may results in deformity of the leaves. Brown necrotic tissue at the leaf edge is known as tip burn. What causes tip burn? Whether you’re a home gardener of a commercial greenhouse grower, this is an important issue to address.

Watch PART 2 of this series covering Light Duration:

OUTER TIP BURN:
Causes of outer tip burn are not as clear as inner tip burn. Outer tip burn seems to relate to insufficient water or high salt levels. Low humidity and excessive air flow can contribute to this problem.

INNER TIP BURN:
Causes are easier to explain and relate to poor calcium supply in young, developing leaves. But often, it’s not because the nutrient solution lacks calcium. Instead, the cause is linked to insufficient evapotranspiration.

As 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.

TEST BED – PRODUCT LINKS (# CommissionsEarned):
Lettuce – Tom Thumb: http://shrsl.com/bzt8 (Botanical Interests)
SANSI – White LED Light: http://amzn.to/2zEUQzT (Amazon)

My PPFD Meter (Apogee Instruments):
► https://amzn.to/3ebJiHZ

Test 1: In my initial test, I used a 15w Red/Blue LED from SANSI. The spectral composition of the lights caused a shade avoidance response. This test was ultimately ruined due to unexpected elongation of the lettuce plants.

Test 2: In this 2nd test, I switched to full spectrum white LEDs (SANSI, 15w). The plant growth was dramatically different, resulting in a successful experiment.

Light 1: 9.5″ Distance – 575 PPFD @ 16hr = 33 DLI
Light 2: 13.5″ Distance – 278 PPFD @ 16hr = 16 DLI
Light 3: 22″ Distance – 175 PPFD @ 16hr = 10 DLI

Solution: 6.2pH 1720 EC
Experiment Day 1: 09/18/17
Harvest: 10/16/17

Fresh Weight (tops only):
575 PPFD = 80g
278 PPFD = 55g
175 PPFD = 9g

The lettuce grown under 278 PPFD white light grew very well. It exhibited fast, healthy growth without any deformities from inner tip burn.

SOLUTIONS TO INNER TIP BURN:
1) Increase Air Flow – A fan can be used to increase air flow, resulting in higher plant transpiration rates. This, in turn, results in more calcium being pulled through the plants roots and into the leaves.

2) Decrease Light Intensity – Reducing light intensity reduces the rate of growth. This, in turn, reduces calcium requirements, allowing leaves to develop without defect.

Lettuce is considered a low light plant. Typical maximum DLI values can be from 15 to 17 DLI. Ideal PPFD is from 250 to 300, with a maximum of 400 PPFD. This is just a general guide and can vary with cultivar and environmental factors.

HORTICULTURE REFERENCES:

Hydroponic Lettuce Handbook:
http://www.cornellcea.com/attachments/Cornell%20CEA%20Lettuce%20Handbook%20.pdf
© Cornell University CEA Program

e-GRO Alert:
http://www.e-gro.org/pdf/2015_431.pdf
Volume 4, Number 31 April 2015

#Hydroponics #GrowLights #GrowingTips #IndoorGrow #Lettuce #UrbanGardening #knowledge

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