How long should I keep my grow light on? – House Plant Journal

How long should I keep my grow light on?

Prerequisite: Light Meter

Without a light meter, you’ll never be able to make sense of how much light your plants are getting. It’s like buying an oven without knowing what temperature you’re setting it to; like a car with no speedometer.

Any of these will do:
(Amazon Affiliate Links)
Dr. Meter (shown above): https://amzn.to/34qxtfx
Dr. Meter (version 2): https://amzn.to/2HE6F2A
URCERI Light Meter: https://amzn.to/3kvfcDr

Q: What about PAR?
A: If you already have a PAR sensor, then you can use that and refer to the PPFD column instead of the foot-candles column. The conversion factor of 0.17 is applicable for most ***WHITE LED*** grow lights – I verified with my spectrometer! This conversion will not work for pure blue/red LED grow lights – and since they have no real advantage over white LED grow lights, just get the white ones.

White LED Grow Light

Foot-Candle to PPFD conversion factor: 0.17

How to use this chart:
1) Turn on your grow light.
2) Using a light meter, take a reading from the top of the plant (in foot-candles or in PPFD).
3) Cross reference with duration (in hours) to get the desired DLI.

Q: Wait – what is DLI?
A: It stands for Daily Light Integral and is a measure of how much light is received in a 24 hour period. More details HERE and HERE

Intensity Duration     
Foot-CandlesPPFD (approx.)1 h2 h4 h8 h12 h16 h
508.50.03060.060.10.20.40.5
100170.06120.120.20.50.71.0
200340.12240.240.51.01.52.0
400680.24480.491.02.02.93.9
6001020.36720.731.52.94.45.9
8001360.48960.982.03.95.97.8
10001700.61201.222.44.97.39.8
15002550.91801.843.77.311.014.7
20003401.22402.454.99.814.719.6

Q: That’s great but where do I find desired DLI for my plant?

A: Before I give you an answer, you must understand the concept of ***being precise but not prescriptive***. Calculating the DLI from your grow light is being *more precise* about light than not measuring at all or thinking “medium light” has any concrete meaning. But plants can grow in a wide range of DLI – it’s not as if the plant will surely die if the DLI is off by 0.1 mol/d. There’s also the issue of personal opinion, as in, whether you think the plant still looks nice. So even if you measure light precisely, it should not be taken as a prescription to guarantee perfect plants.

I’m working on my own set of values for different plants based on observations from *INDOOR* environments but here’s some documented values for *GREENHOUSE* production – and remember that “Minimum acceptable quality” is for commercial production of the plant. For recreational maintenance, the bar is lower, which is great news for us houseplant enthusiasts.

DLI for commercial greenhouse production of various plants: Purdue University: Commercial Greenhouse Production

Here are some grow lights that have worked well for my houseplants: FULL ARTICLE HERE

Here’s a very powerful grow light, capable of a DLI of over 20 mol/day: FULL ARTICLE HERE

Comparing With Natural Light Sources

Just for some context, here are a few measurements from natural light sources and situations. One CRITICAL thing to note: natural light is dynamic – meaning it changes throughout the day. So the actual DLI for any one day can vary a lot whereas, for grow lights, it’s a simple matter of multiplying the hourly PPFD by the number of hours the light is on.

Clear Sky – Through Window, No Direct Sun

Foot-Candle to PPFD conversion: 0.18
Key variable: the ANGLE OF VIEW dictates the strength of the indirect light (bigger windows = higher indirect light readings)

Intensity Duration     
Foot-CandlesPPFD (approx.)1 h2 h4 h8 h12 h
5090.03240.060.10.30.4
100180.06480.130.30.50.8
200360.12960.260.51.01.6
400720.25920.521.02.13.1
6001080.38880.781.63.14.7
8001440.51841.042.14.16.2

Direct Sun – Through Window

Foot-Candle to PPFD conversion: 0.19
Key variable: window transparency

Intensity Duration     
Foot-CandlesPPFD (approx.)1 h2 h4 h8 h
40007602.73605.4710.921.9
50009503.42006.8413.727.4
600011404.10408.2116.432.8

Direct Sun – Outside

Foot-Candle to PPFD conversion: 0.2
Key variables: the sun’s angle in the sky (time of day, atmospheric conditions, latitude all play a part)

Intensity Duration     
Foot-CandlesPPFD (approx.)1 h2 h4 h8 h12 h
800016005.760011.523.046.169.1
900018006.480013.025.951.877.8
1000020007.200014.428.857.686.4
1100022007.920015.831.763.495.0

If you’re looking for a logical and sensible approach to houseplant care, I’ve created the perfect online course for you:

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One response to “How long should I keep my grow light on?”

  1. Table 2 DLI Requirements for Various Greenhouse Crops by Purdue Extension does not reference the species for Calathea. Do you have any information pertaining to Calatheas?

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