Let there be light. And lots of it – Troubleshooting Pepper Problems

Starting seedlings can be hard. Starting them during an insufferably grey spring in Chicago is even harder. I am still trying to revive my ostry cyklon peppers under some LED grow lights and while heat is one of the components, so is light. Perhaps even more so given that light emits energy which in turn produces heat. As an aside, I’m not taking any chances and germinating a few more seeds. Okay. So. How much light does one of our baby pepper seedlings need?

To start that discussion, we first need a concept of brightness and a unit of measurement for it. In comes the idea of  “lux”. A lux is a international scientific standard unit of measurement for illuminance and luminous emittance, equal to one lumen for a square meter. Whoa– maybe some vocab first?

  • LED – Light-emitting diode. It is speculated that led lights have the potential to offer greater efficiency, longer lifetimes and wavelength specificity. 
  • lumen – a unit of luminous flux in the International System of Units perceivable by a human, that is equal to the amount of light given out by a source of one candle intensity radiating equally every direction.
  • illuminance – the amount of light level measured on plane surface perceivable by the human eye. Correlates to human brightness perception. 
  • luminous emittance – the luminous flux per unit area emitted from a surface. (so how much bounces off). 
  • lux (lx) –  lumens per square meter. One lx = one lumen per square meter. 

As I understand it, the lux is the unit of measurement that answers the question “how many candles would have to be crammed into a square meter to produce equivalent brightness?” . Here are some examples for frame of reference when we start measuring around the house.

Common Lux Light Levels Observed In Nature Table
Recommended Light Levels (Illuminance) for Outdoor and Indoor Venues – National Optical Astronomy Observatory

For pepper seedlings we want to reproduce the equivalent of direct sunlight through most of the day for our peppers. Damn tropical plants. With that amount of brightness the chlorophyll in our plants’ leaves will have the best conditions for photosynthesis which in turn creates energy for plant growth.

But wait, lumens aren’t everything! Consider that when we talked about lumens, we discussed human perception of brightness. Not plants. Humans see more light on the yellow/ green wavelengths but plants utilize red/blue wavelengths which the human eye is poor at perceiving. Plants use prefer lights beginning at 450 nm (nanometers). 450 – 650 nm rays are required for plant photosynthesis, the production of food from light, water, carbon dioxide through the catalytic action of the plant pigment, chlorophyll. The 650 and 730 nm wavelengths control flowering through light-induced changes in the plant pigment, phytochrome.

Graph of visible light on the electro magnetic spectrum

It is worth mentioning that there is currently some research being conducted where scientists are trying to determine what waves have what affects on plants. I was not able to find too much info on that.

The effects of light-emitting diode lighting on greenhouse plant growth and quality — Margit Olle & Akvile Virsile. Agricultural and Food Science, [S.l.], v. 22, n. 2, p. 223-234, june 2013. ISSN 1795-1895.
However, unfortunately, the measure for that Photosynthetically Active Radiation (PAR) is not commonly advertised on those colored LED lights, so we will not be using it in this discussion. Hopefully, in the future this will be how we rate and describe grow lights.  But what little I found suggested that every wavelength plays its own special role in producing a healthy plant. So just defaulting to using lux doesn’t seem to be without merit.

So how bright are our overcast Chicago days rating? I downloaded a simple app on my phone to measure lux. Today was a partially cloudy day and the reader measured between 1,500 – 5,500 lux at 5:30 pm when I get home from work. Hovering my phone parallel to my led grow lamps, at about three inches away,

Fish pepper seedling under led grow light
Fish pepper in a seat of high honor.

I am reading about 100,000 – 114,000 lx. However, every inch I move away from the bulbs, my lux reading drops significantly. At six or so inches, I am lucky to get a reading of 14,000 lx. Given the strength of the LEDs and their consistency, the fact that I can keep them on for how long I see fit, I’m inclined to keep using them. I think it makes sense to put my most sensitive and beloved plants as close to the grow lights as possible. The remainder, I guess will go on the window sill and we pray for some sunny days.

I wish I had records for what the lux light level was like when I was at work. I have an extra RaspberryPi lying around and now am inspired to put together a light logger so I can have a better idea of what kind of suffering I am putting my seedlings through. What about you guys? Have you created any smart gardening tech tools to help you better understand the conditions of your plants? How much — if at all — do you utilize extra lights in your gardening?


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