I'm starting Common Milkweed seeds indoors under an LED panel grow lamp, and I've found this tutorial to be a useful reference, and my seeds have just started to sprout. One thing it says is:

If you’re using a grow light, make sure to lower the bulb closer to the pots, or your seedlings may become leggy as they stretch to find the light.

Is this a real thing? I'm a physicist, and I'd like to understand this issue. Wild plants grow under sunlight, which on a clear day comes from a compact source 93 million miles away. The grow lamp is a diffuse source (or, rather, a collection of many point sources spread over a large solid angle, compared to the sun) a few inches away. How do plants "know" the distance to the source of illumination, and why does it matter if it's 3 inches or 12 inches? If this is a real effect, what is the biophysical mechanism behind it?

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    @benn Why that jab at the end? Yes, I understand the intensity of light, and I found your explanation helpful aside from that. What I didn't understand, as a physicist and not a botanist, is that plants grow more the less light there is. That's not at all obvious if you don't know it.
    – coneslayer
    Mar 26 '20 at 14:00
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    Sorry, it was certainly not meant offensive. My bad! Legginess is not a normal way of growing, under optimal light condition (intensity) a plant would grow more compact (and thus more healthier). You can see legginess growth in plants as a survival mechanism in nature, so that the plant can grow towards a better spot in the sun. Good luck with your cultivation adventure.
    – benn
    Mar 26 '20 at 17:05
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    Not an answer to your question, but a general answer - 1% more light =1% more growth. Light will drop off at >= square of distance so having the light closer makes a big difference. (It might also provide useful heat) My GUESS is that plants grow towards the light as its an evolutionary advantage - especially for a seed or in a forrest.
    – davidgo
    Mar 26 '20 at 19:11
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    In my experience, grow lights often put out too few lumens, or microEinsteins, for full sun plants. For those, I use 5 3700Kelvin compct fluorescents plus a 150 watt high pressure sodium. In a two foot by four foot area, That's still less than half a sun. Can't move the plants too close to the lights, or the heat will burn them. If legginess is not extreme, you can live with it until It's time to harden off the plants outside. Mar 26 '20 at 19:50
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    @coneslayer Phototropism en.wikipedia.org/wiki/Phototropism stems grow towards light. If light is too dim that's most of what they'll do, at the expense of growing thick. Internodal distance (between leaves) also increases. Mar 26 '20 at 23:01

If all normal daylight really came from a "point source 93 million miles away", you would see many different things compared with the way they are.

For example, the sky during the day would be completely black. Shadows on the ground would also be completely black. At sunrise and sunset, the light level would change from "normal" to "pitch black" in a few seconds as the sun disappeared below the horizon. If you were in a room in a house where the window was not facing the sun, it would be completely dark all day. And so on. You can get some idea what that would be like if you go outside on a dark night with only a torch, or the headlights of a car, providing illumination.

In reality, much of the light which reaches the ground is diffuse light which is scattered by the atmosphere and arrives equally from all possible directions. If that amount of light is enough for a plant to grow properly, it will not "grow towards the light" because the light is coming from "everywhere". Of course some plants do "grow towards the sun" to some degree, but the sun is not in the same position in the sky all day!

This is completely different from a grow-light which is not really providing enough brightness, and which is a single fixed light source which the plant can grow towards to try to get more light. The plant doesn't "know" how far away the light is. It just "knows" that it isn't getting enough light, and the brightest light is coming from one particular direction - so it grows in that direction.

You can see this in an extreme way if you try to store vegetables like potatoes in a "dark cupboard" which is not completely dark, because some light is leaking in around the door. If the potatoes begin to sprout, they may grow sprouts several inches long, pointing directly towards the small amount of light that is getting in.

  • OK, so I'm thinking this advice matters more for single-bulb grow lights? Because the LED panel light isn't a single discrete source, and covers the whole "sky" except for seedlings at the edge.
    – coneslayer
    Mar 26 '20 at 11:43
  • Since you're a scientist, just try an experiment. Move one plant to within a couple of inches of the light source, another 12" from the light, and leave the others where they are. Give all plants the same watering/feeding regime Wait a week or so. Alephzero's explanation is basic, but correct.
    – Jurp
    Mar 26 '20 at 12:49

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