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It's Earth and the moon with all parameters as they are in real life with one exception, the moon is twice as reflective as usual. Instead of an albedo of 0.12, the moon has an albedo of 0.24. This change will have huge implications for all kinds of things. What I'm interested in is the growth rate of plants.

With a brighter moon would plants have more energy for growth or is that still too dim to be useful? If the extra light is useful, how useful would it be?

My assumption is that it will have a small but measurable impact on plant growth. My understanding is that photosynthesis shuts down in darkness so having a little more light on nights when the moon is up will provide more energy for the plant to use.

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It's still way too dim to be useful. You'd have less than 1 lux worth of light, but you need a bare minimum of 50 lux to make any difference at all, and preferably 500 lux to see some significant (but small) amount of extra growth.

Plants' light requirements fall into one of the four categories below, depending on the plant. "Most plants will survive illuminance 10 times lower than listed below but will not grow as well or bloom."

Low (500–2,500 lux)
Medium (2,500–10,000 lux)
High (10,000–20,000 lux)
Very High (20,000–50,000 lux)

https://en.wikipedia.org/wiki/Houseplant_care#Data_for_some_common_houseplants

And common lux levels:

Illuminance (lux)   Surfaces illuminated by
0.0001              Moonless, overcast night sky (starlight)[3]
0.002               Moonless clear night sky with airglow[3]
0.05–0.3            Full moon on a clear night[4]
100                 Very dark overcast day[3]
400                 Sunrise or sunset on a clear day.
1000                Overcast day;[3] typical TV studio lighting
10,000–25,000       Full daylight (not direct sun)[3]
32,000–100,000      Direct sunlight

https://en.wikipedia.org/wiki/Lux#Illuminance

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Not really.

Sun is 400,000 times brighter than full Moon. Doubling and even quadrupling Moon's brightness would still have negligible effect on plant's life. However, it may be a game changer for nocturnal animals.

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    $\begingroup$ And for humans. Double the albedo and the average human could likely read a book under a full moon. It might redefine nightlife. It would certainly give a whole new twist to the "Rustler's moon" (enough light to rustle cattle, not enough light to get caught; used to desribe the sliver just before and after a new moon). $\endgroup$
    – JBH
    Dec 12, 2017 at 2:21
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Your third paragraph is actually far more important that you may realise, plants need the rest time without photosynthesis at night.

If you did supply 24h light something significant in plant biology would need to change to cope. I think in hydroponic farms with artificial lighting this is typically ~6 hours darkness for optimal growth.

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    $\begingroup$ I've never heard this before. Do you have a link where I could read more about it? $\endgroup$
    – Green
    Dec 12, 2017 at 11:53

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