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An odd title perhaps, but a valid question. Do plants know what time of day it is and does it make much difference when we choose to start and finish our artificial photoperiods?

I've got two pretty much identical set ups in different rooms. Each tank is an ADA Mini-M 19L/5.5g with ADA Aquasky LED lights, pressurised CO2 and Eheim 2213 filters. One room gets a lot of light in the mornings due to an east facing window (indirect light filtered through an opaque roller blind). The other room stays quite dim. Lights were on timers set to come on at 14:00 and turn off at 21:00 for a seven hour photoperiod.

I've had problems getting good growth from the plants in the brightly lit room, but haven't had any real issues in the more dim room despite pretty much identical set ups. Could it be that the brighter lighting in one room was 'waking up' the plants earlier in the day resulting in them making less use of the later part of the artificial photoperiod?

I've recently made a change to the photoperiod in the room that gets a lot of morning light. The lights now come on at 11am and go off at 9pm, for a 10 hour photoperiod. Growth has been much better, but this could also be due to the increased photoperiod.

Curious to hear if anyone else has observed anything similar.
 

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I just read somewhere (don't remember if it was on this forum or elsewhere) that if you have the photoperiod set for quite some time and then alter it by an hour, for a time afterwards certain plants will close up their leaves at what previously was the final hour of their 'day'. So somehow they are tracking time. Doesn't really answer your question, but I thought it was interesting.
 
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