Photo period revisited - The Planted Tank Forum
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post #1 of 3 (permalink) Old 03-14-2018, 06:11 PM Thread Starter
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Photo period revisited

I know this question has been asked many times. In CO2 tanks, there is consensus to limit the photo period to 8 hours. The explanation is that plants shut down photosynthesis in about 8 hours as evident by some stem plants closing up. Any extra light will just be a waste or worse, trigger algae growth.

I have questions on the consensus:

Question 1: Many aquarium plants are tropical. The average day light hour in the tropic is 12 hour. So longer photo period is normal in nature, and is supported by Walstad method.

Question 2: A few stem plants response is not an evidence that all plants respond the same. I keep mostly slow growing fern, Anubias, Crypts and Buce and they don't necessarily shut down as fast growing stems after 8 hours but I have no way to tell. I have never observed closing and opening of my slow growing stems: Rotala rotundifolia and Luwigia repens.

Question 3: If indeed plants shut down in 8 hours continuous photo period, wouldn't splitting a long photo period into two promotes more growth? Walstad method advocates for a siesta period to recovery CO2 which obviously doesn't apply to CO2 tanks. But wouldn't a siesta period fools the plants into doubling daily growth spurt.

What is your opinion or experience on this. I love to be able to increase growth on slow growing plants without increasing light intensity that may trigger algae.
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post #2 of 3 (permalink) Old 03-14-2018, 06:25 PM
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You can't really use the Walstad method as a point of comparison since those setups limit light. They usually don't work with high light demanding plants. That's why they could run 12 hrs. The 8 hr period is more for algae containment then whether the plant is still doing it's thing.

Personally if conditions are right 8 hours should be plenty to get crypts, ferns to grow pretty fast with co2 and good light. What you can do is just provide 1 to 2 hours of very intense light. That's why I usually do and it seems to work well to grow plants well and contain algae issues.
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post #3 of 3 (permalink) Old 03-14-2018, 10:34 PM Thread Starter
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I have no intent to draw comparison with Walstad method as I understand the two setups are heaven and earth different.

I understand having shorter photo period is to control algae in high light systems. But I have low/medium light, not substantially different from Amano Nature aquariums that are between 35 to 50 PAR from Tom Barr's measurements.

Yes, with CO2, low light plants will grow faster, but only relative to non-CO2 condition and still much slower than fast growing plants. The benefit of faster growth in slow growing plants is to outgrow algae, which I have not found the growth fast enough in low light CO2 system.

Another benefit of having a longer photo period is for me. I prefer longer viewing time of my tank than leave it in darkness most of the day. Similarly, split photo periods give me more viewing time in the morning and evening when I am home, and hopefully promote faster plant growth and less algae by splitting long photo period.

I post this to explore the pros and cons of having longer photo period, and split photo periods, in low light CO2 system, and hopefully get feed back from people who have tried it.
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