AF interviews: Diana Walstad

The «siesta regimen»

Q. In the first version of your book you suggest a photoperiod of 12/14 hours, in the last edition a interweaved photoperiod (4 hours of light, 3 of pause and 4 of light), in other contexts I read that you suggest 9h. We usually suggest a photoperiod of 8/9 hours.

What do you think is the best photoperiod and how can the choice be influenced by other factors such as CO₂ intake and / or a higher/lower fertilization?

A. I have never tried or even considered keeping plants with a photoperiod of less than 12-14 hours. The Siesta Regimen (4-5 hours of light, 3-4 of pause and 4-5 of light) still has a 12-14 hour photoperiod. In Aquarium Plants (2003), the author Christel Kasselmann (page 52) recommends a 12 to 13 hour photoperiod to «accommodate as many plants as possible».  And experimentally, she found that plants could handle a 10 hour photoperiod temporarily, but that some species eventually collapsed entirely after a few weeks. In the 2013 edition of my book, I explain why at least 12 hours is best and cite experimental scientific studies backing this up. I wrote even more on this topic in The Aquatic Gardener (2009, vol. 22, no. 1).

That said, it is hard to argue with success. If your group is getting good plant growth with 8-9 hours, that’s worth investigating. If the plants get 8 hours of overhead lighting but the tanks receive window light or even room lighting, the plants might still be «sensing» a 12 hour photoperiod. I could also speculate that CO₂ injection, fertilizer dosing, and the intense light of artifical tank setups—with their optimal growing conditions—could over-ride the plant’s hormonal response to a short daylength. (Generally, a short daylenth like 8 hours signals many plant species to «down-size» and hibernate with the onset of winter.)

I would be curious to learn more details about tanks that are getting good plant growth with an 8-9 hour photoperiod.  Plant species, CO₂ injection, fertilizer dosing, window light, etc.

Q.With the «Siesta Regimen» you have 8/10 hours of artificial light and 3/4 hours of natural light (during the Siesta); when you suggest 12/14 hours, do you mean artificial light plus natural light or only artificial light?

A. No. Just artificial light. Artificial light is more relevant in most situations.

Q. Could more than 8 hours of continuous artificial light without «Siesta» pause in illumination, advantage algae against plants?

A. Yes.  Algae are more adept than plants at getting CO₂. Around noon, CO₂ levels decline to very low levels due to morning photosynthesis. With continuous light, algae has an advantage over plants all afternnon. In contrast, with a Siesta (no light for a few hours), algae cannot grow and CO₂ has time to recover to levels that plants can use when the lights come back on.  (The CO₂ regeneration during a Siesta is shown in my book’s Figure XI-2 on page 179., 2013 edition)

Many people like the Siesta Regimen for saving electricity. I keep some tanks with Siesta and some without.  My outdoor tubs get about 14 hours of continuous natural light—no Siesta. All plants are doing well.

There are many ways to light tanks, just as there are many light sources (CFLs, LEDs, etc) that will work. Many plants can still grow even with a photoperiod shorter than 12 hours.  Plants are adaptable.

Q. In the «Siesta Regimen» method the substrate has an enormous importance in allowing the recovery of the dissolved CO₂ in water. After how long since the launch of the tanks you began to record a model of recovery of CO₂ dissolved in water as shown in the XI-2 graph of your book?

A. Reasonable question. However, it would be nice if people who doubt what I wrote in my book would start doing some experiments themselves rather than quibble and question.
I went back to my original CO₂ experiments. The results shown in the book’s figure are those from one of my shrimp bowls that I set up in June 2009. I did the CO₂ measurements in Dec 2009, so the substrate was about 6 months old. The bowl setup is described in my article Small Planted Tanks for Pet Shrimp.
I did about 10 separate measurements on different days. Some tanks had a bigger CO₂ recovery than others; but they all showed a recovery. In the following table there are CO₂ measurements I made on 12/7/2009 of 4 separate tanks.

14 hour photoperiod: 7 am lights on, noon off, 4 pm on, 9 pm off. Measurements done on 12/7/2009 tank conditions: all stable and doing well; unheated bowls had temp < 66°F
– 50 gal was set up in June 2008
– 55 gal was set up around 2003-2004 with plants either in pots or attached to rocks
– Bowls were set up in June 2009

As you will see, the substrates of the 55 gal and the 50 gal were over a year old.
Please note, that I did not do the Siesta experiments to prove any preconceived theory. I was actually quite surprised by the results.