Taylor and Others,
I would like to put in a favorable word for Pistia in aquaria. Yes, they do require some light, but not so much as you might imagine. I have a very healthy growth in a small aquarium here with a single screw-in type fluorescent light (25 watt equivalent) on a timer (8 hours per day) and about five or six inches above the water line. This is sufficient light to grow Pistia and have them flower regularly. One aspect of Pistia is that they are an invasive weed where the weather is warm, as Julius indicates. For aquarium growth this means that in time they will cover the surface and shield the water from the light so you don't get green water. The second interesting thing about them is that they form amazing roots with feathery rootlets. These penetrate down into the water column and make an excellent refuge for baby fish as well as a spawning medium for egg laying species. Each plant, when in a happy situation, will also produce runners that then create new plants vegetatively. I imagine that in a warm climate these runners and new plants will form a tough web of life over the surface.
In the summer you can put them outside if you live in a cold place like I do. The placement outside results in the immediate death of all your leaves since the light intensity (and probably air movement also) is completely different from that indoors. Soon enough, however, new leaves will emerge from tiny growth centers in the middle of each plant that are tough and will flourish all summer in full sun. The plants will get bigger outside than they do inside. In the fall you can pull them inside again when temperatures start to dip into the 40s Fahrenheit. This rejuvenation will give them renewed vigor.
If you also have a few Cryptocoryne or Anubias, or other semi-aquatic species, you can grow both them and Pistia in the same tank, provided the water level is low enough to permit the others to push their leaves above the water line and the layer of Pistia. This makes a nice combination, sort of what you might see in the real world where Pistia and other plants co-exist.
By the way, I have found that Pistia are nitrogen pigs. By this I mean that they crave soluble fixed nitrogen such as you find in fertilizers. This suggests that Pistia may also have a beneficial function where they are considered weeds. That is, they will remove soluble nitrogen from bodies of water, rendering them less polluted, and concentrate these nutrients in their tissues.
I have been observing Pistia intensely for a couple of years and find them much more interesting than I had imagined. One thing that has eluded me so far is how they get pollinated. Julius indicates that they throw tiny seeds. I have not yet seen seeds form in indoor cultivation, despite apparently healthy and fully mature male and female parts. This might imply an interesting insect that performs this task that I don't have in my basement. Does anyone have any ideas on this aspect?
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