Can you use algae, water plants (e.g. water lillies), hydroponics or fungi for schach?

Question

My understanding is that schach must be from a plant that specifically grows from the ground. If this is not so, would there be any problems using:

• algae / seaweed / water lilies / swamp scum - these grow or float in water
• fungi or moss - such as mushrooms or other similar items that are not plants but grow on the ground or the bark of trees
• hydroponics - plants grown in the air in a greenhouse

Answer 1

See: אהלי שם : אוצר פסקי המועדים who writes that one who uses water grown plants has what to rely on, though ideally one should avoid using them.

This comes at the end of a discussion (and list of sources) on plants grown in a pot. Seemingly, the same would apply to your other categories.

Answer 2

My understanding of the issues at hand tells me that your first and second groups are not valid for scach, while your third group might be subject to a machlokes.

• Algae and the like are not גדולי קרקע, as they do not grow in the ground.
• Mushrooms (a form of fungus), even when they grow on the ground, are not considered to be גדולי קרקע with regard to brachos, because we make the bracha of shehakol rather than ha'adamah, because they do not derive nourishment from the ground.
• Hydroponics might depend if you make the bracha of ha'adamah on them or not -- if you do, then they are גדולי קרקע; if you make a shehakol, then they might¹ not be גדולי קרקע.

This is my own, unsourced understanding of the issues at hand. If you disagree, please feel free to leave constructive comments to let me know where I went wrong.

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¹ _{The reason that I wrote "might not be," is because a case might be made that these plants are, by their very essence, גדולי קרקע and not subject to change based on where this particular specimen grew. I can't say for certain whether or not that is true, but my (unbased) feeling is that it would depend on where this particular specimen grew.
In retrospect, this may have been the intent of the question :P}

Answer 3

Rav Abadi OY 1: 224 was asked whats the din of Shach which comes from an atzitz sheino nakov (plant in a pot without any holes) is it considered detached or attached to the ground? He answers that he isn't so sure ,but simply it makes sense it is considerd detached in all aspects . However, still one shouldnt use it from a rabbinic standpoint because the Rabbaniom made a decree to give maaser of fruits from a non-preforated pot. He ends off by saying he hasnt seen anyone discuss this issue.