Moss harvesters are in a sense removing “old-growth” mosses, which cannot replace themselves nearly as quickly as they are removed. This is, by definition, unsustainable harvest. Their loss will have consequences we cannot foresee. When the mosses are taken, their web of interactions goes along with them. Birds, rivers, and salamanders will miss them.
Gathering Moss: A Natural and Cultural History of Mosses
Go Native! North Carolina 
Why is moss harvested?
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These days moss is used for the floral arrangement industry (planters, wreaths, hanging baskets, etc.), craft projects, terrariums, movie sets, garden designs, bonsai gardening. I think in the past it may have been used as a packing material. Some reports say that millions of pounds of moss are harvested from American forests each year. It’s actually a big underground market, and much of it is exported.
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The moss that I remember in potted orchids was Spanish moss, which rehydrated and grew out in the yard. I have seen moss in bonsai, but is was just collected from around the farm where the stock was grown. They stay in their dishes for so long, that some of them get their own moss.
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