Several ways. Thread is one way. If you get a color that blends in with the plant and driftwood, you wont see it. By the time the thread rots, the plant should have attached itself. Fishing line is another way. I think it takes several hundred years to completely rot, but it's easily cut and removed once the plant has taken hold of the wood. I've actually used zip ties once to attach java fern to a piece of driftwood. I just went back and clipped it off with scissors after a month and a half or so. I can't say that I'd recommend it though, as it is an eye sore if you don't hide it just right.
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