I don't think it's poisonous - I have lots of mounted plants that have
"grabbed" the back of the adjacent slab without an issue. The outside is a
lot more porous, so maybe that's the reason for the preference.

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Ray Barkalow - First Rays Orchids - www.firstrays.com
Plants, Supplies, Books, Artwork, and Lots of Free Info!
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"Wendy" wrote in message
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I have noticed that over the years I have mounted some orchids
on the inside of a round hollow piece of cork.
The plants struggle & I have lost a couple.
The inside of the cork has a dark lining if you will on it.
Could this be a poison to the plants?
Cheers Wendy