when do you know 30' bamboo is mature enough to harvest?

Question

I tried when it was new ,but it was so wet it rotted ,

Answer 1

Harvest it when it's the diameter you desire and as BNLC said let it dry. Lay it out in the sun on something flat, if you want it flat. As it dries it will harden in that position.

Answer 2

you can harvest any time, but you need to make sure it dries out before you use it in wet areas. then you should seal it before use as well. It is easier to work while green, so harvest, make cuts and bends or whatever, then dry and seal