I am involved in a project to put some small wooden structures like
nesting boxes out in some woodland. The Desire is that they should be
100% (or at least 99%) biodegradable, so they last as long as poss but
ultimately rot away to nothing. Fixing them in trees with synthetic
ropes or straps is therefore obviously a complete no-no. So traditional
rope is required.

There's a bewildering array of different materials - coir, hemp, flax
etc and some treatments as well like tarred rope. Does anyone know what
is a good choice here for just lasting a few years out in the weather?
Is it better to have thicker rope for lasting, or just strong enough to
do the job?

TW