On 01/11/2020 16:31, TimW wrote:
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?

The best rope for outdoor use is undoubtedly Manila. It can last up to
10 years either in dry or in wet but windy conditions. Not having a
chance to dry out will, however, shorten its life. It needs to be
pre-shrunk, by keeping in water overnight, when it will lose up to 10%
of its length. Failing to do that will result in it shrinking when it
first gets wet.

Sisal or hemp are unlikely to last more than three years outdoors and
may last much less.

Tarred rope requires regular re-treatment to avoid the rope rotting and
I suspect that pine tar is probably not something you want around
nesting boxes.

--
Colin Bignell