Different plants that produce different fabric in different biomes. This has 2 reasons:

1. Encourage exploring to find them.
2. Being able to make string regardless of the biome you spawn in.

With renewable I mean either as crop or by spreading.
I was thinking:

• Flax (Linen)
• Cotton
• Rushes (Jute)
• Hemp

Some are already in the mod I think?

Only cotton is so far, and as it stands right now, everything just spawns in every biome (not ideal, sure, but it's too much work and I really don't have the effort to put it into place).
As far as spreading, the plantball should cover any possible needs for plant duplication.