By Barbarak
North Yorkshire,
United Kingdom 
I bought a blechnum Volcano. A Brazilian tree fern. It came in a 9cm pot and is about 5" tall. I intend to container grow it initially. My question is do I repot when the roots fill the pot or will it be better going into a large pot straightaway?
19 Jun, 2019
I think I'd wait until the roots are coming out the pot rather than filling the pot.
they are not plants that I grow. But the photo of it looks like it is a very nice plant.
here is the RHS advice page
https://www.rhs.org.uk/Plants/296496/i-Blechnum-brasiliense-i-Volcano/Details
19 Jun, 2019