Hi
I grow them outdoors but i certainly wouldn't call them "slow"lol. Will get 8/10 feet tall with equal spread in under 5 years . I grow button fern and Boston fern under mine ,not because I want to but because they pop up lol. gary

Ten feet tall??? That's not right, Gary (you're confusing the species under discussion with Cyathea cooperi). No, Dicksonia antarctica is extremely slow growing and a trunkless nursery sporeling would be unlikely to exceed even SIX INCHES in five years! For that reason if no other, it's not practical to make it share its tub with a companion plant - either the poor Dicksonia will be swamped or its preference for shade and very high permanent moisture will not suit the companion plant. But it doesn't mean you can't decorate the tub with a moveable feast of some small bromeliad pots say - just don't literally plant together or make them share root space. I don't think you'll be able to mix any other ferns in this area until the tree fern is a lot bigger...always assuming you can get it through the winter. It's tricky overwintering tree ferns indoors (unless you have a frost-free glasshouse) so maybe start planning for this well in advance.

And the compost? Tree ferns like a slightly acid mixture and high in organic matter (and which will need enriching every season with generous re-mulching). Ericacious peat-based compost and bark-based mixtures blend together very well at a 50:50 ratio.