I've always used aluminium (cant afford copper
)
My little ones in a 50:50 mix grit / compost, I was told they like it gritty n well drained so I made it up as I went
,
With the larger ones I had in the same compost last year I noted exceptional Growth, and its the first to come out this year.
I think I might use a mix like the one I'm experimenting with: 3:2:1 : Cat Litter: Grit: Multi-Purpose, It seems really good for moisture retention, and drainage, the Cat litter (not the one which breaks up) holds the moisture for ages, while there is no water sitting in the pots, My Box was suffering a bit at the end of last year because I cut it's feed a bit too early, so its new growth went a bronze colour, but Since I got it in the new compost its gone bright green! So the small amount of Multi-purpose must have enough nutrients in it to suit a tree like that. I can't tell for certain until they all start growing properly, But hopefully alls well!
-Matt-