Japanese Garden Juniper (Juniperus procubens 'Nana') 22 yrs

I guess the pots must be quite weighty, so that the trees aren't top-heavy and fall over ?

Good question, I've never picked one up. Mostly made from ceramic or clay, I think.

Yeah your right Tastyg, however the tree roots are wired in place by passing the wire through the drainage holes and twisting the ends together to tighten. Also remember the damp potting medium gives the pot some stability too.