JennyC writes
"Robert"
We are planning on growing some dwarf Cannas in containers this year and
would be grateful for advice.

I'm not familiar with the species you mention Robert, but any container will
soon be too small !
They expand enormously, growing extra rhizomes.
6" seems a bit close to me unless they are the dwarf types.
Jenny

They are listed as 'small' which I understand to be around 2' tall and
are apparently suitable for planting in containers. We had thought of
using 18"/20" terracotta pots.

Sounds OK. Mine are all in pots. Though I use plastic as it's lighter for on the
roof garden. Terracotta will however keep them from blowing over as its a
heavier base.

Do they send up more than one shoot per rhizome as a norm?
Hart say that a single rhizome will produce typically 4 to 8 rhizomes in
a year - is this realistic?
-- Robert

My experience is that they are VERY prolific. The rhizomes form new ones fanning
out from the original one. I've had them split hefty plastic flower pots of 60
cm diameter!

Jenny