In message , nambucca
writes
Planted some cannas last year in large pots sunk in ground
........overwintered in greenhouse ........today found they have huge tubers
.........can i split them in some way to give even more plants

Yes. I am surprised they didn't split the pots already.

Choose chunks that have a couple of growing points and break at the
narrowest points. In a good year they will flower planted directly in
borders given a head start under glass. And with a free root run you get
a huge crop of tubers - difficult sometimes to dig out in one piece.

Regards,
--
Martin Brown