Just time and patience, I think. The last time thatr I split a clump it was years before they really got going again. I am now forbidden to split any more clumps of iris in the garden:-(

Thanks, Bulbaholic. As long as I know they will come on again, I can wait. Maybe next year....

I find that the trouble with big clumps is that as they become more congested I find that less and less plants are flowering. I'm thinking of trying a two stage process, by tackling say half the clump at a time and by removing plants carefully instead of digging up the whole lot, splitting it and then replanting some. My theory is that the single plants left will feel free to do their own thing and, overjoyed at having bags of space will flower their heads off. Next year, same again to the other half of the plants. Needless to say I will sprinkle a general fertiliser round the plants that are left in position. Be interesting to see if this works and may avoid the problem Bulbaholic has experienced.