they do need a lot of space so they wont do well in a tub. How old are they? do they get plenty of sun?

How long has it been since you dug up the crown/s, split and replanted? How old are the plants, and ave you given them a good dollop of composted manure mulch in spring at all?

They have grown in the same place more than 5 years and get reasonable mount of light. I give them compost regularly. But I think the growing place is coming too full... How often I should dug old?/ new? grow up? and what time of the year?

I've split them successfully when dormant, but its best not to pull any stems from them the first season after moving. Do it when the leaves have died down in the autumn.

Agree,dig up in autumn,chuck out any weak or poor parts, split large crowns and replant. And don't pick next year.

Thanks. Will wait until autumn....