You get what you pay for, Johnathon. In your question you have already given the answere; the tubers were plug plants that have been grown on for one or more years and have taken up space and time in the nursery, so entailing extra cost to give a more mature plant.

I'm still confused just a little. The tubers are wee brown, dry looking thingies which I bought from Thompson and Morgan, just three and a half cm wide. How can they be mature plants? And how can they be more mature or more advanced than the plugs are?

Like any good Goverment official, Johnathon, I am afraid that I cannot comment on specific cases. Although, once again, T&M and price?

Will my little three and a half cm wide tubers which T and M have sent me grow to reach full sized plants this year? Or will they only reach full size and full flowering potential next year?