Collie,
I have two month old plants six foot high in my front garden .
A pal sent me three tubers .. don't know how they are growing underground..
Our front garden is hopefully soon to be landscaped ( we have just done the back forty acres
)
They are growing in rubbish ground of heavy coalmine bluish clay heavily manured with horse muck so should do well in most places. They are the round type.
As mentioned I've bee told to harvest them when the tops start to die back ..drop me your address if you want a few FOC when I take them up and replant some of the harvest.
David
PS.
I'm told that once they get established they are like weeds and very hard to eradicate , they evidently make a good summer screen if planted in three rows two foot wide.