I don't know the 'proper' answer but, when it was introduced at Alnwick, the gardeners started it in the greenhouse and planted it out after all risk of frost had gone. By that time it was about 30cm high.

It seemed to be OK. Flowered, ponged, died back and came again the following year.

Good answer poison. If the professionals do it in a certain way then it's best to follow that for a similar result.

Furthermore though if you are going to be leaving the plant in the round after it's died back, you need a good free draining soil so it doesn't sit dormant in a pool of water. Then if you provide a good layer of mulch above (leaf mulch etc) that should protect the roots from the frost - though obviously it will take longer to get going than if you lifted it, potted it up and kept it inside the greenhouse for the winter before repeating the process.