My P. trifoliata are in a field fully exposed to wind and everything. They have taken -5 F every winter for 5 winters. I don't have my calculater along, but that is colder than -10 C. I don't think cold will be a problem with established plants.
My way has been to buy fruit at Hong's Landscape in Wichita, Kansas. His 2 trees are over 15 years old, and Mr. Hong doesn't remember where he got them and doesn't have any information on which variety he has.
So I harvest the seeds and put about 25 in a 1 gallon pot and let them grow inside for their first winter. By spring, the 4 or 5 month old seedlings are planted where they are to grow and tended like tomato plants until mid August to get as much growth on them as possible. In mid August, I quit feeding them and reduce water to harden them for winter. During winter I water a little if it hasn't rained or snowed for a month. I dought you need to water.
There may be better ways to do it, but this has worked for me. Once established, they are tough plants.