-20 C is roughly -4 F zone 6 , more or less
The green bark of 'Flying Dragon' does photosynthesize, they can survive quite well on just the photosynthesis from the green bark of stems, without leaves.
'Flying Dragon' is the asexually propagated clone of Poncirus (Citrus) trifoliata. If you propagated your trees from seed they are not clones of the original. They should just be called Trifoliate Orange, or Poncirus trifoliata. The 'Flying Dragon' is a single clone with highly contorted stems. All stems bend and twist between internodes, sometimes doing 'U' turns between internodes. The normal form has a growth pattern much like an orange or lime. Straight segments of growth between internodes.
Citrus seed sometimes do a "trick" and produce viable seed without a pollinator. This process is "apomictic" and results in something that is not exactly a clone of the fruit bearing parent, but much of the fruit parent's genome comes through with only limited re-assortment. These seeds are not clones, so technically should not carry the clonal name, in this case 'Flying Dragon' but they might carry the most notable trait of 'Flying Dragon' its contorted growth. This is an area of confusion. Not much we can do to sort this out. But if your Poncirus trifoliata came from seed, it technically can not be 'Flying Dragon', it is just Trifoliate Orange.
I've seen mixed notes on hardiness for trifoliate orange, some say zone 6, some say zone 5. I would expect it is probably pretty safely winter hardy through zone 7, and into zone 6, thru 6b. If sited well it might be okay further north in a protected location.