The zone is 5-6, in EU.

Ok, that's what I figured. (btw be sure on an American WWW site to mention if you mean +1 C or +1 F, we continue to use this archaic non-metric measure...!)

Like I said, just let it recover on its own schedule. If this or related cultivars become established plants in your local area though, you're going to have to investigate why it keeps getting burnt to this extent. Which does set the plant back, and is obviously unsightly. Do you have some kind of evergreen nearby, you are waiting to grow to provide additional wind and sun protection? I'm guess the wrapping was needed for winter protection. Some yaku cultivars are only zone 7 hardy...is it the pure species? Here is a website with some evaluations of rhododendrons in a cold North European climate: http://www.mustila.fi/en/plants/rhododendron

I wrapped against the sun. But I did it already in fall, maybe after that they didn't get any water (maybe the coating did not let any water trough) and now the because of the direct sunlight temperature is getting too high and soil is still frozen.. My neighbor has the same plants, but under pines, perfect location and they are fine. I think I have to replant them in spring.

It is Yakushima Polaris, Nova Zembla is quite okay, not that damaged. I think the location is not right.

Thank you again.

When did the problem start? That looks like freeze damage to me. How cold did it get before it started to look like this? If you had a very cold spell combined with very dry soil (not enough watering) this is exactly what happens.

Hi, you're right, I think it was about -15C briefly (3 weeks ago). And after that about -8C for a week. I took the covers off, to let moist into the soil.

"Yakushima Polaris" sounds like a Hachmann cultivar to me. In North America, some Hachmann hybrids have proven to be not as hardy as advertised. Not saying that is the case here, though.