Hi, just chiming in with my project of trying to grow outdoor yuzus in Helsinki/zone 6b.
- Started on March 2022 after ordering fresh yuzu for some seeds. Germination March-April.
- Moved small plants on my balcony during May for the summer. They seem to grow better indoors with lamps, but I'm low on space indoors.
- On October I decided to split plants into two groups. About a third I'm trying to make survive on my balcony during the winter. I'm planning to use various methods and see how they react to them. Some will be left fully exposured to the weather. Some will be stored on open and closed styrofoam containers. Some will be wrapped in frost-protecting blanket. Two thirds of the plants I brought indoors to grow some better roots & stem, and I will repeat the outdoor procedure next winter. Some of the plants I will plant outdoors next spring and see if they will survive at all in 6b zone.
The plants I'm trying to make survive the winter: https://imgur.com/a/g7wkLIE
Last winter Helsinki experienced temperatures going as low as -18c. My balcony stays about five degrees higher than this. Freezing temperatures lower than -25c are very rare in vicinity of the ocean, but occasionally do happen.
Welcome to the forum and to the wonderful world of citrus.
Your seedlings look strong and healthy. However, you will have to be patient for them to flower (5-7+ years). If you plant them too small and they freeze back, you might never see flowers.
If you don't know it already, you should learn how to graft and sow some Poncirus rootstocks. It saves you so much time (and money too). As far as I know, there are no or few restrictions within the EU, so you should be able to source budwood from other growers fairly easily.
Poncirus trifoliata is the only citrus that could theoretically handle your zone without protection. Have a look at less awful selections like Poncirus+ or Swamp Lemon.
And finally, citrus coldhardiness depends on a lot of factors. They don't like too much winter sun followed by freezes or prolonged freezes. It doesn't matter that a Yuzu can take -12 C or lower. If your daytime highs stay below 0C for several days or even weeks, it will cark it much sooner.