Sorry Mogsie. Zone 5 in the US means temps typically stay above zero F and go up to 90F summer. Not sure the exact temp ratings, and it also has to do with the type of landscape (i.e. mountains, valleys, sand, oceans, etc.) and where I live is farm ground so pretty flat in this State and Zone.
I'm really bad at converting C to F, so I looked it up. Your 25 summer temps are only 77 and so far you have not gotten below 32. We are very mild here this winter, too. It's been down to about 10F (-12C) this year, but even when we get below 10, it's usually not for very long. However, our winters typically have temps below freezing 32F (0C) for extended periods of time, so ponds would freeze and remain frozen for 1-2 months usually. It's going to have to get pretty cold now to get the ponds (regular farm ponds and our smaller koi/goldfish ponds) frozen and stay that way, but it sure might still do that. It's nearly the end of January, but we could get bad cold and snow yet February and March. Even have had snow in April, but not very often, and it would thaw usually within a day or two.
So, if you're that temperate where you live, and those plants do well, I wonder if they would do ok in my area. Your pond usually DOES freeze, right, just having a mild winter so not much yet this year?