I have mine in the ground and it does well getting bigger every year. it is fully hardy so pop it in the ground and it should be fine. personally I wouldn't grow it in the pond margins.

https://www.rhs.org.uk/Plants/96505/i-Iris-chrysographes-i-black-flowered/Details

Mine is in the garden too. It's in 'average' soil (not wet) and is extremely happy

Many thanks for both your replies, I'll release it from its pot and pop it into the soil where it is. The label suggests it likes to be kept moist and as the Chilterns have been dry and dusty for much of the summer I played safe by standing its pot in a bowl of shallow water which was topped up most evenings. The guidelines on the label suggested to me that they are not as tolerant as you have both found them to be, perhaps they are right about very young plants.

I find it to be bomb-proof in the garden! I grow it in a moderate, neutral soil in Scotland. Last summer was very dry and hot (now a distant memory) and this summer very much the opposite! It has been in situe for many years an produces more rhizomes each year so is now a large plant.

That is very reassuring Bulbaholic, thank you :-) All too often the plants I fall in love with don't care for the conditions in the Chilterns, usually it is the clay they are adverse to, so I am probably inclined to cosset even robust new plants too much ! Here's hoping my black iris does as well as yours, my fingers are crossed. ;-)