This grower is fairly sure of this plant identification.
Yellow flag iris, Water flag
Iris pseudacorus (Yellow flag iris). This species will produce blooms which are a pale to deep yellow and sometimes have a dark purple or brown zone radiating from the centre of the petals. The leaves are attractive and glossy green. This plant is often found in the wild in Northern Europe.
They are fully hardy, but some groups may thrive only in specific growing conditions. All irises are best planted in late summer and early autumn. Outdoors you can grow them in well drained, fertile, neutral or slightly acid or alkaline soil in full sun to lightly dappled shade. In order for irises to thrive, most of them need long, hot summers. Irises that require protection from rain or frost should be grown under glass.
Bog, aquatic, border, cutting bed.