Why yellow flag iris earns its place

Yellow flag iris, Iris pseudacorus, is a British native that has quietly worked its way back into garden favour, and with good reason. It carries bold, deep yellow flowers through late May and June, held on stiff stems that reach 1.2–1.5 metres in a good wet spot. The foliage is just as useful: fans of upright, sword-shaped, grey-green leaves that stay smart from spring until the first hard frosts, giving a border strong vertical shape when many herbaceous plants are still finding their feet.

Unlike many moisture-lovers, it does not sulk when conditions are less than perfect. It will grow in a boggy border, at a pond edge, in a rain garden or a soakaway, and it copes with winter flooding and the occasional dry summer spell. Give it damp ground and it asks for very little else.

Choosing the right spot

Sun is the key to a generous display. An open position with six hours or more of light gives the most flowers and the strongest leaf colour; plants in deep shade survive happily but flower sparsely. Any soil will do, including the heavy clay that defeats so much else, as long as the ground stays reliably damp. The ideal is waterlogged for part of the year and merely moist for the rest.

At a pond margin, the rhizomes will sit happily in 5–15 centimetres of water. Deeper than that and growth slows. In an unlined setting, yellow flag can spread widely through its creeping rhizomes, so baskets are worth the trouble — they also protect a pond liner from the hard, sharp growing tips.

In a wet border or rain garden, simply choose the lowest, dampest patch. Ground that squelches in winter is not a problem to be fixed; it is an opportunity.

Planting and containing it

Bare-root rhizomes and pot-grown plants both establish well from late autumn to early spring, while the soil is cool and wet. Set the rhizome so its top is just visible at the surface — level with the soil, not buried. This matters more than anything else, because a deeply buried rhizome will rot. Space plants 45–60 centimetres apart and water them in well, even if the ground is already wet.

In a basket, use a heavy loam-based aquatic compost rather than a fluffy peat-free blend that floats away, and top the surface with gravel to stop the compost washing out and birds flicking it about.

Yellow flag is a spreader. In a small garden, keep it in check with a length of rigid edging sunk 30 centimetres deep around the clump, or grow it in a large bottomless pot buried to the rim. Divide clumps every three to five years in late summer to keep flowering strong and stop the centre dying out.

Care through the year

• Spring: clear away collapsed leaves and old stems, then mulch around — never over — the rhizomes.
• Summer: deadhead spent flowers if you do not want seedlings in every damp corner. Leave a few if you fancy the sculptural seed pods, which brown and split in autumn.
• Late summer: divide overcrowded clumps once flowering has finished.
• Autumn and winter: leave the seed heads standing for structure, then cut the foliage back in late winter before new growth begins.

The sap and rhizomes are toxic if eaten, so wear gloves when dividing and wash your hands afterwards. Keep children and pets from chewing the roots, and never leave rhizome pieces where livestock might reach them.

The wildlife it brings in

This is where yellow flag earns its keep. The stiff, upright leaves make perfect emergence ladders for dragonflies and damselflies. A nymph that has spent a year or more underwater climbs a leaf after dark, splits its skin and pumps up its wings before its first flight. Without plants of this kind standing in or beside the water, that final stage has nowhere to happen. Adults then use the leaves as basking perches and hunting lookouts.

In June, the flowers are worked by bumblebees, honeybees and hoverflies, while the dense clumps give frogs, toads and newts somewhere to hide between bouts of activity. A clump beside a pond is one of the most reliable places in the garden to watch wildlife at close range.

Good companions for a damp border

Yellow flag looks best in a loose colony, with lower-growing plants around its feet. Try meadowsweet, purple loosestrife, marsh marigold, ragged robin, water mint, water forget-me-not, soft rush and sedges. They mix well, flower at different times and keep the ground covered so weeds have less room. Keep the most vigorous — purple loosestrife especially — contained or divided regularly.

One last caution: outside its native range, in parts of North America and Australasia, this iris is a serious wetland invader. In the UK it belongs here, but avoid tipping rhizome fragments into wild ponds, ditches or rivers. Keep it in the garden, enjoy it there, and let the dragonflies do the travelling.