Marginal pond plant
Yellow flag iris
Iris pseudacorus L.
A vigorous, clump-forming native marginal iris for pond edges that flowers May–July; plant in shallow water and contain/manage in small ponds to prevent it dominating.
PLANT PROFILE
About Yellow flag iris
Yellow flag iris (Iris pseudacorus) is a rhizomatous, clump-forming marginal (emergent) perennial commonly used on UK pond margins. It is robust and readily establishes in wet substrate, producing tall sword-like leaves and yellow flowers, and is valuable for wildlife — Freshwater Habitats highlights its value for water voles and it is frequently included in wildlife-pond planting lists.
Depth guidance from the supplied sources varies slightly: Freshwater Habitats places yellow flag in the marginal group with a planting depth of about 2–10 cm (push rooted plants into shallow sediment), while RHS lists the species in a 5–15 cm suitable water-depth band. For planting, buy small rooted plants and press them into the sediment at the margin or plant divisions; RHS recommends propagation by division (midsummer–early autumn). In small ponds the species is not recommended unless contained: RHS advises planting vigorous marginals in containers (for example small pots) for small pools to restrict spread.
Management is important because the species can spread and dominate if unchecked. Both Freshwater Habitats and RHS recommend regular thinning, cutting back tall emergents, removing old foliage in autumn, cutting seed heads before they ripen, and careful disposal of removed material to avoid spreading fragments. GOV.UK guidance on invasive and non-native plants (England & Wales) is cited for disposal practice — although Iris pseudacorus is not listed on the statutory England & Wales invasive-plant list in the supplied sources, the general advice not to release pond plants into the wild and to dispose of plant material carefully applies.
Note that RHS flags the plant as potentially harmful if eaten, so handle with care (use gloves). It is hardy (RHS H7) and will die back to overwintering rhizomes, re-shooting in spring.
POND SUITABILITY