Yellow flag iris

Sun

Full sun to partial shade; flowers best in full sun.

Water

Prefers consistently wet soil or shallow standing water. Established plants tolerate occasional drying, but do not need deep water.

Soil

Moisture-retentive loam or clay, including muddy pond margins; tolerates a wide range of soil pH.

Height

3-5 ft (0.9-1.5 m)

Spread

2-3 ft (0.6-0.9 m) initially; spreading rhizomes can form much larger colonies.

Flowers

Late spring to early summer; yellow flowers with darker markings on the lower petals.

Foliage

Broad, upright, sword-shaped green to grey-green leaves in dense fans; usually die back in winter.

Growth

Fast

Care needed

High

• Pollinators
• Deer resistant
• Heat tolerant
• Low allergen

Northern hemisphere months. In Australia, shift them by six.

Difficulty

Hard

Toughness

Very tough

Pick your zone, country, light and whether you have pets.

1. Planting day

   1 ft (0.3 m)

2. After 1 year

   2.5 ft (0.76 m)

3. After 2 years

   4 ft (1.2 m)

   full size

About 2 years to full size.

From a typical nursery size, at an average rate for this kind of plant. A rule of thumb, not a forecast.

Cats and dogs

Toxic

Native to

the UK

Invasive in

the US, Canada and Australia

In the garden

Spreads vigorously: plan to contain it.

Growth rate

Fast

• Check local restrictions before planting; avoid it where it is invasive or could escape into waterways.
• Where appropriate to grow, keep the roots in wet soil or shallow water, with the crown close to the water surface.
• Do not routinely feed plants in pond margins; extra nutrients encourage excessive growth and algae.
• Remove flower stems before seed capsules develop, and inspect nearby ground for seedlings.
• Lift and divide congested clumps in spring or after flowering, wearing gloves; collect every rhizome fragment.

Pruning. Cut flower stems down to the base immediately after flowering to prevent seed production. Remove dead leaves in late autumn or winter, wearing gloves because the sap can irritate skin.

Late winter

Remove remaining dead foliage and check that clumps have not spread beyond their intended area.

Spring

Where growing it is appropriate and legal, divide overcrowded clumps and remove unwanted rhizomes completely.

Late spring

Check leaves for sawfly larvae and remove them by hand if feeding damage is substantial.

Early summer

Cut off spent flower stems before seed capsules form.

Summer

Maintain wet soil during dry weather and remove seedlings or escaped shoots promptly.

Autumn

Collect loose rhizome fragments and seed heads; dispose of them according to local invasive-plant guidance, never in waterways.

• In its native range, use small, managed groups at a large pond margin for upright foliage and yellow spring flowers.
• Pair with lower-growing wetland plants to contrast broad vertical leaves with softer ground-level growth.
• Use in a managed bog garden as a strong vertical accent, allowing access for regular thinning.
• In suitable native-range gardens, repeat clumps along a damp boundary to create a seasonal ribbon of colour.

Garden styles it suits: Bog garden, Cottage garden, English country and Architectural minimal.

Plant it with: Marsh marigold, Meadowsweet and Water avens.

Wildlife: Flowers provide pollen and nectar for visiting insects, including bees. Dense invasive stands can displace native wetland plants and reduce habitat diversity.

• Uncontrolled spread: rhizomes and floating seeds can establish dense colonies. Remove seed heads and escaped growth; pond baskets do not reliably prevent escape.
• Iris sawfly: larvae, particularly in the UK, chew leaf edges and may strip foliage. Hand-pick larvae when practical.
• Leaf spot: brown or streaked patches develop on leaves. Remove affected dead foliage and thin crowded clumps to improve airflow.
• Poor flowering: deep shade or severe crowding reduces bloom. Move to a sunnier position or divide the clump where continued cultivation is appropriate.

Toxic to people, cats, dogs and horses if eaten. All parts are potentially harmful, especially the rhizomes; ingestion can cause mouth irritation, drooling, vomiting, abdominal pain and diarrhoea. Sap can irritate skin. Seek medical or veterinary advice after ingestion.

Where it spreads. Spreads aggressively by rhizomes and waterborne seed. It is invasive across much of the United States, including the Pacific Northwest, California, the Northeast and Great Lakes region; in parts of Canada, including British Columbia, Ontario and Quebec; and in southeastern Australia, including parts of New South Wales, Victoria and Tasmania. Sale or planting is restricted in some jurisdictions.

• Northern blue flag (Iris versicolor)

  Has violet-blue flowers with yellow-and-white markings rather than wholly yellow flowers; foliage alone can be difficult to distinguish.

• Southern blue flag (Iris virginica)

  Usually has violet-blue flowers with yellow signals; yellow flag has yellow flowers and typically forms especially robust, dense clumps.

• Sweet flag (Acorus calamus)

  Has aromatic leaves and a small, greenish, finger-like flower spike rather than showy iris flowers.

The species name pseudacorus means 'false Acorus', referring to the resemblance of its leaves to those of sweet flag.

Propagation. Usually propagated by dividing rhizomes in spring or after flowering, but do not propagate where invasive or restricted. Seed also germinates readily and is a major route of escape; remove seed heads rather than allowing self-sowing.

Accepted botanical name

Iris pseudacorus

In the US and Canada

Introduced in Canada and the lower 48 states.

Native range

Albania, Algeria, Austria, Baleares, Baltic States, Belarus, Belgium, Bulgaria, Central European Rus, Corse, Czechoslovakia and Denmark.

Measured

Soil pH 7 to 7.5. Average height 66 cm.

Sources: GBIF (CC BY 4.0), USDA PLANTS and Trefle (CC BY 4.0). The care and design notes are LandscapeGPT's own.