Blue flag iris

Sun

Full sun to partial shade; flowering is best with at least six hours of sun where moisture is plentiful.

Water

Keep consistently moist to wet. Established plants tolerate shallow standing water up to about 4 in (10 cm) above the rhizomes.

Soil

Humus-rich, moisture-retentive soil, preferably acidic to neutral; tolerates wet clay.

Height

2-3 ft (0.6-0.9 m)

Spread

1-2 ft (0.3-0.6 m), gradually widening through rhizomes

Flowers

Late spring to early summer; violet-blue flowers with purple veins and yellow-and-white markings on the falls.

Foliage

Upright fans of green to blue-green, sword-shaped leaves; dies back in winter.

Growth

Moderate

Care needed

Low

• Pollinators
• Deer resistant
• Cold hardy
• Wildlife
• Low allergen

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

Difficulty

Easy

Toughness

Fairly tough

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

1. Planting day

   1 ft (0.3 m)

2. After 1 year

   1.5 ft (0.45 m)

3. After 2 years

   2 ft (0.6 m)

4. After 3 years

   2.5 ft (0.75 m)

   full size

About 3 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 US and Canada

In the garden

Spreads vigorously: plan to contain it.

Growth rate

Moderate

• Plant in spring or early autumn with the rhizomes just below the soil surface.
• Water during dry spells so the root zone never dries out completely.
• Mulch moist-border plants with leaf mould, keeping the leaf bases uncovered.
• In border soil, apply a light compost dressing in spring; avoid fertilising plants in ponds.
• Divide congested clumps after flowering every three to five years.

Pruning. Remove spent flower stems at the base unless you want seed. Cut leaves back near ground level once they have yellowed in autumn, and remove dead foliage rather than leaving it around the crowns.

Early spring

Clear any remaining dead leaves and check that the planting area remains moist.

Spring

Plant new clumps and top-dress border-grown plants lightly with compost.

Late spring

Check for chewed leaves or water-soaked streaks that may indicate iris borer in affected North American regions.

Early summer

Remove faded flower stems if seed is not wanted.

Summer

Maintain moisture and divide crowded clumps after flowering.

Late autumn

Cut back yellowed foliage and remove debris around the plants.

• Plant in groups along a pond margin for vertical foliage and blue flowers.
• Combine with marsh marigolds and royal ferns in a bog garden for contrasting leaf shapes and successive seasonal interest.
• Use in the reliably moist lower parts of a rain garden, not on its dry upper edges.
• Mix with sedges and cardinal flowers in a naturalistic wetland planting.

Garden styles it suits: Bog garden, Wildlife garden, Native meadow, Cottage garden, New perennial (prairie) and Blues and silvers.

Plant it with: Marsh marigold, Royal fern, Cardinal flower, Swamp milkweed, Tussock sedge and Joe-Pye weed.

Wildlife: Flowers provide nectar and pollen for bees, including bumblebees. Dense clumps offer cover for small wetland animals.

• Dry soil causes browning leaves and poor growth: water deeply and move the plant to a wetter position if necessary.
• Iris borer occurs in parts of North America: remove infested leaves or rhizomes and clear dead iris foliage in autumn.
• Fungal leaf spots cause brown patches: remove badly affected leaves, improve spacing and avoid overhead watering.
• Bacterial soft rot makes rhizomes soft and foul-smelling: lift the clump, discard rotten sections and replant only firm, healthy pieces.

Toxic if eaten by people, cats, dogs or horses. All parts contain irritant compounds, with the highest concentrations in the rhizomes; ingestion can cause drooling, vomiting, abdominal pain and diarrhoea. Sap may irritate skin. Keep rhizomes away from children and animals, and seek medical or veterinary advice after ingestion.

• Yellow flag iris (Iris pseudacorus)

  Has yellow rather than violet-blue flowers and generally forms larger, more vigorous clumps. It is invasive in many regions, so identify plants before sharing divisions.

• Siberian iris (Iris sibirica)

  Usually has much narrower, grass-like leaves and flowers held on slender stems above the foliage. It prefers moist border soil rather than prolonged standing water.

Iris comes from the Greek word for rainbow, while versicolor means variously coloured, referring to the flower's contrasting colours and markings.

Propagation. Divide rhizomes after flowering, retaining healthy roots and at least one leaf fan on each piece. Seed can be sown fresh outdoors; germination often follows a period of winter cold, and seedlings take several years to flower.

Accepted botanical name

Iris versicolor

In the US and Canada

Native in Canada, the lower 48 states and St. Pierre and Miquelon.

Native range

Connecticut, Labrador, Maine, Manitoba, Maryland, Masachusettes, Michigan, Minnesota, New Brunswick, New Hampshire, New Jersey and New York.

Measured

Average height 79 cm.

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