Sweet flag

Sweet flag: how do you recognise it?

Habit

Upright fans of sword-shaped leaves up to 1 m tall, rising from a creeping rhizome.

Leaves

Narrow and sword-shaped, up to 2 cm wide, fresh green with a clear midrib and a margin that is wavy in places.

Scent

When bruised, leaves and rhizome smell spicy, of tangerine and cinnamon.

Flower

Rare and inconspicuous: a greenish-yellow spadix jutting out sideways from a leaf-like stem. It does not set seed here.

Roots

A thick, fleshy, creeping rhizome that spreads through the mud.

Plants: April–June · Pruning: March and October

Why grow it?

• Crisp, upright leaves give structure to the pond edge from spring to autumn.
• The bruised leaves smell fresh, of tangerine and cinnamon.
• Grows both in the marsh zone and in shallow water up to 20 cm deep.
• Tough and fully hardy, even in nutrient-rich water.
• The dense leaves shelter frogs, and dragonfly larvae climb up them to emerge as adults.
• The variegated form brightens a dark corner of the pond.

Sweet flag: where does it grow best?

• Sun or partial shade; it grows densest in full sun.
• Marsh zone or shallow water: 0–20 cm of water above the basket.
• Loam, clay or peat; acid, neutral or chalky: it is not fussy.
• Allow 40–60 cm per plant, and in a small pond one aquatic basket per plant.

Light

Sun and partial shade

Soil type

Loam, clay or peat

Acidity

Not fussy

Moisture

Wet

Size

1 m tall, 0.6 m wide

Hardiness

Hardy (to -30 °C)

Goes well with

• Yellow flag: Yellow flowers in May–June with similarly shaped leaves.
• Flowering rush: Pink flower umbels in summer, in the same shallow water.
• Marsh marigold: Yellow spring flowers at its feet, in the marsh zone.
• Water forget-me-not: Pale blue flowers that weave between the stiff leaves.
• Water plantain: Airy white flower sprays beside the crisp foliage.

Sweet flag: when and how to plant it?

Plant in April–June.

1. Plant from April to June, as the water warms up.
2. Use an aquatic basket at least 20–30 cm wide, filled with heavy, low-nutrient aquatic soil or clayey garden soil without fertiliser or compost.
3. Lay the rhizome horizontally just below the soil, with the fan of leaves above, and firm in well.
4. Top with a layer of gravel so the soil does not wash out and fish do not dig in it.
5. Lower the basket to 0–20 cm below the water surface; set young plants a little shallower at first.
6. Keep 40–60 cm between plants.

Care

Water

Must never dry out. In dry summers, top up the pond with rainwater.

Feeding

Not needed: fertiliser in the pond encourages algae.

Spread

Check each spring whether the rhizome is growing over the rim of the basket and cut those pieces off.

Repotting

Every 3–4 years, lift it from the basket, divide it and replant in fresh aquatic soil.

Dead leaves

Remove yellowed leaves from the water in autumn, so they do not rot there.

Tip: Always plant it in a sturdy aquatic basket at a water depth of 0–20 cm: that keeps the thick rhizome in check and makes it easy to divide.

Sweet flag: when to prune it?

Pruning months: March and October.

When: In October, cut off the dead leaves above the water; in March, the rest of the old foliage, before the new shoots appear.

How:

• Cut yellow and brown leaves back to a few centimetres above the water surface.
• In autumn, leave some foliage as shelter for insects and clear it in March.
• Always remove the cut leaves from the pond.

Caution: Wear gloves if you handle the rhizome a lot and wash your hands afterwards: it is poisonous.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Division

In April–May, cut the rhizome into pieces of 5–10 cm, each with a growing point and a few leaves, and plant each piece in its own aquatic basket.

Seed

Sweet flag does not produce viable seed here: it is propagated by division only.

Sweet flag: how does it get through the winter?

Hardy to −30 °C.

• Fully hardy, to about −30 °C: no protection needed.
• The rhizome overwinters in the mud; most of the foliage dies back.
• A basket in shallow water may freeze solid: the rhizome survives it.

Diseases and pests

• Spreading: The rhizome grows over the rim of the basket and roots into the pond floor. Cut off runners every spring.
• Aphids: In summer, aphids may settle on young leaves. Hose them off with a strong jet of pond water; frogs, fish and insects deal with the rest.
• Brown leaf tips: These appear naturally in late summer and after frost. Trim them off: it is not a disease.
• Reverting: The variegated form may produce plain green, more vigorous shoots. Cut them out to keep the variegation.

Through the year

• March: Cut old leaves back to above the water.
• April: Plant or divide. Cut off runners outside the basket.
• May: Plant. The new leaves grow quickly.
• June: Plant. Watch for aphids.
• July: Top up the water in dry spells.
• August: Top up the water in dry spells.
• October: Cut off dead leaves above the water and remove them from the pond.
• December: Rest.

Edible and useful

Not edible. Sweet flag root was once candied and used to flavour bitters and liqueurs, but it contains beta-asarone, which has caused cancer in laboratory animals: do not use sweet flag as a herb, tea or liqueur. It often grows next to yellow flag, which is poisonous too and has almost the same leaves when not in flower; you can tell sweet flag by the spicy scent of a crushed leaf and its wavy edge. Keep children, dogs, cats and grazing animals away from the pond edge and wear gloves when dividing the rhizome. If any is swallowed, call a poison centre (in Belgium 070 245 245) or your doctor at once.

Did you know?

• Sweet flag reached western Europe in the 16th century by way of Turkey. Because it does not set seed here, it has spread only through pieces of rhizome along rivers and canals.
• In the past, the fragrant leaves were strewn on the floors of churches and houses and the rhizome was used as a flavouring; because of its toxic compounds, this is no longer advised.
• Sweet flag belongs neither to the irises nor to the arums, but has a family of its own, one of the oldest branches of the monocots.