Acorus calamus - Sweet Flag, Aromatic Pond Marginal Plant

Acorus calamus, known as sweet flag, is a tall marginal plant grown for its upright, bright green, sword-shaped leaves, which smell of citrus when crushed. It looks like an iris in leaf but does not have showy flowers.

Sweet flag at a glance

Botanical name	Acorus calamus
Common name	Sweet flag
Plant type	Upright, rhizomatous perennial; dies back in winter
Height and spread	60–90 cm tall, 50–100 cm wide
Flowers	Small, brownish, horn-like spike in early summer; only plants growing in water flower
Position	Full sun
Soil	Wet soil or shallow water
Pond depth	0–15 cm of water over the crown
Hardiness	RHS H7 – very hardy (below -20°C)
Native to	Not UK native; introduced to Britain by 1582

Is sweet flag native to the UK?

No. It was brought to England by 1582 and now grows wild in places, but it is not a native plant. It does not set seed in Britain and spreads only by its roots.

Where to plant sweet flag

A sunny pond shelf with up to 15 cm of water over the crown
Wet ground at the edge of larger ponds
Formal ponds, for strong upright leaves

How to plant and care for it

Plant in a wide aquatic basket in spring or summer, as the roots spread sideways. Cut back old leaves in autumn and divide every few years to stop clumps becoming congested.

Is sweet flag poisonous?

Yes. It contains beta-asarone, which is toxic, and it should not be eaten.

Which size should I buy?

Bare-root plants (7 cm, 9 cm, 1 litre and 3 litre) arrive without a pot or compost and are the most economical way to plant a group. Potted plants (9 cm, 1 litre and 3 litre) arrive growing in our peat-free compost and give a faster display. Packs of 3 cost less per plant. The 2x Pot & Compost option is two plants supplied with pots and compost.