Description
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.
