Species · genus Acorus · Acoraceae

Calamus

Acorus calamus L.

Also known as: Common Sweet Flag · Myrtle Flag · Flagroot · Sweet Calamus · Sweet Flag

Growth habit

perennial or rhizomatous geophyte

Native range

Kazakhstan

Acorus calamus — reference photo — Kostka Martin · CC0

Cultivars of A. calamus

The named forms growers keep — each with its own page, each honest about how much we know.

No named cultivars catalogued under A. calamus yet. When growers document a stable, named form, it earns its own page here.

Guide

The species-level account. Cultivar pages build on this and say where they differ.

Sweet Flag, a Pond Perennial

Acorus calamus, known as sweet flag, is a spreading, deciduous marginal aquatic perennial with strap-shaped, aromatic bright green leaves. It reaches roughly 60 to 90cm in height, and in midsummer produces small, unremarkable brown flowers held just below the tips of leaf-like stems.

Source: RHS

Sun, Water, and Soil

Sweet flag wants full sun, ideally a south- or west-facing spot, and tolerates both exposed and sheltered positions. It belongs at a pond margin, sitting in very moist to wet, poorly-drained soil, and grows best in water no deeper than 22cm. It is untroubled by soil pH, taking to acid, neutral, or alkaline conditions, and is rated hardy throughout the UK.

Source: RHS

Dividing the Rhizome

Propagation is by division of the rhizome, done at the start of the growing season. Newly divided pieces should be potted up and only planted out once they are fully established.

Source: RHS

Form and Foliage

This is a clump-forming plant with linear, sword-shaped foliage typical of the Acorus genus. Its ultimate height and spread both sit in the range of 0.5 to 1 metre, reached over two to five years. The leaves carry a distinct aromatic quality, and the summer flower spikes are brown rather than showy.

Source: RHS

Largely Untroubled

Sweet flag is generally free of both pests and diseases, which makes its care rhythm mostly a matter of position and moisture rather than vigilance against attackers.

Source: RHS

Cutting Back and Dividing

Cut the foliage back after flowering has finished. Divide the clump every few years as well, since congestion builds up over time at the pond margin.

Source: RHS

Care recipe

The species record — the baseline every cultivar inherits unless it says otherwise.

Care notes are early — not yet verified

Light: ai:claude-opus-4-8
Water: ai:claude-opus-4-8 (range 2-5d)
Humidity: ai:claude-opus-4-8
Soil: ai:claude-fable-5
Care: ai:claude-fable-5

Light

Bright, indirect

Water

≈ every 5 days

Humidity

High

Temperature

10–24 °C

Soil mix

Heavy, water-retentive mix; tolerates boggy conditions

Propagation

Rhizome division

Fertilizing

Light monthly feed during active growth

Growth rate

Moderate

Flowering

Rarely flowers in cultivation

ToxicityAuthoritative · species

Not yet documented

This is where toxicity lives — every cultivar of this species inherits it and none can override it.

Reference images

Every image credited. Licenses link to their deeds.

Sources: ai:claude-opus-4-8 · ai:claude-opus-4-8 (range 2-5d) · ai:claude-fable-5

Kept true by its growers

Every fact here carries its source. Grow Calamus yourself? What you’ve observed — how it drinks, how it holds its color, a name that’s off — and a clear photo of your plant are exactly what keep this page honest. Contributions are reviewed before anything changes.