How big does Japanese sweet flag grow?

About 30 cm × 30 cm after ten years (height × width). Its ultimate size is 10–30 cm × 15–30 cm.

What soil does Japanese sweet flag grow in?

It tolerates these soils: average garden soil, clay, acidic.

Is Japanese sweet flag hardy?

Hardy down to -23 °C (USDA zone 6, RHS H7). It gets through winter everywhere at lower altitudes, but may not survive unprotected in a mountain garden.

When does Japanese sweet flag flower?

Flowering: May–July, green, yellow.

How much water does Japanese sweet flag need?

Water need: high. Drought tolerance: low.

Japanese sweet flag

Acorus gramineus

Japanese sweet flag (Acorus gramineus) is an ornamental grass that reaches 30 cm × 30 cm after ten years. Light: full sun, part shade. Hardy down to -23 °C. Drought tolerance: low.

Illustration of Japanese sweet flag — An illustration of the typical shape of the mature plant, not a photo.

What it's good for, and what it isn't

A low evergreen perennial that looks like a grass but is not one: its fan-shaped tufts of narrow, glossy leaves are decorative all year, and the yellow or white striped forms brighten shadier corners too. It belongs at a pond edge, in shallow water, in a permanently moist bed or in a container, and looks best in part shade. Never let it dry out, because the leaf tips brown at once; it spreads slowly and manageably and does not overrun its neighbours.

Watch out for

In soil that dries out the leaf tips scorch and the leaves wither; in blazing sun and hot summers it only stays good-looking in constantly wet soil or shallow water, so in this climate a spot sheltered from midday sun is better. Hardiness is borderline in Central Europe: sources rate it reliable to roughly -20 °C, and in a hard winter without snow the foliage browns or dies back and the crown may be damaged, so mulch it with leaves in such places and protect container plants from freezing through. Comb or cut out the browned leaves in spring. It thickens slowly by short rhizomes and is not considered invasive; in a pond basket it does not spread at all.

Key facts

Light: Full sun, Part shade
Water need: High
Care: Low
Size after 10 years: 30 cm × 30 cm
Hardiness: down to -23 °C
Foliage: Evergreen
Flowering: J F M A M J J A S O N D May–July, green, yellow

Group	Ornamental grass
Light	Full sun, Part shade
Size after 10 years	30 cm × 30 cm
Ultimate size	10–30 cm × 15–30 cm
Hardiness	down to -23 °C (USDA 6, RHS H7)
Soil	Average garden soil, Clay, Acidic
Foliage	Evergreen
Flowering	May–July, green, yellow
Water need	High
Drought tolerance	Low
Maintenance	Low
Root aggressiveness	Low
Suckering	no data
Spreading	Low
Distance from buildings	no data
Distance from paving	no data
Litter	not typical
Planting distance	25 cm
For dogs	no data
For cats	no data
For children	no data
Typical price	no data

Toxicity data is for information only. If your pet or child has eaten a plant, contact a vet or a doctor.

Frequently asked questions

Data updated: October 8, 2026 · How our data is made

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Ground cover plants Evergreen plants Plants for containers Ornamental grasses

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