Japanese sweet flag (sekishō) Acorus gramineus
Japanese sweet flag (sekishō) (Acorus gramineus) is an evergreen perennial grass in the Acoraceae family, native to Japan, China, Korea and Indochina. It grows 15–35 cm (6–14 in) tall and 20–40 cm (8–16 in) wide, wants part sun to part shade and high water, and is hardy to about -23 °C (-9 °F). Space plants about 20 cm (8 in) apart.
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Japanese sweet flag (sekishō) at a glance
| Type | Perennial grass, evergreen |
|---|---|
| Family | Acoraceae |
| Mature height | 15–35 cm (6–14 in) |
| Mature spread | 20–40 cm (8–16 in) |
| Spacing | 20 cm (8 in) centre to centre |
| Sun | Part sun to part shade, at least 3 h of direct sun a day |
| Water | High; drought tolerance none, waterlogging tolerance high |
| Soil pH | 5–7.5 |
| Cold hardiness | About -23 °C (-9 °F) once established (roughly USDA zone 6) |
| Heat stress | Likely above about 33 °C (91 °F) |
| Flowering | from about May for 2–3 weeks (northern hemisphere; later in cold climates, earlier in warm ones) |
| Wind tolerance | Low |
| Pollinator value | None |
| Edible | Not grown as food |
| Native to | Japan, China, Korea and Indochina |
| Common in gardens of | 🇯🇵 Japan, 🇨🇳 China, 🇰🇷 South Korea, 🇺🇸 United States, 🇬🇧 United Kingdom |
Is Japanese sweet flag (sekishō) toxic?
- People: mildly toxic
- Cats: mildly toxic
- Dogs: mildly toxic
- Parts: rhizomes, leaves
Contains β-asarone, a suspected carcinogen; do not take it as a herbal remedy. Mild stomach upset if chewed.
If a child or pet may have eaten part of a plant, call a poison centre or vet straight away.
Companion planting: what to grow with Japanese sweet flag (sekishō)
No well-documented helpful pairing. Group it with plants that like part sun to part shade and high water.
Questions about Japanese sweet flag (sekishō)
- How far apart should I plant Japanese sweet flag (sekishō)?
- About 20 cm (8 in) apart, centre to centre. A mature plant spreads 20–40 cm (8–16 in).
- Does Japanese sweet flag (sekishō) need full sun?
- Part sun to part shade: at least 3 hours of direct sun a day.
- How much water does Japanese sweet flag (sekishō) need?
- High water. Drought tolerance is none and waterlogging tolerance high.
- How cold-hardy is Japanese sweet flag (sekishō)?
- It tolerates about -23 °C (-9 °F) once established, roughly USDA zone 6.
- When does Japanese sweet flag (sekishō) flower?
- From about May for 2–3 weeks (northern hemisphere; later in cold climates, earlier in warm ones).
- Is Japanese sweet flag (sekishō) poisonous to cats and dogs?
- For cats it is mildly toxic; for dogs mildly toxic; for people mildly toxic. Contains β-asarone, a suspected carcinogen; do not take it as a herbal remedy. Mild stomach upset if chewed.
- What grows well with Japanese sweet flag (sekishō)?
- There is no well-documented companion pairing for Japanese sweet flag (sekishō). Group it with plants that like the same sun (part sun to part shade) and water (high).
Sources and evidence
Evidence grade B (B: extension service or peer-reviewed).
- Missouri Botanical Garden Plant Finder, Missouri Botanical Garden
- Extension Gardener Plant Toolbox, NC State Extension
- Flora of China (eFloras), Missouri Botanical Garden & Harvard University Herbaria
Notes and assumptions
- Low fans of glossy grassy leaves, often gold-striped (‘Ogon’), at stream and pond edges or between stepping stones; leaves smell spicy when crushed.
Reference values for planning, not horticultural, veterinary or medical advice. Varieties, soils and local climates differ: check with a local nursery or extension service.
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