Japanese indigo (ai, dyer’s knotweed) Persicaria tinctoria
Japanese indigo (ai, dyer’s knotweed) (Persicaria tinctoria) is an annual herb in the Polygonaceae family, native to Southern China and Vietnam. It grows 50–90 cm (20–35 in) tall and 30–50 cm (12–20 in) wide, wants full sun and high water, and is hardy to about 2 °C (36 °F). Space plants about 30 cm (12 in) apart.
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Japanese indigo (ai, dyer’s knotweed) at a glance
| Type | Annual herb |
|---|---|
| Family | Polygonaceae |
| Mature height | 50–90 cm (20–35 in) |
| Mature spread | 30–50 cm (12–20 in) |
| Spacing | 30 cm (12 in) centre to centre |
| Sun | Full sun, at least 6 h of direct sun a day |
| Water | High; drought tolerance none, waterlogging tolerance low |
| Soil pH | 6–7.5 |
| Cold hardiness | About 2 °C (36 °F) once established (roughly USDA zone 10) |
| Heat stress | Likely above about 34 °C (93 °F) |
| Flowering | about 14–20 weeks after planting, for 4–7 weeks |
| Days to maturity | About 90 days from planting |
| Wind tolerance | Low |
| Pollinator value | Moderate |
| Edible | Not grown as food |
| Native to | Southern China and Vietnam |
| Common in gardens of | 🇯🇵 Japan, 🇨🇳 China, 🇰🇷 South Korea, 🇻🇳 Vietnam, 🇬🇧 United Kingdom, 🇺🇸 United States |
Is Japanese indigo (ai, dyer’s knotweed) toxic?
- People: not known to be toxic
- Cats: not known to be toxic
- Dogs: not known to be toxic
Not reported as poisonous; not used as food.
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 indigo (ai, dyer’s knotweed)
No well-documented helpful pairing. Group it with plants that like full sun and high water.
Questions about Japanese indigo (ai, dyer’s knotweed)
- How far apart should I plant Japanese indigo (ai, dyer’s knotweed)?
- About 30 cm (12 in) apart, centre to centre. A mature plant spreads 30–50 cm (12–20 in).
- Does Japanese indigo (ai, dyer’s knotweed) need full sun?
- Full sun: at least 6 hours of direct sun a day.
- How much water does Japanese indigo (ai, dyer’s knotweed) need?
- High water. Drought tolerance is none and waterlogging tolerance low.
- How cold-hardy is Japanese indigo (ai, dyer’s knotweed)?
- It tolerates about 2 °C (36 °F) once established, roughly USDA zone 10. As an annual it is usually planted out after the last frost.
- When does Japanese indigo (ai, dyer’s knotweed) flower?
- About 14–20 weeks after planting, for 4–7 weeks.
- Is Japanese indigo (ai, dyer’s knotweed) poisonous to cats and dogs?
- For cats it is not known to be toxic; for dogs not known to be toxic; for people not known to be toxic. Not reported as poisonous; not used as food.
- What grows well with Japanese indigo (ai, dyer’s knotweed)?
- There is no well-documented companion pairing for Japanese indigo (ai, dyer’s knotweed). Group it with plants that like the same sun (full sun) and water (high).
Sources and evidence
Evidence grade B (B: extension service or peer-reviewed).
- Plants of the World Online (taxonomy, native range), Royal Botanic Gardens, Kew
- Natural Dyes: Sources, Tradition, Technology and Science (Cardon, 2007), Archetype Publications, London
- Flora of China (eFloras), Missouri Botanical Garden & Harvard University Herbaria
Notes and assumptions
- Formerly Polygonum tinctorium. Cut the leafy stems twice in summer for the blue: fermented leaf compost (sukumo) is the base of Japanese aizome, still grown in Tokushima (Awa indigo). Frost-tender annual; needs rich, moist soil.
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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