Mountain hydrangea (yama-ajisai) Hydrangea serrata
Mountain hydrangea (yama-ajisai) (Hydrangea serrata) is a deciduous perennial shrub in the Hydrangeaceae family, native to Mountain woods of Japan and Korea. It grows 80 cm–1.5 m (31 in–4.9 ft) tall and 80 cm–1.5 m (31 in–4.9 ft) wide, wants part sun to part shade and high water, and is hardy to about -23 °C (-9 °F). Space plants about 1 m (3.3 ft) apart.
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Mountain hydrangea (yama-ajisai) at a glance
| Type | Perennial shrub, deciduous |
|---|---|
| Family | Hydrangeaceae |
| Mature height | 80 cm–1.5 m (31 in–4.9 ft) |
| Mature spread | 80 cm–1.5 m (31 in–4.9 ft) |
| Spacing | 1 m (3.3 ft) 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 none |
| Soil pH | 5–6.5 |
| Cold hardiness | About -23 °C (-9 °F) once established (roughly USDA zone 6) |
| Heat stress | Likely above about 30 °C (86 °F) |
| Flowering | from about June for 6–10 weeks (northern hemisphere; later in cold climates, earlier in warm ones) |
| Wind tolerance | Low |
| Pollinator value | Moderate |
| Edible | Not grown as food |
| Native to | Mountain woods of Japan and Korea |
| Common in gardens of | 🇯🇵 Japan, 🇰🇷 South Korea, 🇬🇧 United Kingdom, 🇺🇸 United States, 🇩🇪 Germany, 🇳🇱 Netherlands, 🇫🇷 France |
Is Mountain hydrangea (yama-ajisai) toxic?
- People: mildly toxic
- Cats: mildly toxic
- Dogs: mildly toxic
- Parts: leaves, flowers, buds
Hydrangeas contain cyanogenic glycosides (ASPCA): vomiting and diarrhoea if eaten. Amacha leaf tea is made only from a sweet cultivated form.
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 Mountain hydrangea (yama-ajisai)
No well-documented helpful pairing. Group it with plants that like part sun to part shade and high water.
Questions about Mountain hydrangea (yama-ajisai)
- How far apart should I plant Mountain hydrangea (yama-ajisai)?
- About 1 m (3.3 ft) apart, centre to centre. A mature plant spreads 80 cm–1.5 m (31 in–4.9 ft).
- Does Mountain hydrangea (yama-ajisai) need full sun?
- Part sun to part shade: at least 3 hours of direct sun a day.
- How much water does Mountain hydrangea (yama-ajisai) need?
- High water. Drought tolerance is none and waterlogging tolerance none.
- How cold-hardy is Mountain hydrangea (yama-ajisai)?
- It tolerates about -23 °C (-9 °F) once established, roughly USDA zone 6.
- When does Mountain hydrangea (yama-ajisai) flower?
- From about June for 6–10 weeks (northern hemisphere; later in cold climates, earlier in warm ones).
- Is Mountain hydrangea (yama-ajisai) poisonous to cats and dogs?
- For cats it is mildly toxic; for dogs mildly toxic; for people mildly toxic. Hydrangeas contain cyanogenic glycosides (ASPCA): vomiting and diarrhoea if eaten. Amacha leaf tea is made only from a sweet cultivated form.
- What grows well with Mountain hydrangea (yama-ajisai)?
- There is no well-documented companion pairing for Mountain hydrangea (yama-ajisai). 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).
- RHS Plant Finder, Royal Horticultural Society
- Missouri Botanical Garden Plant Finder, Missouri Botanical Garden
- ASPCA Toxic and Non-Toxic Plant List, ASPCA
Notes and assumptions
- A daintier, hardier cousin of the mophead hydrangea with flat lacecap heads; blue in acid soil, pink in alkaline. Some botanists treat it as a subspecies of H. macrophylla.
Reference values for planning, not horticultural, veterinary or medical advice. Varieties, soils and local climates differ: check with a local nursery or extension service.
Related plants
- Bigleaf hydrangea Hydrangea macrophylla
- Mock orange Philadelphus coronarius
- Panicle hydrangea Hydrangea paniculata
- Smooth hydrangea 'Annabelle' Hydrangea arborescens
- Climbing hydrangea Hydrangea anomala subsp. petiolaris
- Slender deutzia (himeutsugi) Deutzia gracilis
- Yellow wax bells (kirenga-shōma) Kirengeshoma palmata
- Rough-leaved hydrangea Hydrangea aspera