Thomson’s rhododendron Rhododendron thomsonii
Thomson’s rhododendron (Rhododendron thomsonii) is an evergreen perennial shrub in the Ericaceae family, native to Eastern Himalaya (Nepal, Sikkim, Bhutan, south-east Tibet). It grows 2–5 m (6.6–16 ft) tall and 2–4 m (6.6–13 ft) wide, wants part sun to part shade and moderate water, and is hardy to about -15 °C (5 °F). Space plants about 3 m (10 ft) apart.
Plan Thomson’s rhododendron in your garden →
Free. Draw your beds, then watch it grow through the year in 2D and 3D.
Thomson’s rhododendron at a glance
| Type | Perennial shrub, evergreen |
|---|---|
| Family | Ericaceae |
| Mature height | 2–5 m (6.6–16 ft) |
| Mature spread | 2–4 m (6.6–13 ft) |
| Spacing | 3 m (10 ft) centre to centre |
| Sun | Part sun to part shade, at least 4 h of direct sun a day |
| Water | Moderate; drought tolerance low, waterlogging tolerance none |
| Soil pH | 4.5–6 |
| Cold hardiness | About -15 °C (5 °F) once established (roughly USDA zone 7) |
| Heat stress | Likely above about 28 °C (82 °F) |
| Flowering | from about April for 3–4 weeks (northern hemisphere; later in cold climates, earlier in warm ones) |
| Wind tolerance | Low |
| Pollinator value | Moderate |
| Edible | Not grown as food |
| Native to | Eastern Himalaya (Nepal, Sikkim, Bhutan, south-east Tibet) |
| Common in gardens of | 🇬🇧 United Kingdom, 🇮🇪 Ireland, 🇳🇿 New Zealand, 🇺🇸 United States, 🇮🇳 India |
Is Thomson’s rhododendron toxic?
- People: moderately toxic
- Cats: severely toxic
- Dogs: severely toxic
- Parts: leaves, flowers, nectar (honey)
All rhododendrons contain grayanotoxins: eating leaves or flowers causes vomiting, drooling, weakness and heart problems in pets and livestock; honey from some species ("mad honey") poisons people.
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 Thomson’s rhododendron
No well-documented helpful pairing. Group it with plants that like part sun to part shade and moderate water.
Keep apart or take care
- caution With Mountain laurel: Mountain laurel and rhododendrons are hosts of Phytophthora ramorum (sudden oak death), which spreads from infected foliage between neighbouring plants. Evidence B
- caution With English lavender: Acid-loving plants need pH 4.5–5.5, lavender prefers neutral to alkaline soil; one bed cannot suit both. Evidence B
Questions about Thomson’s rhododendron
- How far apart should I plant Thomson’s rhododendron?
- About 3 m (10 ft) apart, centre to centre. A mature plant spreads 2–4 m (6.6–13 ft).
- Does Thomson’s rhododendron need full sun?
- Part sun to part shade: at least 4 hours of direct sun a day.
- How much water does Thomson’s rhododendron need?
- Moderate water. Drought tolerance is low and waterlogging tolerance none.
- How cold-hardy is Thomson’s rhododendron?
- It tolerates about -15 °C (5 °F) once established, roughly USDA zone 7.
- When does Thomson’s rhododendron flower?
- From about April for 3–4 weeks (northern hemisphere; later in cold climates, earlier in warm ones).
- Is Thomson’s rhododendron poisonous to cats and dogs?
- For cats it is severely toxic; for dogs severely toxic; for people moderately toxic. All rhododendrons contain grayanotoxins: eating leaves or flowers causes vomiting, drooling, weakness and heart problems in pets and livestock; honey from some species ("mad honey") poisons people.
- What grows well with Thomson’s rhododendron?
- There is no well-documented companion pairing for Thomson’s rhododendron. Group it with plants that like the same sun (part sun to part shade) and water (moderate).
Sources and evidence
Evidence grade A (A: strong evidence).
- Plants of the World Online (taxonomy, native range), Royal Botanic Gardens, Kew
- Flora of Nepal (online flora and checklist), Royal Botanic Garden Edinburgh with Nepalese and international partners
- RHS Find a Plant (plant profiles and growing guides), Royal Horticultural Society
Notes and assumptions
- A plant of the cool, misty Himalayan and west Chinese mountains: it wants moist, humus-rich soil and shelter from hot sun and drying wind, and sulks in hot, humid summers.
- Needs acid, lime-free soil (like other rhododendrons and camellias): in chalky ground grow it in a large pot of ericaceous compost.
- Waxy blood-red bells set in cup-like green calyces, over round blue-green leaves, with smooth, peeling cinnamon and grey bark. Introduced by Hooker from Sikkim and named after his travelling companion, the botanist Thomas Thomson.
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
- Blueberry Vaccinium corymbosum
- Rhododendron Rhododendron
- American cranberry Vaccinium macrocarpon
- Lingonberry Vaccinium vitis-idaea
- Bilberry Vaccinium myrtillus
- Satsuki azalea Rhododendron indicum
- Japanese andromeda (pieris) Pieris japonica
- Heather (ling) Calluna vulgaris