Rhododendron thomsonii

RHS Plant Profile
Thomson's rhododendron

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An evergreen shrub or small tree, usually up to about 3m high in cultivation, though it can reach up to 6m in its native habitat. Bark is smooth, reddish and peeling, and leaves are rounded to oval, and matt dark green, with a bluish tint when young. Produces loose clusters of 3-10 bell-shaped, rich crimson flowers in mid to late spring

Position

Partial shade

Soil Types

Clay, Loam, Sand

Max Height

2.5-4 metres

Max Spread

2.5-4 metres

Size

Time to Maturity
5–10 years
Max Spread
2.5-4 metres
Max Height
2.5-4 metres

Growing Conditions

Clay Loam Sand
Moisture
Moist but well–drained
pH
Acid

Position

Partial shade
Aspect
North–facing or West–facing
Exposure
Sheltered
Hardiness

Hardiness Ratings

All ratings refer to the UK growing conditions unless otherwise stated. Minimum temperature ranges (in degrees C) are shown in brackets

  • H1A: under glass all year (>15C)
  • H1B: can be grown outside in the summer (10 - 15)
  • H1C: can be grown outside in the summer (5 - 10)
  • H2: tolerant of low temperatures, but not surviving being frozen (1 to 5)
  • H3: hardy in coastal and relatively mild parts of the UK (-5 to 1)
  • H4: hardy through most of the UK (-10 to -5)
  • H5: hardy in most places throughout the UK even in severe winters (-15 to -10)
  • H6: hardy in all of UK and northern Europe (-20 to -15)
  • H7: hardy in the severest European continental climates (< -20)
H5

Colour & Scent

Season Stem Flower Foliage Fruit
Spring
Summer
Autumn
Winter

Botanical Details

Family
Ericaceae
Native to GB/Ireland
No
Foliage
Evergreen
Habit
Columnar upright, Spreading branched
Potentially harmful
Harmful if eaten. Wear gloves and other protective equipment when handling. Pets (dogs, cats, rabbits, tortoises) Harmful if eaten - for further information and contact numbers regarding pets, see the HTA guide to potentially harmful plants
Genus
Rhododendron can be evergreen or deciduous shrubs or trees, with simple leaves, sometimes with a dense colourful indumentum of hairs on the lower side, and funnel-shaped, bell-shaped or tubular flowers that may be solitary or in short racemes
Name Status
Correct
Plant Range
Myanmar to Himalaya and Tibet

How to Grow

Cultivation

Grow in moist but well-drained, humus rich, acidic soil, in partial or dappled shade. Choose a site away from frost pockets and sheltered from strong winds; flowers may need protection from late frosts. Do not allow plants to dry out, and mulch annually, ideally with leaf mould. See rhododendron cultivation for more detailed advice

Propagation

Propagate by semi-ripe cuttings in late summer, by layering in autumn, or by grafting in winter

Suggested planting locations and garden types

  • City and courtyard gardens
  • Cottage and informal garden
  • Flower borders and beds
  • Wall side borders

Pruning

Minimal pruning required, see pruning group 8 (evergreens)

Pests

May be susceptible to vine weevil, rhododendron and azalea whitefly, rhododendron leafhopper, Pieris lacebug, scale insects, caterpillars and aphids

Diseases

May be susceptible to powdery mildews, honey fungus, silver leaf and Phytophthora, as well as more specific Rhododendron diseases