Rhododendron 'Humboldt'

RHS Plant Profile

rhododendron 'Humboldt'

Shrubs

An evergreen shrub, to about 1.5m high, with an upright habit and oval-shaped, leathery dark green leaves. Produces clusters of up to 20 very pale purple flowers speckled with dark red, in late spring and early summer

Position

Full sun, Partial shade

Soil Types

Clay, Loam, Sand

Max Height

1-1.5 metres

Max Spread

1-1.5 metres

Size

Time to Maturity: 10–20 years
Max Spread: 1-1.5 metres
Max Height: 1-1.5 metres

Growing Conditions

Clay Loam Sand

Moisture: Moist but well–drained
pH: Acid

Position

Full sun Partial shade

Aspect: South–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): H7

Colour & Scent

Season	Stem	Flower	Foliage	Fruit
Spring
Summer
Autumn
Winter

Botanical Details

Family: Ericaceae
Native to GB/Ireland: No
Foliage: Evergreen
Habit: Bushy
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: Unresolved

How to Grow

Cultivation

Grow in moist but well-drained, humus rich, acidic soil, in sun or light shade. Choose a site away from frost pockets and sheltered from strong winds. Mulch annually, ideally with leaf mould. See rhododendron cultivation for more detailed advice

Propagation

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

Suggested planting locations and garden types

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