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Rhododendron yakushimanum
yakushima rhododendron
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Overview
A compact, dome-shaped evergreen shrub reaching 3-4 feet (90-120 cm) tall with a spread of 4-5 feet (120-150 cm) at maturity. Leaves are oblong-elliptic, 2.5-5 inches (6-13 cm) long and 1-1.5 inches (2.5-4 cm) wide, dark green and recurved above, with a dense, thick indumentum (woolly coating) on the undersides that is initially white, aging to tawny or cinnamon-brown. New growth emerges with a silvery-white tomentose covering that is shed as leaves mature. Flowers are bell-shaped, 1.5-2 inches (4-5 cm) across, opening from deep rose-pink buds to pale pink and fading to white, borne in rounded trusses of 8-12 flowers. This color progression produces trusses with multiple shades simultaneously. Bloom occurs in mid to late spring. Growth rate is slow, adding 1-3 inches (2.5-8 cm) per year; plants require 10-15 years to approach mature dimensions. Branching is dense, forming a symmetrical dome without pruning. Root system is shallow and fibrous. More heat-tolerant than many rhododendron species, performing adequately into zone 8 where summers are not excessively hot and humid. Foliage may develop leaf spot in regions with prolonged summer humidity.
Native Range
Endemic to Yakushima Island in southern Japan, where it occurs at elevations above 5,000 feet (1,500 m) in exposed, wind-swept habitats. Found on rocky outcrops and ridgetops in montane cloud forest zones, adapted to high rainfall, cool temperatures, and acidic substrates.Suggested Uses
Commonly planted in woodland borders, foundation plantings, and as specimen plants at 4-5 foot (120-150 cm) spacing. Grows in containers of at least 7 gallons (26 L) with acidic potting mix. Widely used as a parent in breeding programs; many compact hybrids in commerce derive from this species.How to Identify
Appearance
Size & Dimensions
Height3' - 4'
Width/Spread4' - 5'
Reaches mature size in approximately 15 years
Bloom Information
Mid-May through early June in zones 5-7; late April through May in zones 7-8. Individual flowers last 10-14 days. Total bloom period extends 3-4 weeks. The color progression from pink buds to white open flowers extends the visual interest of each truss beyond the typical single-color bloom.Detailed Descriptions
Flower Description
Deep rose-pink buds fading to pale pink then whiteFoliage Description
Dark green above with thick white to cinnamon-brown indumentum below; new growth silvery-whiteGrowing Conditions
Sun Requirements
Requires 3-6 hours of direct sunlight daily
• Full Sun: 6+ hours of direct sunlight
• Partial Shade: 3-6 hours of direct sunlight
• Full Shade: Less than 3 hours of direct sunlight
Care & Maintenance
Care Guide
Requires acidic soil with pH 4.5-6.0; amend alkaline soils with sulfur or iron sulfate before planting. Water consistently during the first two growing seasons, providing 1 inch (2.5 cm) per week in the absence of rainfall. Established plants tolerate brief dry spells but leaf margins curl inward as a drought stress indicator; irrigate when curling is observed. Maintain 2-3 inches (5-8 cm) of acidic mulch. More sun-tolerant than many rhododendrons; accepts morning sun with afternoon shade or dappled light throughout the day. Phytophthora root rot and Botryosphaeria dieback are the most significant disease threats in poorly drained or stressed conditions.Pruning
Rarely requires pruning due to naturally symmetrical dome shape. Remove dead or damaged branches at any time. Spent flower trusses can be removed by snapping at the base after bloom to redirect energy from seed production. Hard pruning is possible but unnecessary on this compact species; if attempted, cut to just above a leaf whorl in late spring.Pruning Schedule
J
F
M
A
M
J
J
A
S
O
N
D
late spring
Maintenance Level
lowContainer Growing
✓ Suitable for container growing
Minimum container size: 7 gallons