Rhododendron stenopetalum 'Linearifolium', spider azalea
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Deciduous Shrubs

Rhododendron stenopetalum 'Linearifolium'

spider azalea

Ericaceae

Japan (Honshu); mountain forests and rocky slopes

At a Glance

TypeShrub
HabitUpright
FoliageSemi-evergreen
Height3–6 feet (0.9–1.8 m)
Width3–5 feet (0.9–1.5 m)
Maturity10 years

Growing Zones

USDA Hardiness Zones

6 - 9
Zone 6
Zone 7
Zone 8
Zone 9
These zones indicate the coldest temperatures this plant can typically survive.
Frost Tolerancehardy

Key Features

Attracts Pollinators
Container Friendly
Maintenancelow

Overview

Rhododendron stenopetalum 'Linearifolium' is a semi-evergreen azalea grown for its highly unusual narrow, thread-like foliage and deeply divided flowers. Shrubs reach 3–6 feet (0.9–1.8 m) tall and 3–5 feet (0.9–1.5 m) wide at maturity; growth rate is slow, 4–6 inches (10–15 cm) per year. The habit is upright and openly branched. Foliage is the defining characteristic: leaves are extremely narrow and linear, 1.5–3 inches (3.8–7.5 cm) long and only 0.1–0.2 inch (2.5–5 mm) wide — narrower than any other commonly cultivated rhododendron or azalea. Leaves are arranged in whorls of 5 at branch tips, dark green, semi-evergreen with some leaf retention through mild winters. Flowers appear in April–May: rosy-pink to purple-pink, with petals deeply divided into narrow strap-like segments rather than forming a solid funnel, giving an open, spidery appearance 1.5–2 inches (3.8–5 cm) across. The combination of thread-like foliage and spidery flowers makes this one of the most distinctive azaleas in cultivation. All parts are toxic to humans and animals if ingested.

Native Range

Rhododendron stenopetalum is native to Japan, primarily on Honshu, in mountain forests and rocky slopes. The cultivar 'Linearifolium' was selected for maximum leaf and petal reduction. It is not native to the Pacific Northwest.

Suggested Uses

Planted as a specimen in woodland gardens, Japanese-inspired gardens, and collector borders where the thread-like foliage and spidery flowers can be viewed up close, spaced 3–4 feet (0.9–1.2 m) from adjacent plants. The narrow foliage provides texture contrast throughout the growing season; spring flowers add further interest. Suitable for container culture in partly shaded positions. Best suited to partly shaded, moist, acidic west-side Pacific Northwest conditions.

How to Identify

Rhododendron stenopetalum 'Linearifolium' is identified by its extremely narrow, thread-like linear leaves 1.5–3 inches (3.8–7.5 cm) long and only 0.1–0.2 inch (2.5–5 mm) wide, arranged in whorls of 5 at branch tips; and rosy-pink to purple-pink flowers with deeply divided, strap-like petals giving a spidery, open appearance. No other commonly cultivated azalea has leaves this narrow.

Appearance

Size & Dimensions

Height3' - 6'
Width/Spread3' - 5'

Reaches mature size in approximately 10 years

Colors

Flower Colors

pink
purple

Foliage Colors

dark green

Fall Foliage Colors

no change

Bloom Information

Bloom Period

~3 weeks
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Spring
Flowers appear in April–May. Each cluster produces 2–4 rosy-pink to purple-pink flowers 1.5–2 inches (3.8–5 cm) across, with petals divided into narrow strap-like segments rather than forming a solid funnel. Bloom period lasts 2–3 weeks.

Detailed Descriptions

Flower Description

rosy-pink to purple-pink with deeply divided strap-like petals

Foliage Description

dark green; extremely narrow thread-like leaves

Growing Conditions

Sun Requirements

Partial Shade
Full Shade
Requires 2-6 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

Soil Requirements

pH Range4.5 - 6.0(Acidic)
357912
Soil Types
loampeat
Drainage
well drained

Water & Climate

Water Needs

Medium

Frost Tolerance

hardy

Time to Maturity

8–12 years

Care & Maintenance

Care Guide

Water weekly during the first two growing seasons; established shrubs prefer consistent moisture and do not tolerate drought. Prefers cool, moist, acidic, well-drained soils with high organic matter; mulch annually to maintain moisture and acidity. Partial shade is recommended — direct afternoon sun stresses the narrow foliage and bleaches flower color. No serious pest or disease problems in cool Pacific Northwest conditions. Semi-evergreen — some leaves may drop in cold winters but the plant recovers in spring. All parts are toxic to humans and animals if ingested.

Pruning

Prune immediately after flowering to preserve the following year's buds. Minimal pruning needed — the open habit and slow growth require little intervention. Remove dead or crossing branches at any time.

Pruning Schedule

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late spring

Maintenance Level

low

Container Growing

✓ Suitable for container growing

Minimum container size: 5 gallons

⚠️ Toxicity Warning

Toxic to pets and humans