Can I grow this outdoors?

Only in mild climates where winter temperatures stay above about -5 °C. Protect it from hard frost.

Why are the leaves turning yellow?

This is often a sign of iron chlorosis due to soil that is not acidic enough.

Plant guide

Sims azalea.

Rhododendron simsii

Pronounced roh-doh-DEN-dron SIM-see-eye

Also known as Formosa Azalea, Red Azalea, Rhododendron

Rhododendron simsii — Photo by oleggoodzon (CC BY) via gbif

This beautiful shrub is a staple in many gardens for its vibrant, showy flowers. It thrives in cooler, humid environments and adds a splash of color to your landscape. You will love how it transforms your space when it blooms.

Another photo of Sims azalea — Photo by luccasfernandes (CC BY-SA) via iNaturalist

At a glance

Care level: Moderate
Plant type: Shrub
Light: Partial shade
Indoor light: Bright indirect
Water: Every 3 to 7 days
Mature size: 2 to 3 meters
Growth rate: Medium
Hardiness: USDA 8 to 10
Bloom time: Late winter to mid spring

Care

Keep the soil consistently moist but never soggy to avoid root rot. Watch for yellowing leaves, which often signal that the soil pH is too high or the plant needs more iron. Provide plenty of humidity to keep your plant happy and blooming.

Watering: Water when the top inch of soil feels dry to the touch.
Feeding: Use an acid loving fertilizer during the active growing season.
Pruning: Prune immediately after flowering to shape the plant and encourage bushiness.
Repotting: Repot only when the plant becomes root bound in its current container.
Soil: Acidic, well draining soil
Soil pH: 4.5 to 6.0
Temperature: -5 to 30°C
Humidity: 60 to 90%
Drought tolerance: Low
Lifespan: Perennial, typically 10 to 20 years with proper care.
Deer and rabbits: Low

Signs it needs repotting

Roots growing out of the drainage holes
Water running straight through the pot
Stunted growth during the growing season

Safety

People: The plant contains grayanotoxins which are poisonous to humans and pets if consumed.
Pets: This plant is toxic to cats and dogs if ingested.
Children: Keep this plant out of reach of children, as all parts are toxic if chewed or swallowed.
Contact allergens: The leaves and stems can cause mild skin irritation in sensitive individuals.
Airborne allergens: The pollen is generally not considered a major allergen.
Edibility: All parts of this plant are toxic if ingested and can cause serious illness.

How to identify it

Evergreen to semi evergreen foliage
Funnel shaped flowers in clusters
Small, leathery, dark green leaves
Hairy stems and leaf undersides

Height: 0.6 to 2m
Spread: 0.6 to 1.5m
Leaves: dark green, evergreen
Flowers: pink, red, white, purple, 3 to 6cm

Problems to watch for

Lace bugs · These small insects suck sap from leaves, causing yellow stippling.
Root rot · Fungal infection caused by overly wet, poorly drained soil.

By symptom

Plant problems by symptom

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Growing it

Propagation: cuttings, seed
When to plant: spring, early autumn
Overwintering: Move containers to a sheltered, cool location indoors if frost is expected. Keep the soil barely moist during the winter resting period.
In containers: It is excellent for containers, provided you use an acidic potting mix and a pot at least 30 cm wide.

Stem cuttings

1. Take a 10 cm cutting from new growth in summer.
2. Remove the lower leaves and dip the end in rooting hormone.
3. Place in a moist, peat based medium.
4. Cover with a plastic bag to maintain humidity until rooted.

Group these with other acid loving plants to make soil management easier.

Grows well with

Japanese Maple · They share similar requirements for acidic soil and partial shade.
Ferns · Ferns provide a lovely textural contrast and thrive in the same moist environment.

Similar species

Kurume Azalea · These are smaller, more compact relatives often used in similar garden settings.

Science

Family: Ericaceae
Genus: Rhododendron
Order: Ericales
Class: Magnoliopsida
Native range: Eastern Asia, specifically China and Taiwan.
Conservation: It is not currently listed as endangered in the wild.

It grows in the understory of mountain forests where it benefits from dappled light and moist, acidic soil.

Value to wildlife: nectar source
Pollinators: bees, butterflies
Uses: container plant, ornamental shrub, patio plant

Through the year

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Flowering

Fruiting

Foliage

Ecological indicators

Salinity: 1/9
Soil reaction: 3/9
Light: 4/9
Temperature: 6/9
Moisture: 7/9
Nutrients: 4/9

Taxonomy resolved against GBIF.

Questions people ask

Can I grow this outdoors?: Only in mild climates where winter temperatures stay above about -5 °C. Protect it from hard frost.
Why are the leaves turning yellow?: This is often a sign of iron chlorosis due to soil that is not acidic enough.

Where the name comes from

The genus name comes from the Greek words for rose and tree. The species name honors the British botanist John Sims.

Good to know

It is often called the Indian Azalea.
It thrives in acidic soil conditions.
The flowers are funnel shaped and quite showy.
It requires consistent moisture to bloom well.

Also in

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This guide was generated with AI on October 19, 2026 and checked against GBIF taxonomy. It is general information, not horticultural or veterinary advice. Read about how we build these guides.