Why are my leaves turning yellow?

This is often a sign of iron chlorosis caused by soil that is not acidic enough.

Do they need full sun?

Most prefer partial shade, especially in the afternoon, to prevent leaf scorch.

Plant guide

Rhododendron.

Rhododendron L.

Pronounced roh-duh-DEN-drun

great rhododendron — Photo by Michael J. Papay (CC BY) via iNaturalist

Rhododendrons are stunning woody shrubs known for their massive, showy flower clusters. They thrive in cool, moist environments with acidic soil. Most varieties prefer dappled shade to protect their foliage from harsh afternoon sun.

At a glance

Care level: Moderate
Plant type: Shrub
Light: Partial shade
Indoor light: bright-indirect
Water: Every 5 to 10 days
Mature size: 1 to 6 meters
Growth rate: medium
Hardiness: USDA 5 to 9
Bloom time: Mid to late spring

Care

Keep the soil consistently moist but never waterlogged to prevent root rot. Mulch around the base to maintain root coolness and moisture. Watch for yellowing leaves which often signal poor drainage or incorrect soil pH.

Watering: Water deeply whenever the top few centimeters of soil feel dry.
Feeding: Apply an acid loving fertilizer once in early spring after flowering.
Pruning: Prune immediately after flowering to shape the shrub and remove dead wood.
Repotting: Repot only when the plant has clearly outgrown its container.
Soil: Well draining, acidic, organic rich soil
Soil pH: 4.5 to 6.0
Temperature: -15 to 25°C
Humidity: 50 to 80%
Drought tolerance: low
Lifespan: Perennial, typically 20 to 50 years or more.
Fragrance: light
Deer and rabbits: low

Signs it needs repotting

Roots growing out of the drainage holes.
Water sitting on the surface for a long time.
Noticeable decline in vigor or growth.

Safety

People: This plant contains grayanotoxins which are poisonous to humans and pets if eaten.
Pets: All parts of the plant are highly toxic to cats and dogs if ingested.
Children: Keep this plant away from small children as ingestion of any part can lead to severe poisoning.
Contact allergens: The leaves can cause mild skin irritation in sensitive individuals.
Airborne allergens: The pollen is generally not considered a major allergen for most people.
Edibility: All parts of the rhododendron are highly toxic if ingested and can cause serious illness.

How to identify it

Large, leathery evergreen leaves
Showy, bell shaped flower clusters
Woody, multi stemmed growth habit
Leaves often have scaly undersides

Height: 1 to 6m
Spread: 1 to 4m
Leaves: dark green, evergreen
Flowers: pink, purple, red, white, yellow, 3 to 10cm

Common problems

Rhododendron borer — Larvae tunnel into the stems and branches, causing wilting and dieback.
Phytophthora root rot — A fungal infection that causes the roots to decay, leading to yellowing leaves and plant death.
Lace bug — These insects suck sap from the underside of leaves, causing a stippled or yellowed appearance.

Growing it

Propagation: cuttings, seed, layering
When to plant: spring, autumn
Overwintering: Apply a layer of mulch around the base to protect roots from freezing temperatures. Ensure the soil stays moist but not waterlogged before the ground freezes.
In containers: They grow well in large containers with excellent drainage, requiring at least a 15 gallon pot for long term health.

Stem cuttings

Take a 4 to 6 inch cutting from new growth in late summer.
Remove the lower leaves and dip the base in rooting hormone.
Plant in a sterile, well draining mix and keep in a humid environment until rooted.

Group plants that all enjoy acidic, well draining soil to make maintenance much easier.

Grows well with

Pieris — They share similar requirements for acidic soil and partial shade.
Ferns — Ferns provide a lovely textural contrast and thrive in the same woodland conditions.
Hostas — These shade lovers fill the space beneath rhododendrons perfectly.

Keep away from

Lilac — Lilacs prefer alkaline soil which is the opposite of what rhododendrons need.

Similar species

Azalea — Azaleas are technically a type of rhododendron but are often grouped separately in gardening.
Mountain Laurel — They share a similar growth habit and preference for acidic woodland soil.

Science

Native range: Widely distributed across the Northern Hemisphere, particularly in Asia and North America.
Conservation: Some rare species are protected, but most common garden varieties are not of conservation concern.

They typically grow in cool, moist, acidic woodland environments where they provide essential cover for birds.

Value to wildlife: nectar source, nesting cover
Pollinators: bees, hummingbirds
Uses: ornamental shrub, woodland garden accent, privacy screen

Through the year

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Flowering

Fruiting

Foliage

Ecological indicators

Salinity: 1/9
Soil reaction: 3/9
Light: 4/9
Temperature: 5/9
Moisture: 6/9
Nutrients: 4/9

Questions people ask

Why are my leaves turning yellow?: This is often a sign of iron chlorosis caused by soil that is not acidic enough.
Do they need full sun?: Most prefer partial shade, especially in the afternoon, to prevent leaf scorch.

Where the name comes from

The name is derived from the Greek words rhodon, meaning rose, and dendron, meaning tree. It was coined by Linnaeus to describe these woody plants with rose like flowers.

Good to know

Rhododendrons are the national flower of Nepal.
The name comes from the Greek words for rose and tree.
Some species can grow into massive trees in their native habitats.
They prefer acidic soil to thrive and produce vibrant flowers.

Is your rhododendron looking unwell?

This page is a general guide. Pocket Botanist diagnoses problems from a photo of your actual plant, works out watering for your light and conditions, and reminds you when care is due.

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