Repotting guide

When & how to repot Tree rhododendron (Rhododendron arboreum)

Also called Tree rhododendron, Burans, Lali gurans.

More about tree rhododendron

About Tree rhododendron

Rhododendron arboreum · also called Tree rhododendron, Burans · flowering

Rhododendron arboreum is the tallest species in the genus, forming a small tree with stunning deep-red to pink or white flower trusses from late winter to early spring. Native to Himalayan forests from 1,200–3,600 m, it is the national flower of Nepal and needs cool, moist, acidic conditions to thrive.

Mature size: 4–12 m tall (13–40 ft) in ideal conditions; typically 3–6 m in UK gardens

Watch for — Vine weevil: C-shaped white grubs eat roots of container-grown plants from late summer to spring, causing sudden wilting and plant collapse. Apply biological control nematodes (Steinernema kraussei) to moist compost in August–September.

How to tell tree rhododendron needs repotting

Repotting on a calendar is less reliable than reading the plant. For tree rhododendron, watch for these signs:

Roots growing out of the drainage holes, or the rootball lifting the plant proud of the rim.
Soil that has shrunk away from the pot sides and no longer holds water.
The pot is unstable because the plant has grown top-heavy.
Old, compacted, broken-down mix that stays wet too long — for a succulent that is a rot risk, so refresh it even if the pot size is fine.

For the underlying biology of a pot-bound root system and why it stalls a plant, see our guide to spotting and fixing a root-bound plant.

How often to repot tree rhododendron

Every 2–3 years, into bone-dry mix. Tree rhododendron's growth habit — upright, tree-forming evergreen shrub to small tree — sets the pace. Rhododendron arboreum is the tallest species in the genus, forming a small tree with stunning deep-red to pink or white flower trusses from late winter to early spring. Native to Himalayan forests from 1,200–3,600 m, it is the national flower of Nepal and needs cool, moist, acidic conditions to thrive.

What size pot to step tree rhododendron up to

Use a pot only one size up — or even the same pot with fresh gritty mix if the roots have room. Tree rhododendron stores water and rots in a large pot of slow-drying soil. A tight terracotta pot that dries fast is far safer than a generous plastic one. Never up-pot a succulent by several sizes.

Not sure of the exact diameter? Our pot size calculator takes the current pot and root spread and tells you the right next size — it deliberately recommends a single step up, never a big jump.

The best time of year to repot tree rhododendron

Spring or summer, while tree rhododendron is in active growth and warm, is best — roots recover fastest then, and the plant is not sitting in cool damp soil. Avoid repotting a succulent in winter dormancy.

Step-by-step: repotting tree rhododendron

Repot dry. Do not water tree rhododendron for several days first. Working with dry roots and dry mix dramatically lowers the rot risk for a succulent.
Pick a snug, fast-draining pot. Choose terracotta one size up at most, with a drainage hole. Have gritty acidic, humus-rich, freely draining ready.
Tip it out and clean the roots. Slide the plant out, crumble off the old soil, and trim any black, mushy or dead roots with clean snips.
Pot into dry mix. Set tree rhododendron at its original depth in dry gritty mix, firming gently. Do not bury the stem deeper than it was.
Wait a week before watering. Leave it completely dry and out of harsh sun for about 7 days so any damaged roots callus. Only then water lightly.

Aftercare

Keep tree rhododendron completely dry and out of fierce sun for about a week so any nicked roots callus before they meet moisture; watering a freshly repotted succulent is the classic way to rot it. Then resume the normal lean, dry rhythm. Do not fertilise for about 3 weeks — fresh mix already carries nutrients and feeding freshly disturbed roots scorches them.

The right soil mix for tree rhododendron

Tree rhododendron wants acidic, humus-rich, freely draining. Optimal pH 4.5–5.5. In the wild, grows in deep leaf-mould soils over granite or sandstone. Mix 50% composted bark or ericaceous compost with 50% gritty sand for container culture. Lime-rich soils cause rapid chlorosis. Always use fresh mix when you repot — reusing old, broken-down soil reintroduces the compaction and poor drainage you are repotting to fix.

Repotting tree rhododendron — frequently asked questions

How often should you repot tree rhododendron?

Every 2–3 years, into bone-dry mix for tree rhododendron. Repot tree rhododendron every 2–3 years into a snug pot of acidic, humus-rich, freely draining, ideally in spring or summer. Let it sit in dry soil and do not water for about a week afterwards so any nicked roots can callus. Over-potting and watering straight away is what rots succulents.

What size pot does tree rhododendron need?

When is the best time of year to repot tree rhododendron?

Should you water tree rhododendron after repotting?

No — not straight away. Repot tree rhododendron into dry mix and wait about a week before the first watering so any damaged roots callus over. Watering a freshly repotted succulent is the single most common way to rot one.

Should you fertilise tree rhododendron after repotting?

Not immediately. Wait about 3 weeks after repotting tree rhododendron. Fresh mix already contains nutrients, and feeding freshly cut or disturbed roots burns them. Resume your normal feeding routine once you see new growth.