Rhododendron

Rhododendron: how do you recognise it?

Habit

Evergreen shrub of 1–4 m, broad and densely branched.

Leaf

Alternate, often in false whorls at the shoot tips, leathery, untoothed, 8–20 cm. Azaleas have smaller, often deciduous leaves.

Bark

Greyish-brown.

Flower

Large, funnel-shaped flowers in trusses, May–June, in all kinds of colours.

Fruit

Woody capsules.

Difference from cherry laurel

Cherry laurel has leaves with tiny glands, white flower spikes and black cherries.

Flowering: April–June · Pruning: June–July

Identify by feature: Evergreen leaves

Looks like it, but isn’t

• Cherry laurel Prunus laurocerasus: leaves with tiny glands, white flower spikes and black cherries. Difference from cherry laurel

Why grow it?

• Spectacular flowers in May–June, in almost every colour.
• Evergreen: a full, dark green shrub for partial shade all year round.
• Bumblebees visit the flowers; the shrub gives shelter to birds.

Rhododendron: where does it grow best?

• Partial to light shade, for example under tall oaks or pines; harsh midday sun scorches the leaves.
• Acid, humus-rich, moist but free-draining soil; no lime and no wet feet.
• Allow 2–3 m width, and keep it away from a walnut: its roots release a substance that rhododendrons don’t tolerate well.
• On chalky or heavy soil: choose a rhododendron grafted onto a lime-tolerant rootstock (for example Inkarho), or plant it in a large container of acid soil.

Light

Partial shade or shade

Legal basis

Acid soil

Moisture

Normal

Size

4 m tall, 3 m wide

Hardiness

Throughout Belgium, no protection needed

Goes well with

• Japanese skimmia: Likes the same acid soil and partial shade, and adds red buds in winter.
• Japanese pieris: Evergreen, with white flower sprays in early spring and red young leaves.
• Scots pine: Light shade and acid needle litter: a rhododendron’s natural setting.

Good and bad companions

Goes well with

• Tolerates shade and suits planting under large trees such as: english oak, sessile oak, pin oak, turkey oak, red oak, holm oak
• Like acid soil, just like this plant: heather, japanese skimmia, raspberry, highbush blueberry, witch hazel, broom, gorse, camellia

Best kept away from

• Walnut releases juglone, to which this plant is sensitive: walnut

Rhododendron: when and how to plant it?

1. Plant in spring (March–April) or early autumn (September–October).
2. Dig a wide, shallow planting hole and mix the soil with peat-free ericaceous compost or leaf mould from oak or pine.
3. Plant shallowly: the top of the root ball should be level with the ground or just above. Rhododendrons planted too deep struggle.
4. Soak a dry root ball in water first and water well after planting, preferably with rainwater.

Care

Watering

In dry spells use rainwater: tap water is often limy. The shallow roots dry out quickly.

Feeding

An ericaceous or rhododendron fertiliser in March–April; none after July.

Mulching

A layer of pine bark, pine needles or oak leaves keeps the soil cool, moist and acid. Don’t hoe around the shrub: you will damage the roots.

Deadheading

Snap off the faded trusses after flowering, so the plant doesn’t waste energy on seed.

Tip: Choose a garden variety or a compact species instead of Pontic rhododendron, and water only with rainwater. If you would rather have something native for acid soil and partial shade, consider holly, alder buckthorn or bilberry.

Rhododendron: when to prune it?

Pruning months: June–July.

When: Straight after flowering (June–July).

How:

• Carefully snap off the faded flower trusses; the new buds sit just below.
• An old, leggy shrub can be rejuvenated over three years: each year, cut a third of the oldest branches back hard.
• Cut back an overlong branch to just above a whorl of leaves: it will sprout again there.

Caution: Don’t prune in autumn or winter: next year’s flower buds have already formed and you would cut them off. Wear gloves and never give prunings to animals: they are poisonous.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Layering

In spring, bend a low branch to the ground, nick it lightly where it touches the soil and cover it with acid soil; after one or two years you can cut it free with its roots.

Rhododendron: how does it get through the winter?

Hardy throughout Belgium: no protection needed.

• Most garden rhododendrons are fully hardy.
• In frost the leaves roll up and droop: a natural protection, and they recover when it thaws.
• In dry winters, water on frost-free days: evergreen leaves lose water in winter too.

Diseases and pests

• Vine weevil: Notches like half-moons in the leaf edges, made at night by the adult beetles; the grubs eat the roots. Apply nematodes against the grubs in spring or late summer.
• Bud blast: Flower buds turn brown to black and become covered in tiny black bristles; the fungus gets in through the egg-laying of the rhododendron leafhopper. Snap off affected buds and throw them away.
• Yellow leaves: Yellow leaves with green veins point to lime in the soil. Water with rainwater, use an ericaceous fertiliser and mulch with pine bark.
• Rootstock suckers: On older grafted shrubs the rootstock (often Pontic rhododendron) can send up shoots with different leaves and lilac flowers. Cut or pull them off at the base.

Through the year

• March: Feed with ericaceous fertiliser and mulch; planting possible.
• April: Planting; nematodes against vine weevil grubs.
• May: Flowering.
• June: Snap off faded trusses and prune.
• July: Last month for pruning; stop feeding.
• August: Rainwater in dry spells; nematodes against vine weevil grubs.
• September: Planting possible again.
• December: Leaves rolled up in frost: that’s normal.

What can you use it for?

Garden and landscape

Evergreen flowering shrub for partial shade on acid soil, under oaks and pines.

For wildlife

Bumblebees visit the flowers; the shrub provides shelter.

Caution: All parts are poisonous, including the nectar. On chalky soil it turns yellow.

Good for wildlife

Bumblebees

Nectar and pollen for bumblebees, also from deep flowers and early in spring.

Birds

Berries, seeds or insects as food, or a place to nest.

Did you know?

• All parts are poisonous. Pontic rhododendron runs wild in woodland on acid, sandy soils.
• Honey from Pontic rhododendron around the Black Sea is poisonous (‘mad honey’); even in antiquity Xenophon’s soldiers fell ill from it.
• The name comes from Greek: rhodon means rose, dendron means tree.
• The area around Ghent is world-famous for growing azaleas; the ‘Ghent azalea’ is protected as a regional product by the European Union.
• The Greek historian Xenophon already described how soldiers on the Black Sea coast were intoxicated by honey from rhododendron flowers.