Rhododendron

Rhododendron: how do you recognise it?

Habit

Compact, spreading shrub about 1 m tall and 1.5 m wide, densely branched.

Leaves

Almost round to broadly oval, 2–4 cm, with a heart-shaped base; dark green above, paler and bluish beneath. Young leaves are bronze.

Flowers

Hanging bells, 3–4 cm, in small groups at the tips of the branches, soft pink, in April–May.

Roots

A fine, shallow root system: the plant feeds from the top layer of humus and dislikes compacted or waterlogged soil.

Evergreen

The leaves stay on the shrub all year, so it keeps its structure in the garden even in winter.

Flowering: April–May · Plants: March–April and September–October · Pruning: March and May

Why grow it?

• Graceful flowering in April and May, with hanging pink bells that draw in bees early in the season.
• Evergreen, with round leaves and bronze young growth as an extra spring highlight.
• Stays compact (about 1 m tall and 1.5 m wide), so it suits a small garden or a large pot.
• Needs hardly any pruning: a short job after flowering is enough.
• A good companion for other acid-loving plants, such as pieris and Japanese maple.

Rhododendron: where does it grow best?

• Partial shade: dappled light under tall trees is ideal. In deep shade it flowers poorly, in full sun the leaves scorch.
• Acid soil (pH 4.5–6), rich in humus and open: sand, loam or peaty soil with plenty of leaf mould. On chalky soil the leaves turn yellow.
• Evenly moist but well drained: no wet feet, but no soil that dries out either.
• Sheltered from cold wind and from a spot where morning sun would thaw frozen buds too quickly.
• Allow 1–1.5 m between plants.

Light

Partial shade

Soil type

Sand, loam or peat

Acidity

Acid

Moisture

Normal

Size

1 m tall, 1.5 m wide

Hardiness

Hardy (to -20 °C)

Goes well with

• Pieris: Also acid-loving and evergreen, with white bells and red young leaves in spring.
• Japanese maple: Its fine foliage and light shade suit the same partly shaded spot.
• Ferns: Fresh green foliage under the shrub keeps the soil cool and moist.
• Hosta: Broad leaves in the same partial shade and on humus-rich soil.
• Wood anemone: A white spring flower under the shrub, on the same humus-rich soil.

Rhododendron: when and how to plant it?

Plant in March–April and September–October.

1. Plant in March–April or in September–October, when the soil is neither frozen nor waterlogged.
2. Soak the root ball first in a bucket of water until no more air bubbles rise.
3. Dig a wide, shallow hole, about twice as wide as the root ball but no deeper, and mix the soil with leaf mould or ericaceous compost.
4. Plant shallowly: the top of the root ball may stand slightly above ground level, because the roots lie close to the surface.
5. Water in generously straight away, preferably with rainwater.
6. Spread a 5–8 cm layer of mulch of leaf mould, pine bark or rotted leaves around the plant, keeping it clear of the stem.

Care

Water

Keep the soil lightly moist at all times, especially in the first year and in dry summers. Use rainwater if you can: hard tap water makes the soil less acid. Regular watering in late summer matters, because that is when next year’s flower buds form.

Feeding

Give an organic feed or a special rhododendron feed once a year, in April–May. Do not feed after July, so that the shoots can ripen.

Mulching

Top up the layer of leaf mould or pine bark every spring or autumn: it keeps the roots cool, moist and acid.

Spent flowers

Snap off the faded flower trusses by hand, taking care not to damage the young shoots beneath. The plant then wastes no energy on making seed.

Weeds

Do not hoe or dig around the plant, because the roots are shallow. Pull weeds by hand or let the mulch do the work.

Tip: Plant it shallowly and water with rainwater whenever you can: planting too deep and using hard water are the two most common mistakes. Cover the buds with fleece on late frosty nights.

Rhododendron: when to prune it?

Pruning months: March and May.

When: It needs hardly any pruning. Remove dead or damaged branches in March and snap off the faded trusses straight after flowering, in May.

How:

• Snap off the faded flower trusses just below the flower, without touching the young buds next to it.
• Cut dead, frosted or crossing branches back to healthy wood, just above a side shoot or bud.
• If you want to make it smaller or renew it, do so in March and in small steps, spread over two to three years.
• Keep prunings out of reach of children and animals, because the foliage is poisonous.

Caution: Never prune after June: you would remove next year’s flower buds. Even pruning in March costs you part of that spring’s flowering.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Layering

In spring, bend a low, supple branch down to the ground, lightly wound the bark at the bend, peg it down and cover it with leaf mould. After one to two years it will have rooted and can be separated.

Semi-ripe cuttings

In July–September take cuttings of 8–10 cm, remove the lower leaves and insert them in an acid, open compost, preferably with rooting hormone. Keep them moist and sheltered. Rooting takes months and does not always succeed.

Seed

The seed is very fine: sprinkle it in spring on the surface of moist, acid seed compost without covering it, and keep it cool and light. Seedlings grow slowly and may differ from the parent plant.

Rhododendron: how does it get through the winter?

Hardy to −20 °C.

• Hardy to about −20 °C: in Belgium the shrub usually comes through winter without protection.
• The flower buds are the most vulnerable: during late night frosts cover the shrub with horticultural fleece (not plastic) and remove it in the daytime.
• Water on frost-free, dry days: an evergreen shrub keeps losing moisture through its leaves in winter. Leaves that curl up in frost are normal.
• In a pot: stand it against a sheltered wall and wrap the pot in hessian or bubble wrap, because the roots freeze faster than in the open ground.

Diseases and pests

• Yellow leaves with green veins: Iron deficiency (chlorosis) caused by soil that is too chalky. Use rainwater, mix acid leaf mould into the topsoil and give a feed for acid-loving plants.
• Vine weevil: Half-moon notches along the leaf edge; the white grubs eat the roots and the plant declines. In August–September apply nematodes against the grubs while the soil is still warm.
• Rhododendron lace bug: The upper side of the leaves is speckled grey-yellow, the underside has black, sticky spots. Choose a shadier spot, rinse the underside of the leaves with water and remove badly affected leaves.
• Frost damage to buds: Brown, dead buds or flowers after a late frosty night. Plant in shelter and cover the shrub in frost. Only cut away what is dead once the plant has started growing again.
• Root rot: The plant declines and the foliage droops in wet, compacted soil. Provide free-draining soil, plant shallowly and, on heavy clay, choose a raised bed.

Through the year

• March: Cut out dead branches. Plant if the soil is not frozen. Top up the mulch.
• April: Flowering begins. Cover the buds with fleece on frosty nights. Plant.
• May: Flowering. Afterwards snap off the faded trusses and give a feed for acid-loving plants.
• June: New shoots grow. Water in dry weather, preferably with rainwater.
• July: Water in dry weather. Stop feeding.
• August: Water regularly: next year’s buds are forming. Apply nematodes against vine weevil.
• September: Plant. Water in dry weather.
• October: Plant. Spread a mulch of leaf mould.
• November: Protect pots from frost. Use acid leaf mould as mulch.
• December: Rest. Water on dry, frost-free days.

Edible and useful

Not edible. Rhododendrons and azaleas are poisonous in all their parts, including the flowers and nectar: they contain grayanotoxins, which cause drooling, vomiting, weakness, a slow heartbeat and low blood pressure. Goats, sheep and alpacas above all, but also horses, cattle, rabbits and guinea pigs become seriously ill from them, sometimes after only a small quantity of leaves; dogs and cats are at risk too. So never throw clippings over the fence of a field or into a pen or hutch, do not give them to animals, and do not pick greens for rabbits among the shrubs; if in doubt, call your vet at once. In children who put a leaf or flower in their mouth or suck nectar from the flowers, as they do with dead-nettle, it usually goes no further than a burning mouth and nausea, but teach them not to do it. If any is swallowed, call a poison centre (in Belgium 070 245 245) or your doctor at once.

Good for wildlife

Bees

Nectar and pollen for honeybees and wild bees.

Did you know?

• The species is named after J.C. Williams of Caerhays Castle in Cornwall, a great collector of rhododendrons.
• Azaleas are rhododendrons too: they belong to the same genus, which has about 1000 species.
• Rhododendron literally means ‘rose tree’, from the Greek words rhodon (rose) and dendron (tree).