Shrub (ericaceous)
How to grow rhododendrons
Rhododendron species and hybrids
Rhododendrons need acid soil, pH 4.5 to 6.0, that stays moist but never waterlogged, and dappled shade out of drying wind. Plant in September to October or March to April in a wide, shallow hole with the rootball at or just above soil level: burying the stem base is the commonest way to kill one. Mulch with leaf mould every spring, water in dry summers, and on chalk grow a dwarf type in a pot or raised bed of ericaceous compost.
- Hardiness
- Fully hardy (most)
- Position
- Part shade to dappled
- Soil
- Acid, humus-rich, moisture-retentive
- Ideal pH
- 4.5 to 6.0
- Water
- High, never let it dry; rainwater on hard mains
- Feeding
- Light feeder
Sowing and planting rhododendrons
WhenPlant Sep to Oct or Mar to Apr, shallow; flowers Apr to Jun; deadhead after
Starting off
Plant container-grown stock Sep to Oct or Mar to Apr, teasing out any circling roots. Layering a low branch, pegged into ericaceous compost in Apr and severed 15 to 24 months later, is the surest home method. Semi-ripe cuttings 8 cm long with a basal wound, taken Aug to Sep, root slowly at 18 to 20°C with bottom heat; seed gives variable offspring. On chalk, grow in a 60 cm deep raised bed of ericaceous compost rather than fighting the soil.
Spacing and planting
Dwarf yakushimanum hybrids reach 1 m by 1.2 m; standard hybrids 2 to 3 m by 2 to 3 m; so plant 1 to 2.5 m apart. Dig a wide, shallow hole three times the rootball width and only as deep, setting the rootball at or slightly above soil level, since burying the stem base is the commonest killer. Dwarf types suit a 40 to 50 cm pot of ericaceous compost.
Varieties worth growing
- 'Cunningham's White'
- Old, very tough hybrid to about 2.5 m with white flowers in May; copes with exposure, town air and soil closer to neutral than most.
- 'Percy Wiseman'
- Yakushimanum hybrid, a dense dome about 1 m high with peach flowers fading to cream; the right size for a small garden or a large pot.
- 'Nova Zembla'
- Strong red trusses in May on a very hardy plant to about 2 m; one for cold, open gardens.
- 'Praecox'
- Small-leaved and compact, to about 1.2 m, with rosy-lilac flowers as early as February or March; shelter the flowers from frost.
- INKARHO rhododendrons
- Popular cultivars grafted on a lime-tolerant rootstock, bred for neutral soil; worth paying for where the pH is 6.5 to 7 and other rhododendrons yellow.
Rhododendrons month by month
Timings are for central England. Allow two weeks later in the north and on cold, heavy soil.
- January
Nothing to do. Leaves that roll and droop in hard frost are normal and recover.
- February
Pick off blasted buds. Test the soil pH before buying.
- March
Plant. Light feed. Renovate leggy rough-barked plants to 30 to 60 cm.
- April
Plant. Mulch 5 cm with leaf mould. Layer a low branch. Vine weevil nematodes on pots.
- May
Flowering. Water new plants weekly in dry weather.
- June
Snap off spent trusses. Any pruning now. Second light feed.
- July
Water deeply in dry spells with rainwater: next year's buds are forming.
- August
Keep watering. Semi-ripe cuttings. Nematodes on pots late in the month.
- September
Plant from now. Sever layers pegged down the spring before last.
- October
Plant. Remove rootstock suckers. Clear mildewed leaves.
- November
Nothing to do. Raise pots on feet.
- December
Nothing to do. Knock heavy snow off the branches.
Feeding rhododendrons
A light feeder. On acid soil with a yearly leaf mould mulch an established rhododendron needs little or nothing; feed young plants, pots and tired plants at half rate, and never lime.
Low, an ericaceous feed in spring; lime is fatal.
Feeding plan
March, beds
Fruit & Vegetable Fertiliser 4-5-6
Half the label rate: 35 to 55 g/m² under the canopy, watered in. A plant 2 m across covers about 3 m², so 110 to 170 g. This is not an ericaceous feed and does not acidify the soil.
April
Leaf mould, composted bark or pine needles
A 5 cm mulch out to the edge of the canopy, laid on the surface and never dug in.
June, after deadheading
Fruit & Vegetable Fertiliser 4-5-6
25 g/m², about 80 g for a plant 2 m across, and nothing after June. Leave it out for a plant in good growth.
March and June, pots
Fruit & Vegetable Fertiliser 4-5-6
1 to 2 g per litre of compost on the surface, half the label top-dress rate: 30 to 60 g for a 40 cm pot (about 30 litres), 55 to 110 g for a 50 cm pot (about 55 litres).
Autumn or early spring, only if a test shows pH above 6
To lower the pH by one unit: 55 g/m² on sand, 110 g/m² on loam in two doses six months apart, 175 g/m² on clay in three doses; never more than 80 g/m² in one go.
Dr Forest has no ericaceous feed. The plan uses the Fruit & Vegetable Fertiliser 4-5-6 at half the label rate as the nearest fit; it is not an ericaceous feed and does not acidify. Mycorrhizal Fungi Powder is not recommended for ericaceous plants.
Signs to watch
Young leaves yellow with green veins → lime-induced chlorosis from alkaline soil or hard mains water → rainwater, sulphur, an ericaceous feed with chelated iron. Older leaves yellow between the veins in Aug on sandy ground → magnesium leached out → Epsom salts foliar spray at 20 g per litre. Rolled, drooping leaves in a hard frost are normal, but the same in Jul with brown edges → drought on shall
Pests, diseases and problems
Flower buds turn brown or silvery grey in winter, fail to open and grow black bristles
Bud blast, a fungus
Pick off and bin affected buds as soon as you see them, well before summer. Dead buds can stay on the plant for two or three years and carry on spreading spores.
Pale yellow blotches on the upper leaf with buff or grey felt underneath; leaves drop early
Rhododendron powdery mildew
Collect and bin affected and fallen leaves, water the roots in dry spells and open up crowded plantings. Replace a badly affected plant with a different cultivar.
Leaf edges notched all round; young or pot-grown plants wilt
Vine weevil adults on the leaves, grubs on the roots
Nematodes on pots and new plantings in late August or September and again in April. Notching on a big established plant is cosmetic.
Black sooty film on the upper leaf surface; flat brown scales and white waxy egg bands underneath
Cushion scale; the soot grows on its honeydew
Wipe or wash the leaves, and spray the undersides with a plant soap in June and July when the young scales hatch. Feed and water the plant so it outgrows the damage.
Leaves dull, droop and roll in summer on moist ground; the plant slowly dies
Phytophthora root rot in wet or badly drained soil
No cure. Remove the plant with its soil, correct the drainage, and replant on a slight mound or in a raised bed.
Strong shoots with plainer, narrower leaves and mauve flowers rising from the base of a grafted plant
Suckers of Rhododendron ponticum rootstock
Trace each sucker back to its origin below ground and pull or cut it off flush. Left alone they take over the plant.
Pruning
Pruning and care
Flowers on the previous year's wood, so any pruning is done straight after flowering in May to Jun; most need none beyond deadheading. Snap off spent trusses between finger and thumb without damaging the new buds beneath. Leggy, bare-based plants with rough bark (not smooth-barked types) can be renovated to 30 to 60 cm in Mar, then fed and kept moist; recovery takes 2 to 3 years. Mulch 5 cm with leaf mould
Good to knowShallow-rooted, mulch with leaf mould and never let it dry out; deadhead spent trusses.
Growing rhododendrons in pots
Dwarf and yakushimanum hybrids grow for years in a 40 to 50 cm pot; the big hybrids do not. Use a loam-based ericaceous compost, plant with the top of the rootball level with the surface and mulch with bark. A rhododendron in full leaf uses a lot of water, so check the pot daily from May to September and use rainwater where the mains is hard. Top-dress each March with fresh ericaceous compost and repot every three years. Wrap the pot in a hard winter, since the fine roots sit against the pot wall.
Rhododendrons: common questions
Why is my rhododendron not flowering?
Most often because it was dry in July to September, when the flower buds form. Deep shade, a late frost on the buds, bud blast, or pruning later than June are the other causes. Water in late summer, mulch, and give the plant more light if it is under dense trees.
Can I grow a rhododendron in alkaline soil?
Not in the open ground on chalk or limestone. Sulphur will bring a neutral soil down, but on chalk the lime keeps coming back. Grow a compact variety in a pot or a 60 cm deep raised bed of ericaceous compost, water with rainwater, or choose an INKARHO plant for soil near neutral.
Should I deadhead a rhododendron?
Yes, where you can reach. Snapping off the spent truss with finger and thumb straight after flowering stops seed forming and puts the energy into next year's buds. Take care not to break the new shoots just below the truss.
Sources
Growing information on this page was checked against these references in November 2026. Feeding rates are Dr Forest label rates.