Swamp azalea

Swamp azalea: how do you recognise it?

Habit

Deciduous shrub about 2 m tall and wide; it suckers over the years and spreads wider.

Foliage

Small, 2–5 cm, obovate, dark green above and blue-green below. Autumn colour yellow to red.

Flower

Tubular with narrow lobes, white (sometimes pale pink), in clusters in June–July, after the leaves emerge.

Scent

Strong and sweet, clove-like; the outside of the flower tube is covered with sticky hairs.

Roots

Shallow and finely branched, with suckers. They dry out quickly and must not be disturbed by digging or hoeing.

Flowering: June–July · Plants: March–April and September–October · Pruning: July–August

Why grow it?

• Flowers late, in June–July, extending the azalea season into summer.
• A strong, sweet scent that fills the garden on a warm evening.
• Tolerates wet soil as long as it is acid, so it suits places where other azaleas struggle.
• Fully hardy, to about −30 °C, and deciduous.
• The flowers are visited by bees and bumblebees.

Swamp azalea: where does it grow best?

• Half shade is ideal; it also tolerates more sun if the soil stays wet.
• Acid soil (pH about 4.5–5.5): sand or loam rich in humus. Never use lime or chalky compost.
• Normal to wet: it tolerates wet soil better than most azaleas, but not standing, oxygen-poor water for long.
• Sheltered from cold wind. Allow 1.5–2 m of space; ideal in a moist corner or at the edge of a pond.

Light

Partial shade

Soil type

Sand, loam or peat

Acidity

Acid

Moisture

Normal or wet

Size

2 m tall, 2 m wide

Hardiness

Hardy (to -30 °C)

Goes well with

• Royal fern: An imposing fern for the same wet, acid soil.
• Japanese water iris: Flowers in June–July on moist, acid soil, at the same time as the azalea.
• Hosta: Large leaves in the same half shade.
• Ferns: Fresh green foliage in moist, humus-rich soil.

Swamp azalea: when and how to plant it?

Plant in March–April and September–October.

1. Plant in March–April or September–October, when the soil is not frozen.
2. Dig a hole 2–3 times as wide as the root ball, but no deeper. Mix the excavated soil with leaf mould or peat-free ericaceous compost.
3. Soak the root ball in a bucket of water until no more air bubbles rise, then gently tease out the roots.
4. Set the root ball so its top is level with, or slightly above, the soil: never plant deeper than in the pot.
5. Firm the soil and water straight away, preferably with rainwater.
6. Spread a 5–8 cm layer of leaf mould or pine-needle mulch around the base.

Care

Water

Keep the soil evenly moist and water generously in dry weather, preferably with rainwater: tap water often contains lime and makes the soil less acid.

Feeding

In April–May, an ericaceous fertiliser or organic fertiliser. Not after July.

Mulching

Renew the leaf mould or needle mulch every spring. Do not dig or hoe around the shrub: the roots lie just below the surface.

Suckers

Cut off unwanted suckers with a spade in spring, or replant them elsewhere.

Spent flowers

You may snap off spent trusses by hand without damaging the young buds below. It is not essential.

Tip: Plant it near a seat or terrace where you sit in the evening: its scent is strongest on a warm June or July evening.

Swamp azalea: when to prune it?

Pruning months: July–August.

When: It needs little pruning. Do it just after flowering, in July–August, because next year’s flower buds form in summer.

How:

• Snap or cut off spent trusses if you wish.
• Remove dead, weak and crossing branches at the base.
• Shorten overlong branches just above a side shoot to adjust the shape.
• An old, bare shrub can be cut back to 30–50 cm above the ground in March: it usually regrows, but will not flower that year.

Caution: Do not prune late in autumn: you would cut off next year’s flower buds.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Dividing suckers

In spring, cut off a sucker with its roots and plant it elsewhere. This is the easiest method.

Layering

In spring, bend a low, flexible branch down, peg it into acid, loose soil and put a stone on top. Separate it with its roots after one or two years.

Seed

Scatter the fine seed in January–March on moist acid seed compost without covering it, in the light at 15–20 °C. Seedlings grow slowly and do not always resemble the parent.

Swamp azalea: how does it get through the winter?

Hardy to −30 °C.

• Fully hardy, to about −30 °C: no protection is needed in Flanders.
• A layer of leaves or needle mulch around the base protects the shallow roots, especially on young plants.
• In a pot the roots freeze more easily: stand the pot against a wall, wrap it in hessian and water on frost-free days.

Diseases and pests

• Chlorosis: Yellow leaves with green veins on soil that is too chalky. Acidify the soil with needle mulch and leaf mould, water with rainwater and give an ericaceous fertiliser.
• Powdery mildew: A white, powdery coating on the leaves, mainly in late summer in dry weather. Water at the base, leave space between plants and remove affected leaves.
• Vine weevil: Half-moon notches in the leaf edges in summer; the grubs eat the roots. Apply nematodes in August–September to warm, moist soil.
• Drought damage: Brown leaf edges and drooping leaves on soil that is too dry, even though it tolerates wet feet. Water with rainwater and apply a thick mulch.

Through the year

• March: Plant. Ericaceous fertiliser. Renovate old shrubs if needed (no flowers that year).
• April: Plant. Top up the leaf mould or needle mulch. Cut off suckers.
• May: Water with rainwater in dry weather. Buds appear.
• June: Flowering and scent begin.
• July: Full flower. After flowering, remove spent trusses and prune lightly.
• August: Prune lightly after flowering. Water in dry weather. Apply nematodes against vine weevil grubs.
• September: Plant. Collect seed when the capsules open.
• October: Plant. Autumn colour. Top up the mulch.
• November: Rest. Fallen leaves can stay as mulch.

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 name viscosum means ‘sticky’ and refers to the glandular hairs on the outside of the flower tube.
• In the United States it is also called swamp honeysuckle, because the tubular flowers resemble honeysuckle.
• It was used in breeding the Ghent azaleas, passing on its late flowering and scent.