Swamp azalea

Sun

Partial shade is best, especially in hot climates; tolerates full sun where the soil stays moist.

Water

Keep consistently moist. Tolerates seasonal wetness but avoid permanently stagnant, waterlogged soil.

Soil

Acidic, humus-rich soil, ideally pH 4.5-6.0; sandy or peaty soil with reliable moisture suits it well.

Height

3-6 ft (0.9-1.8 m), occasionally 10 ft (3 m)

Spread

3-6 ft (0.9-1.8 m), becoming wider through suckering

Flowers

Late spring to midsummer, depending on climate; fragrant white, sometimes pink-tinged tubular flowers appear after the leaves open.

Foliage

Small oval green leaves, often pale or slightly hairy beneath, turning yellow, orange or reddish purple in autumn.

Growth

Slow to moderate

Care needed

Medium

• Pollinators
• Cold hardy
• Heat tolerant
• Wildlife
• Low allergen
• Fragrant

Northern hemisphere months. In Australia, shift them by six.

Difficulty

Moderate

Toughness

Fairly tough

Pick your zone, country, light and whether you have pets.

1. Planting day

   1.6 ft (0.5 m)

2. After 1 year

   2.1 ft (0.63 m)

3. After 2 years

   2.5 ft (0.76 m)

4. After 3 years

   2.9 ft (0.89 m)

5. After 5 years

   4 ft (1.2 m)

About 7 years to full size.

From a typical nursery size, at an average rate for this kind of plant. A rule of thumb, not a forecast.

Cats and dogs

Toxic

Native to

the US

Growth rate

Slow

• Plant with the top of the root ball level with the soil surface; do not bury the shallow roots.
• Water deeply during dry spells, particularly during the first two growing seasons.
• Apply 2-3 in (5-8 cm) of leaf mould or composted pine bark, keeping it clear of the stems.
• If growth is weak, feed lightly in spring with a fertiliser formulated for acid-loving shrubs; avoid heavy feeding.
• Remove unwanted suckers if you need to keep the shrub within a defined space.

Pruning. Prune lightly just after flowering, removing dead or damaged wood and shortening only branches that need controlling. Avoid routine shearing or late-season pruning, which removes next year's flower buds.

Early spring

Check soil moisture, refresh mulch and apply a light acid-loving plant feed only if needed.

Spring

Plant container-grown shrubs while conditions are cool, and water thoroughly.

Early summer

Enjoy the fragrant flowers, then carry out any light shaping immediately after flowering.

Summer

Water during dry spells and check leaf undersides for lace bugs.

Autumn

Plant or transplant after leaf fall where winters are mild; in cold regions, wait until spring.

Winter

Check that mulch remains in place and that the root area is not sitting in stagnant water.

• Plant beside a pond or stream, above the permanently submerged margin.
• Group beneath an open canopy of deciduous trees with ferns and other acid-loving shrubs.
• Place near a path or seating area where the early-summer fragrance can be appreciated.
• Use in a moist native shrub border, allowing room for a loose colony to develop.
• Combine with white-flowered plants for a softly lit evening garden.

Garden styles it suits: Woodland, Bog garden, Sensory garden, Wildlife garden, White garden and New England.

Plant it with: Cinnamon fern, Royal fern, Summersweet, Highbush blueberry, Mountain laurel and Foamflower.

Wildlife: Flowers attract bees and butterflies; the branching growth provides shelter for small birds. Deer may browse the foliage and buds.

• Yellow leaves with green veins usually indicate soil that is too alkaline: test the pH, avoid lime and alkaline irrigation water, and move to an acidic bed if necessary.
• Lace bugs cause pale stippling and dark specks beneath leaves: reduce drought stress, provide afternoon shade and use insecticidal soap on leaf undersides if infestation is substantial.
• Root rot can develop in stagnant, poorly aerated soil: improve drainage or relocate the shrub, and do not plant it deeper than its original root level.
• Drought and hot afternoon sun can scorch leaves and reduce flowering: water deeply, mulch the root area and provide light shade.

All parts are toxic to people, cats, dogs and horses if eaten. Grayanotoxins can cause vomiting, weakness, dangerous heart disturbances and potentially death; seek urgent medical or veterinary advice after ingestion.

• Sweet azalea (Rhododendron arborescens)

  Also has fragrant white summer flowers, but its young twigs are generally smooth rather than hairy, and its conspicuously red stamens and style contrast with the petals.

• Pinxterbloom azalea (Rhododendron periclymenoides)

  Usually flowers pink in spring before or as its leaves unfold; swamp azalea normally flowers white later, after leaf expansion.

• Coast azalea (Rhododendron atlanticum)

  Usually stays lower, has distinctly blue-green foliage and flowers before or with the emerging leaves, rather than after them.

The epithet viscosum means sticky, referring to the glandular hairs on the flower tubes, while swamp azalea describes its preference for wet habitats.

Propagation. Sow seed on the surface of a moist, acidic seed mix without burying it. Layer low branches or take softwood cuttings in early summer; rooted suckers can sometimes be separated while dormant.

Accepted botanical name

Rhododendron viscosum

In the US and Canada

Native in the lower 48 states.

Native range

Alabama, Arkansas, Connecticut, Delaware, Florida, Georgia, Louisiana, Maine, Maryland, Masachusettes, Mississippi and New Hampshire.

Measured

Soil pH 4 to 7. Average height 448 cm.

Sources: GBIF (CC BY 4.0), USDA PLANTS and Trefle (CC BY 4.0). The care and design notes are LandscapeGPT's own.