Swamp azalea
Rhododendron viscosum · Ericaceae
A native azalea that flowers in early summer, after the leaves, with sticky white trumpets and a strong scent of cloves. It grows in wet, acid soil.
- Size
- 6½ ft tall, 6½ ft wide
- Zones
- 4 to 9
- Light
- Full sun to part shade
- Flowers
- Late June to mid July
- Pets
- Toxic (severe)
- Job in a bed
- Anchor
A bed built around swamp azalea
The planner designed this bed around swamp azalea when this page was made: a 16 × 8 ft (4.9 × 2.4 m) English border in zone 6, part sun. It chose the other plants from the 367 that suit those conditions and its wet soil, and says below why swamp azalea stands where it does.
The plants
- 1 Swamp azalea × 1, white
- 2 Mouse-ear coreopsis 'Nana' × 27, yellow
- 3 Mossy saxifrage × 21, pink
- 4 White mugwort × 4, cream
- 5 Siberian iris 'Caesar's Brother' × 3, purple
- 6 Hosta 'June' × 4, variegated leaves
- 7 Virginia bluebells × 8, blue
- 8 Camassia × 3, blue
- 9 Snowdrop × 12, white
Why this plant
- The bed's only structure. Shrubs and grasses keep their shape when the perennials die down, and it is the only one here.
- The only trumpets. The others are daisies, open bowls, plumes and others. Shape reads at a distance, where colours blur.
Why here
- Anchors a back corner. At 6½ ft it is the tallest plant here, and the tallest close the ends of the bed, where they stand in front of nothing.
- A step in the back row. Next to white mugwort (5 ft): no two neighbours there are the same height, so the back steps up and down like a skyline.
- One plant, alone. A shrub is seen as an object, and needs clear ground around it.
Why now
- In flower for five weeks. From late June to mid July.
- Fills a gap. For four of those weeks nothing else on the plan is in flower.
Planned as an English border: 9 of the 10 groups (90%) are drifts, longer along the bed than they are deep. The back corners are held by swamp azalea and white mugwort, the tallest plants on the plan. Neither end outweighs the other: the visual weight is split 51 to 49, left to right.
The planner's own design check: 10 Excellent, 6 Good, 1 Satisfactory.
Open this bed in the planner Change its size, the sun or the style, move plants about, or swap one for another.
Where swamp azalea goes in a border
At 6½ ft it belongs at the back of a bed. In the planner's ten height classes it is very tall (5 to 6½ ft).
Its job: anchor. A shrub that gives the bed permanent structure. Planted singly, as soon as the front edge is laid; everything else is arranged around it.
It flowers a month after other azaleas, and its scent carries a long way. One of the few rhododendrons that tolerates wet ground.
- Fragrance in early summer
- Wet soil and pond edges
- Extending the azalea season
What to plant with swamp azalea
Plants recorded as good partners for it, and what each pairing does:
- Summersweet: in flower together for two weeks, from mid July.
- Winterberry: its green leaves are a setting for the flowers, before and after.
- Cinnamon fern: its green leaves are a setting for the flowers, before and after; low enough (4 ft) to stand in front of it; happy in the same wet soil.
Questions about swamp azalea
Does swamp azalea need sun or shade?
Swamp azalea grows in full sun (6 or more hours of direct sun a day), part sun (4 to 6 hours of sun, including some afternoon sun) and part shade (2 to 4 hours of sun, mostly in the morning).
It wants moist to wet soil, rich in organic matter, and plenty of water. It copes with soil that stays wet.
What zones does swamp azalea grow in?
USDA zones 4 to 9. It is hardy to zone 4, which means it survives winter nights down to about −20 °F (−29 °C).
Gardening in Canada? Use the USDA zone for your town: Canada's own zone map uses different numbers.
When does swamp azalea flower in my zone?
In zone 5 it flowers late June to mid July. As a summer flower it moves about half a week for every zone: earlier where it is warmer.
| Zone 4 | Late June to late July |
|---|---|
| Zone 5 | Late June to mid July |
| Zone 6 | Late June to mid July |
| Zone 7 | Mid June to mid July |
| Zone 8 | Mid June to mid July |
| Zone 9 | Early June to early July |
Worked out from Hopkins' bioclimatic law: spring arrives about four days later for each degree of latitude north, and one zone is roughly two degrees. A warm spring or a cold one moves any garden two weeks either way.
How far apart should swamp azalea be planted?
5 ft (150 cm) apart, from the middle of one plant to the middle of the next. That is a little closer than its full width of 6½ ft (200 cm), so a group knits into one patch.
It grows 6½ ft (200 cm) tall. Give it the full distance from anything taller, or it will lean out towards the light.
How many swamp azalea plants do I need?
Usually one. At 5 ft (150 cm) across, it is a plant you place, not one you mass. For a hedge or screen, set them 5 ft apart: three plants for a 10 ft run.
In the bed above, the planner used one in one group.
Is swamp azalea poisonous to dogs and cats?
Yes: severe toxicity to people, dogs, cats and horses. All parts, including the nectar, are highly poisonous if eaten and can affect the heart.
Rated from NC State Extension and the ASPCA. Where references disagree, the more cautious rating is used.
Is there a pet-safe plant like swamp azalea?
These come closest to doing what swamp azalea does, and no toxicity is recorded for them:
- Viburnum 'Summer Snowflake': the same job in a bed, the same size and the same colour; flowers mid May to early September, wants moist soil.
- Siberian dogwood: the same job in a bed, the same size and the same colour; flowers late May to early June, wants moist soil.
- Summersweet: the same job in a bed and the same colour; 12 in narrower, flowers mid July to mid August.
Matched the way the planner's Swap button matches plants: the same job in a bed, then size, then flowering time.
Do deer eat swamp azalea?
Mostly not. NC State lists it as resistant to rabbits. No plant is proof against a hungry deer in a hard winter, but this one is low on the menu.
What flowers after swamp azalea?
Swamp azalea is over by mid July. These start as it finishes and carry the same spot on, so it never goes flat:
- Summersweet: from mid July to mid August, 6½ ft tall (a recorded partner).
- Hardy hibiscus: from late July to mid September, 5 ft tall, to stand in front of it and cover its fading stems.
- Russian sage: from mid July to mid September, 4 ft tall, to stand in front of it and cover its fading stems.
What does swamp azalea look like in winter?
Swamp azalea drops its leaves and stands as bare branches. That is when to see its shape, and to prune it if it needs pruning.
Is swamp azalea good for bees and butterflies?
Swamp azalea is visited by pollinators, bees, specialist bees, butterflies and moths.
It is native to the United States.
More Rhododendron
- Azalea 'Delaware Valley White' Rhododendron 'Delaware Valley White'
- Azalea 'Hino-Crimson' Rhododendron 'Hino-Crimson'
- Catawba rhododendron Rhododendron catawbiense
- Evergreen azalea Rhododendron (Kurume hybrids)
- Flame azalea Rhododendron calendulaceum
- Korean rhododendron 'Cornell Pink' Rhododendron mucronulatum 'Cornell Pink'
- Northern Lights azalea Rhododendron (Northern Lights Group)
- Pinxterbloom azalea Rhododendron periclymenoides
- Reblooming azalea Rhododendron (Encore Group)
- Rhododendron 'Cunningham's White' Rhododendron 'Cunningham's White'
- Rhododendron 'Nova Zembla' Rhododendron 'Nova Zembla'
- Rhododendron 'Olga Mezitt' Rhododendron 'Olga Mezitt'
- Rhododendron 'PJM' Rhododendron 'PJM'
- Rhododendron 'Purple Gem' Rhododendron 'Purple Gem'
- Rhododendron 'Roseum Elegans' Rhododendron 'Roseum Elegans'
- Rosebay rhododendron Rhododendron maximum
- Royal azalea Rhododendron schlippenbachii
- Yak rhododendron Rhododendron yakushimanum
In these guides
- Plants for wet soil and rain gardens
- Rabbit-resistant plants
- Garden plants toxic to cats
- Garden plants toxic to dogs
- Native plants for shade
- Plants for butterflies
- Plants for bees
- Plants for a white garden
Zone guides: Zone 4 · Zone 5 · Zone 6 · Zone 7 · Zone 8 · Zone 9
Every fact on record
- Type
- Shrub
- Size
- 6½ ft (200 cm) tall, 6½ ft (200 cm) wide
- Plant
- 5 ft (150 cm) apart
- Height class
- Very tall (5 to 6½ ft)
- Place in the bed
- Back
- Flowers
- Late June to mid July; trumpets; white
- Leaves
- Medium texture, deciduous; green
- Shape
- Upright, clumping
- Hardy in
- USDA zones 4 to 9
- Light
- Full sun, part sun, part shade
- Soil
- Rich, moist to wet
- Water
- High
- Feeding
- None needed
- Family
- Ericaceae
- Comes from
- Southeast and west to Texas
- Attracts
- Pollinators, bees, specialist bees, butterflies, moths
- Stands up to
- Rabbits, heat, wet soil
Checked against NC State Extension and ASPCA poison lists. Flowering times are for zone 5: later in colder zones, earlier in warmer ones. Toxicity and spread information is a general gardening guide, not veterinary or medical advice, and invasive status varies by region. If a person or pet eats a plant, contact poison control or a vet. Check your local invasive species list before planting anything with a spread warning.