Shrub · Ericaceae
Rhododendron
Rhododendron
Give it acidic soil and dappled shade, and each spring the rhododendron opens broad trusses of bell-shaped flowers.
Partial shade
Every 3–7 days
USDA 4–8
6–10 ft · 1.8–3 m
Overview
In spring a rhododendron carries its flowers in dense trusses, each a tight cluster of bell-shaped blooms crowded at the branch tips. It belongs to the heath family, kin to the azalea, and grows wild on cool, shaded hillsides in acidic, leaf-mould soil. The broad leaves and packed flower clusters trace straight back to that shaded, humus-rich ground.
This is a slow, long-lived shrub that fills out to 6 to 10 feet (1.8 to 3 m) over many years. Its broad evergreen leaves stay on the branches straight through winter, wide blades that hold their green while most shrubs stand bare. Those same broad leaves are also the vulnerable part, drying and browning at the edges when cold wind meets frozen roots.
Every part of this shrub is poisonous, from the broad leaves to the bell-shaped flowers to the nectar inside them. All of it carries grayanotoxins, so the plant stays out of reach of grazing animals, pets, and small children.
Care
The short version: dappled shade over the broad leaves, steady moisture at the shallow roots, and acidic, humus-rich soil that still drains. Set the root ball high, keep drying winter wind off the evergreen foliage, and feed only when the leaves show a need.
Light
The broad leaves want dappled or partial shade, the shifting light of a woodland edge. In cool climates they take more sun, and in a dim corner they hold on in deep shade, but under harsh full sun those wide blades scorch and lace bugs move in.
Water
The shallow roots want even moisture without waterlogging, since they sit close to the surface and rot in standing water. Water once the soil is dry 8 to 12 inches deep, about every 3 to 7 days in a dry spell, and keep it up into fall so the evergreen leaves stay plump going into winter.
Soil
Those fine, shallow roots want acidic, humus-rich soil that crumbles and drains, near pH 4.5 to 6.0. Break up heavy clay with organic matter and perch the root ball high, because roots left sitting in wet soil turn dark and rot.
Temperature
The hardy kinds endure a broad span, from about -29°F to 86°F (-34°C to 30°C). The cold alone rarely marks them; the damage shows as bronzed, browned leaf edges when frozen roots and drying wind pull water from the broad evergreen leaves.
Humidity
There is no humidity figure to watch, since plain outdoor air suits the foliage. What keeps the broad leaves from drying and browning is steady soil moisture and shelter from wind, not the moisture in the air.
Feeding
The plant is fed rarely, only when the leaves signal a shortfall. An acidic fertilizer at a 3:1:2 or 3:1:3 ratio such as 12-4-8 goes on after the flowers fade in spring, never at planting.
Pruning
Cut the branches back for shape or size just after the trusses finish, before the next buds swell. Snapping off the spent flower clusters, called deadheading, pushes the plant toward fuller trusses the next spring.
Repotting
Rooted in open ground, a rhododendron seldom needs lifting. Move one only to relocate it, and in a container watch for roots circling the surface or a plant losing its vigor, the sign it needs more room.
Size & spacing
A grown shrub spreads to 6 to 10 feet (1.8 to 3 m) tall and 5 to 8 feet (152 to 244 cm) wide, a broad shrub of evergreen leaves. Spacing plants to that full width opens gaps for air to move between the branches, so the foliage dries and disease finds no hold.
When to plant
A rhododendron goes into the ground in the mild spring or fall, not the heat of summer. The cool seasons let the shallow roots knit into the soil before stress. The bell-shaped flowers open across spring, and on some plants they keep coming into midsummer.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Plant: Spring or fall
- Harvest: Spring (some varieties into midsummer)
Hardiness
USDA hardiness zones: 4–8
Temperature: down to -34°C
Troubleshooting
The clues to a rhododendron's health show plainly on its leaves and roots. Stippled foliage, browned margins, dark rotting roots, and puffy swellings each read as a different problem. Look closely at what the plant is showing and match it below.
Wilting shoots and dark, rotting roots
The trouble is Phytophthora root rot, a soil fungus that spreads through wet, poorly drained ground or around a shrub set too deep. Dig down and the roots look dark and decayed; above ground the shoots wilt, rally briefly after watering, then fail again. Improve the drainage before planting, perch the root ball high, and lift out any shrub too far gone.
Fix:
- Improve drainage before planting and avoid wet, compacted sites or areas near downspouts
- Plant the root ball high and amend soil with organic matter
- Remove and discard severely affected plants
Pale-stippled leaves, black specks beneath
The upper leaf surface turns finely stippled and pale, and the underside is dotted with the shiny black specks of lace bugs and their droppings; the leaves then yellow and brown. The insects work hardest on shrubs in full sun and dry soil, starting in late spring. Coat the leaf undersides with horticultural oil or insecticidal soap, and let ladybird beetles thin them out.
Fix:
- Apply horticultural oil at the 2 percent summer rate or insecticidal soap, thoroughly coating leaf undersides
- Encourage natural predators such as ladybird beetles
Bronze to reddish-brown leaf edges
The leaf margins turn bronze to reddish-brown on the windward side, the mark of winter desiccation as cold, drying wind draws water from the leaves faster than frozen roots can send it up. Water deeply in fall before the freeze, and set a burlap screen about 18 inches out on the windy side. Once new growth pushes in spring, trim the browned tissue away.
Fix:
- Maintain adequate soil moisture in fall before the ground freezes
- Erect a burlap or other windbreak about 18 inches from the plant on the windward side
- Prune out dead tissue in spring once new growth resumes
Spotted leaves and puffy green swellings
Dark spots and blotches scattered across the leaves are fungal leaf spot, cosmetic and no more, while the puffy green or white swellings on the newly opened leaves and flowers are Exobasidium leaf gall. Neither harms the plant much. Pick off the spotted leaves and the galls, open the branches to moving air, and keep the foliage dry.
Fix:
- Prune off and discard unsightly spotted leaves or galls
- Improve air circulation and avoid wetting foliage
Toxicity
Behind the broad leaves and bell-shaped flowers sits a real poison. Every tissue, leaf, stem, flower, and nectar, holds grayanotoxins, and even a few leaves can sicken a pet or throw a grazing animal's heartbeat out of rhythm. The toxin even seeps into honey made from the blooms, the so-called mad honey, so keep the plant and its clippings away from pets, livestock, and children, and reach a vet at once if an animal eats any.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Severe
A cat that bites into a leaf soon drools and vomits, then moves stiffly, weak and unsteady on its feet. A larger dose drops its blood pressure and sets the heartbeat stumbling, and it can end in collapse.
Dogs Severe
A dog that chews any part drools heavily, vomits, and passes diarrhea within a short time. The grayanotoxins drain its strength and throw the heart rate off its beat, and a large mouthful can be life-threatening.
Horses Severe
A grazing horse drools and turns colicky, then moves weak and uncoordinated as the toxin spreads. Its heartbeat grows irregular and it may sink to the ground, with large amounts proving fatal.
Rabbits Severe
No leaf or petal of this shrub is safe for a rabbit. The same grayanotoxins knot the gut, sap the muscles, and throw the heartbeat into disorder, and so small a body is overwhelmed by very little.
Livestock Severe
Cattle, goats, and sheep that browse the foliage drool and vomit, then stand weak as the heartbeat turns abnormal. Hungry animals crop it most, and a serious dose can kill.
Birds Severe
A bird is no safer, since the grayanotoxins fall heavily on its heart and nerves for so small a frame. Even a peck of leaf or flower can leave it weak and huddled, so keep it well away.
People Caution
No leaf or flower here is food, and chewing one brings nausea, a run of drool, and a slowed or irregular heartbeat. Even honey drawn from the blooms, the so-called mad honey, holds the toxin, so keep curious children back from the shrub.
Keep plants, prunings, and fallen leaves away from pets and grazing animals.
Helpline numbers
United States
ASPCA Animal Poison Control: (888) 426-4435 (fee applies)
Poison Control: (800) 222-1222
Canada
Poison Centres: 1-844-764-7669
Varieties
The rhododendrons you meet most are hardy species and cold-bred hybrids, and each shows a different flower color. Here they are up close.
Rhododendron catawbiense
Rhododendron catawbiense, the cold-hardy Catawba native, opens lilac-purple flowers and endures cold to about -25°F (-32°C).
Rhododendron maximum
Rhododendron maximum, the great rosebay, is a large evergreen whose flowers open white to pale pink, and it holds up in deeper shade than most.
Roseum Elegans
Roseum Elegans, a vigorous catawbiense hybrid, covers itself in rosy-lilac blooms and comes through hard winters.
How to propagate
New rhododendrons form slowly, whatever the method. The route you take depends on your patience and on whether you need an exact copy of a named cultivar, since seedlings rarely match the parent's flowers.
LayeringModerate
Spring to summer · One to two years
Layering is the most reliable route. From spring into summer, a low branch is wounded, pressed down against the soil, and pinned there; over one to two years it grows its own roots and can be cut free as a separate plant.
Stem cuttingsDemanding
Late summer · Several months to a year
Cuttings come slowly. Semi-hardwood cuttings taken in late summer, dipped in rooting hormone and held in humid air, put out roots only across several months to a year.
GraftingDemanding
Winter · One growing season
Grafting joins a named cultivar onto a hardy rootstock. Worked in winter, the graft knits together over one growing season, though the join is fine, exacting work for a practiced hand.
SeedModerate
Sow on surface of acidic medium · Several years to flowering
Seed is the slowest of all. Scattered on the surface of an acidic medium, the seedlings come up varied, their flowers seldom matching the parent, and they take several years to first bloom.
Myths
A rhododendron is only mildly poisonous, so a few leaves will not harm a pet.
In fact the whole plant, leaf, stem, flower, and nectar, is laced with grayanotoxins, and even a few leaves can bring on serious heart and nerve poisoning in cats, dogs, horses, and livestock. Treat every rhododendron as dangerous around animals, and call a vet the moment one has eaten some.
Rhododendrons grow fine in ordinary garden soil.
They actually need acidic, moist, well-drained, humus-rich ground near pH 4.5 to 6.0, and in dense clay or alkaline soil it falters. Get the soil right first, since no feeding or watering fixes the wrong pH.