Taxonomy and origin
Rhododendron is a genus of about 1,024 species of woody plants in the heath family, Ericaceae, evergreen or deciduous. It is the largest genus in that family, and the count is unsettled, with estimates from 850 to 1,200. The name comes from the Ancient Greek words for rose and tree. Most species are native to eastern Asia and the Himalayan region, with smaller numbers in North America, Europe, and Australia. Botanists split the genus by what covers the underside of a leaf, either scales, the lepidote character, or hairs known as indumentum. Azaleas belong to this same genus, which catches out gardeners shopping for the two as different plants. They make up two subgenera within it, and what most people mean by the word is the narrower horticultural group: large-leaved evergreens with leathery foliage and heavy trusses of bell-shaped flowers.
Almost nothing sold in an American garden center is a wild plant, since the international register holds more than 28,000 cultivars, and the eastern natives are the backbone of the hardy ones. Catawba rhododendron grows on woodland slopes and ridges along the Appalachian range, where it forms dense thickets; rosebay ranges from Ontario and Nova Scotia south to Ohio, Alabama, and Georgia, concentrated in the southern Appalachians; and Pacific rhododendron is the widespread species of the Pacific Northwest. Many of the hybrids that made rhododendron growing possible in cold gardens descend from Catawba rhododendron, so a shrub bought in Minnesota often traces its constitution to a Blue Ridge ridgeline.
Varieties worth knowing, by hardiness and size
The four cards with a zone rating come first, since hardiness settles the shortlist before anything else; the rest are sized but not zoned by their sources, and where two sources differ both sizes appear. Buy a well branched shrub with four or more limbs close to the ground.
Rhododendron or azalea, and which one to plant
Neither is easier to grow: both want the same cool, moist, well-drained acid soil and the same partial sunlight, so habit and eventual size decide. A rhododendron branches from a central trunk while an azalea comes up multi-stemmed from the ground, and the large evergreens need room, since Catawba rhododendron takes 6 to 10 feet of height and 8 to 10 feet of width and rosebay grows 10 to 20 feet. Give those the back of a shaded bed or a woodland edge, and keep azaleas and the small-leaved rhododendrons for a walk or a foundation strip, where a plant such as 'Ginny Gee' stops at 2 feet. One check works in bloom and one out of it: count the stamens, ten on most rhododendrons against five or six on most azaleas, or turn a leaf over, since azalea leaves never carry scales. Mountain laurel is the third look-alike, among the plants Catawba rhododendron is most often mistaken for.
Bloom time
Rhododendron flowers appear from early spring to midsummer, and which weeks you get is set by the variety and by where you garden. The early small-leaved hybrids start in April, carrying 4 to 9 flowers in each cluster. Catawba rhododendron and the hybrids built on it flower from April into June, with 'Nova Zembla' opening in May, and rosebay closes the season, holding its flowers from June to August. Truss size varies as widely as timing: some kinds carry heads of flowers 12 inches across, others hang their blossoms as individual bells. Next spring's flowers are decided the previous summer, since rhododendrons and azaleas set their flower buds by mid to late summer, which hands a spring display to August weather and to whatever was done with the shears late in the season. Cold takes buds after they form: low winter temperatures damage flower buds and can split the bark near the base of the stem, so an open, windy site loses flowers in a hard freeze even when the plant comes through.
Where to plant it and how much sun it wants
Rhododendrons want direct light for part of the day and shelter from the hottest part of it, and excessive shade normally results in very limited flowering. Dappled sun or high open shade is the ideal, morning sun with afternoon shade is acceptable, and the leaves scorch where the afternoon sun is hot. Farther south the balance shifts toward shelter: plants go out of the wind in dappled shade under a high cover of limbs, and while some varieties take a few hours of morning sun, most need protection from the intense southern sun. Oaks and pines are the companions named most often for that job, holding a high canopy of filtered shade while still passing the light intensity the plants need for growth.
Plant on the north or east side of a building, avoid southern exposures in a foundation bed, and prefer a northern slope, which drains and shelters better than a southern one. Concrete is the hidden trap there, creating alkaline conditions at a pH of 6 or above that are harmful to rhododendron and azalea growth, and the calcium leaching out of sidewalks and foundations turns up in the foliage as yellow or yellow-white leaves. Keep the plant out from under walnuts as well, along with butternuts, pecans, and hickories, whose roots produce juglone that most rhododendrons and azaleas are sensitive to. The reputation for being hard to grow comes down to siting, and the shrub in wet ground is the clearest case, since waterlogged rhododendrons decline and become susceptible to root rot. One rule applies only in fire country, where Catawba rhododendron carries a high flammability rating and should not be planted within the defensible space around a house.
Soil and pH
Rhododendrons are acid-loving plants, and the working target is a soil pH between 4.5 and 6.0, though one extension sets it tighter, at 4.5 to 5.5. Iron is why: it is most available to roots between pH 4.5 and 6.0, and above pH 6.5 there can be plenty of iron in the ground in a form the plant cannot use, held there by an excessive amount of calcium carbonate. To bring a bed down, use wettable sulfur or ferrous sulfate. Aluminum sulfate, the bag that comes out when gardeners are turning a bigleaf hydrangea blue, is toxic to rhododendron and azalea roots. Structure matters as much as chemistry. Rhododendron roots are shallow and fibrous, they must never dry out, and they resent any cultivation around the plant, so bark is the standard amendment: spread about 4 inches of pine bark over an area two to three times the ground the branches will cover, then mix it into the top 6 to 8 inches of soil. In heavy clay, build up instead, because clay collects and retains water and a hole backfilled with a light mixture works as a bucket, so the plant goes above the clay in a mound of better soil.
Planting
Set the plant high, and never lower than the soil around it. The American Rhododendron Society puts the top of the root ball several inches above the surrounding soil and warns that planting too deep can eventually kill the shrub, while the Royal Horticultural Society, reading the same shallow root system, says the roots should be just covered. Neither leaves room for a deep hole, and the disease that punishes one is crown rot, a brown discoloration of the wood in the lower stem near the soil line favored by poor drainage and by plants set too deeply. In ordinary garden soil, at or slightly above the original level is the safe middle, with the soil firmed at the base of the hole so the plant cannot settle later.
Open the root ball before it goes in. On a pot-bound plant, make vertical knife cuts 2 inches or more deep, evenly spaced around the sides of the ball, then loosen those cuts by hand and pull the roots outward. Mulch with 2 to 3 inches of compost, pine bark, or pine straw, carried well beyond the outermost branches and pulled back from the main stem to help prevent disease, leaving 5 to 6 inches clear around the trunk. Use material aged at least a season, because chips and bark still breaking down take the available nitrogen with them, and anything that heats up is not decomposed enough to use. Timing follows the climate: early spring in cold areas with early fall the second choice, and fall in hot areas, which lets the root system establish through the cooler months.
Water
Rhododendrons want steady moisture and free drainage at once, the hardest combination to deliver in an ordinary garden. Water when weekly rainfall falls short of 1 inch, and treat that as a standing rule for anything planted within the last two years; an inch of rain or irrigation a week suits established plants as well. A new plant set out in warm weather on sandy soil can need water twice a week through its first year, and young plants need regular watering through dry spells for the first several years. Before turning on the hose, pull back a small area of mulch beneath the canopy and feel the soil; when it is dry, water to a depth of 8 to 12 inches, running a soaker hose or drip line slowly at the base of the plant. Late summer and fall decide the next season. Keep the water going through dry spells from July onward, when the flower buds for next spring are forming, and carry watering into fall until the temperature drops below 40 degrees Fahrenheit. Drainage sets the limit on all of it, and the balance to reach is regular deep watering on soil that drains freely. Where the tap water is hard, stored rainwater is worth the trouble, since tap water in hard water districts carries too much calcium and reduces the acidity around the roots, though it is satisfactory for a month or two in summer once the rainwater runs out.
Height and spread
Height depends on which rhododendron you buy, and the genus covers a range no other garden shrub matches: the smallest species, a New Guinea plant, grows 4 to 40 inches, and the largest has been reported at 100 feet. Nursery plants sit in far narrower bands, with the Carolina types settling around 5 to 6 feet and forms running from compact to loose. Width is the figure people skip, and in the big evergreen types a mature plant covers about as much ground as it stands tall, which is how a shrub bought for a gap beside the porch ends up leaning over the walk. Read the age a listed size refers to as well, since figures for the southern species describe height at ten years.
USDA hardiness
The hybrids on a garden center bench are generally rated from zone 4 to zone 8, and the rating belongs to the variety, so read the number on the pot. The native species hold the cold end, and rosebay is hardy below anything on the hybrid shelf. Popularity is no guide to toughness: many of the hybrid rhododendrons sold as landscape plants are less cold hardy than the species types. The ironclad hybrids are the exception that opened cold gardens to the plant at all, having proven the most cold hardy and often the most heat resistant of the hybrids. In the coldest states, shop by number and ignore the photograph: across most of New Hampshire that means selecting plants hardy in zones 3 to 5. At the warm end the trouble starts below ground, where heat with wet soil brings on Phytophthora root rot, so hot climates get the raised version of the mound already described, a bed built with 50 percent or more fine pine bark. Most of the widely sold hybrids stop at zone 8, and the plants that carry southern gardens are picked for heat tolerance.
Feeding
Most rhododendrons need very little fertilizer. With the soil prepared and the bed mulched, very little is necessary, and older established plants need less still. Where no soil test is available, the general recommendation is a complete fertilizer with a ratio of 3:1:2 or 3:1:3, something like 12-4-8 or 15-5-15. Timing has one hard edge, and the extensions disagree on where it sits: Oregon State Extension says nothing after mid-July, because late feeding pushes growth that will not harden before winter, while other extension guidance allows feeding to mid-September. Lime and alkaline fertilizers are left out either way. Read the foliage before opening the bag. Pale green or dwarfed leaves point to nitrogen, yellow mottling along the leaf margins indicates excess potassium, and small dark green leaves with dead tips suggest phosphorus is low. For the iron shortage that appears on high-pH soil, foliar sprays of iron sulfate or chelated iron reduce the symptoms quickly, while amendments that acidify the soil are the long-term fix. When the yellowing develops reddish-purple blotches with browning leaf edges, magnesium is the shortage, and Epsom salts are a good source of it.
What the leaves are telling you
One symptom carries several causes: yellow can be shade, soil chemistry, or a rotting root, and a curled leaf in January means something different from one in July. Each entry pairs the tell with the move it calls for.
What actually goes wrong, in order
Fifty-seven gardener threads on Houzz, Ask Extension, Garden.org and two smaller boards were read for this list, weighted toward the first two. The counts belong to the threads and measure nothing about the plant.
Ten threads, the most common complaint of all. One photograph collects three answers, because shaded interior leaves, iron chlorosis on alkaline ground, and roots rotting in wet soil all arrive at the same yellow. Posters with a plant against a wall rarely connect the color to the concrete beside it.
Nine threads, and hardly any mention what was cut the previous June or how dry the previous August was, which is where the answer usually sits.
Nine threads, most posted in spring. The assumption is disease and the first question is which fungicide, when the damage was done over winter by wind on a plant whose roots were in frozen ground.
Seven threads about overgrown, leggy plants, several of them bare stems with foliage only at the top. Answers range from a third of the old wood over three years to cutting the whole plant to a foot from the ground.
Six threads, and the pattern repeats: a shrub moved in bloom, in midsummer, or into a sunnier spot, then a post a month later about a yellow plant that will not settle.
Five threads, usually posted as an emergency, with a drooping plant in the photograph and no history of the site or the watering.
Four threads, and the damage is described as sudden and total: everything stripped within reach over one winter, with only the top of the plant untouched.
Three threads, each opening with a photograph of the upper leaf surface, because the specks that would identify the insect are underneath.
Why it will not bloom
A shrub that leafs out and sets no trusses lost its flowers months earlier, usually in the previous summer.
Pruning and deadheading
Cut a rhododendron in the weeks right after the flowers fade, because next season's blossoms appear on old growth, which makes every later cut a withdrawal from spring. Deadheading needs no tool: break out only the spent blooms, leaving the dormant buds beneath them undamaged, and the truss comes away with a sideways snap of finger and thumb. Shaping a young plant is done the same patient way, by disbudding, which means pinching out the terminal leaf bud on new growth so the dormant buds below it wake and push side branches, a job that can be repeated in fall. Shears do the opposite here, cutting every branch to the same height and removing the flowering wood evenly across the plant. The overgrown plant with bare legs and leaves only at the top is the hard case: rejuvenation pruning restores the shape of an older leggy shrub and brings new growth, so a rhododendron does not have to be replaced when it outgrows its spot. Renewal pruning goes in early spring before new growth appears, and the cost is that year's flowers.
Pruning calendar
Flowering wood and winter hardiness sit on the same calendar, so the date of a cut decides more than the shape.
Toxicity and pet safety
ASPCA lists rhododendron as toxic to dogs, cats, and horses, with grayanotoxin as the toxic principle, and states that ingestion of a few leaves can cause serious problems. Clinical effects usually appear within a few hours and run from drooling, digestive upset, and loss of appetite through weakness and loss of coordination to leg paralysis, a weak heart rate, and recumbency lasting two or more days. Those are emergency signs, fast enough that a call to a veterinarian belongs ahead of any home remedy. The plant keeps its leaves when the rest of the border is bare, so the risk sits at nose height all winter.
The human side of the story is mostly about honey. Grayanotoxin sits in the pollen and nectar, which is why some species poison grazing animals, and people have been made ill by mad honey produced by bees working rhododendron and azalea flowers. Xenophon described Greek soldiers behaving strangely after eating honey in a village surrounded by Rhododendron ponticum in 401 BCE. A medical review of modern cases puts the amounts involved at 20 to 200 grams of grayanotoxin-containing honey, with symptoms starting most often within 20 minutes to 3 hours and lasting 1 to 2 days. In people the poisoning is rarely lethal, while cattle and pet cases sit at the serious end of the record.
Getting it through the winter
Broadleaf evergreens keep losing water through their leaves all winter, on mild sunny days and in high wind, and when the ground is frozen the roots cannot take up enough to make up for it. Snow insulates the leaves buried under it and moderates the soil temperature, so the damaging winters are the cold ones with bare ground. On an exposed site a screen earns its keep. One extension method drives stakes 4 to 5 feet tall about 2 feet out from the drip line, wraps them with burlap, leaves the top open, and takes the screen down in spring once temperatures hold above 28 degrees Fahrenheit; a second sets the barrier about 18 inches from the plant on the windward side. Winter damage reads clearly only in spring, which is why the temptation to cut in March should be resisted. Scrape a little bark from a doubtful branch, and green underneath means it is alive and likely to recover. By the end of June a rhododendron should have produced new leaves, which is when dead branches come off.
Propagation
Only species rhododendrons come true from seed, so a named cultivar has to be raised from semi-ripe cuttings, by grafting, or by layering. Layering needs no equipment and a long wait, taking 15 months to two years to root, and up to three years for some smooth-barked rhododendrons. Everything sold under a cultivar name is a clone of one original plant, which is what makes a label's hardiness claim worth reading: the PJM Group, bred by Peter J. Mezitt and his son in the 1930s to be exceptionally winter hardy, reaches gardens as a set of named selections including 'Aglo', 'Landmark', 'Olga', 'Pink Diamond', 'PJM Elite', and 'PJM Regal'.
Pests and diseases
Lace bugs do the most visible insect damage. They feed on the undersides of the leaves, and the result shows on top as pale speckling, called stippling, while the underside develops a fly-specked look from egg coverings and droppings. Azalea lace bug adults run about one eighth of an inch long, with sculptured wings that give them their lacy appearance, and there are usually three or more generations a year in South Carolina; the rhododendron lace bug is slightly larger and yellow, and leaf undersides are worth checking from early May. A heavy infestation turns leaves yellow and then brown, and a shrub standing in full sun can be killed by one. Notching along the leaf margins, worst on the interior leaves, is a different insect, the black vine weevil.
The rhododendron borer announces itself as drought. Off-color, wilted leaves on one branch are the early symptom, and the cause is a clearwing moth larva, pale yellow with a dark head, growing to about half an inch as it tunnels through the inner tissues and moves into the sapwood by late fall. No insecticide reaches a larva inside a branch, so the treatment is mechanical: cut below the active larvae and destroy the wilting branches in late summer or early spring before the adults emerge. Phytophthora is the disease that kills outright, a water mold that thrives in poorly drained or wet conditions and causes one of the most common landscape disease problems on rhododendron and azalea. Chemical control is ineffective once symptoms appear in the landscape, the fungus survives in the soil and cannot be eradicated once the soil is contaminated, and even resistant plants can succumb.
Dieback is the everyday disease of hybrids. Botryosphaeria dothidea is the fungus behind it, it is the most common disease of rhododendron in the landscape, and it usually takes one branch at a time on a plant that otherwise looks healthy. Petal blight starts as tiny irregular spots that give the flowers a freckled look, white on colored petals and brown on white ones, and its sclerotia can survive as long as two years in the soil to restart the cycle in spring. Leaf and flower gall thickens and curls leaves and buds in April and May, turning them pale green to white before they harden, and a white coating on both leaf surfaces is powdery mildew. Container plants collect a list of their own, with vine weevil especially troublesome in pots, alongside rhododendron leafhopper, scale insects, bud blast, azalea gall, and honey fungus.
Growing in pots
A container is the workaround where the ground is alkaline, heavy, or too shallow to plant into. Soil-less ericaceous media, peat-free ones included, suit rhododendrons, with one caution: those mixes lose their structure over time, and the result is poor drainage and an airless root environment that browns the leaves and kills them back. That makes re-potting a scheduled job. Move the plant into fresh potting compost every other year in early spring as soon as growth shows, replace the top 2 inches of compost in the intervening year, and trim off up to a third of the roots if the plant is going back into the same pot.
Fragrance and colors
Scent depends on parentage. Some hybrids carry fragrant flowers, among them the Loderi hybrids, raised by crossing Rhododendron fortunei with Rhododendron griffithianum, and a plant bought out of bloom gives no clue which group it came from, so scent is a reason to shop in flower. Color is where the genus has the range: the published span covers pure white, cream and yellow, apricot and salmon, soft pinks, deep reds, lavender, and deep purple and blue. That anchors almost any scheme, whether the bed is built around purple garden flowers or around pink ones, and the evergreen foliage holds it together after the flowers are gone.
A national flower and two state flowers
The genus holds two United States state flowers at once. Rhododendron arboreum, laligurans in Nepali, is the national flower of Nepal, where the flower is considered edible and eaten for its sour taste, pickled to keep for months, and sold as juice. Washington's came from a vote taken before women there could vote in elections: they selected the coast rhododendron by ballot in 1892 to enter it in a floral exhibit at the 1893 World's Fair in Chicago, and the legislature made the designation official in 1959. West Virginia's is rosebay, the most widespread rhododendron of the Appalachian Mountains, and it appears on the state flag. Two at once is uncommon company among the flowers grown in American gardens.
What gardeners get wrong
Five beliefs account for much of the damage done to healthy plants, and every one sounds like sense.
The myth: Curled, drooping leaves in midwinter mean the plant is dying or thirsty.
The reality: The curl is not caused by insects or disease, and plants recover when the weather warms again. Its tightness tracks the temperature closely enough to work as a natural thermometer, with leaves curling harder as the reading drops.
The myth: Rhododendrons need full shade because they are woodland plants.
The reality: They do well with direct light for at least part of the day, and excessive shade normally results in very limited flowering. A dark corner behind the garage produces a green, healthy, flowerless shrub.
The myth: Yellow leaves mean the plant is hungry and wants fertilizer.
The reality: Weather alone discolors leaves and mimics a nutrient deficiency, with sun scorch doing it in hot spells and frost injury in winter. Soil pH, wet roots, and exposure each produce their own yellow.
The myth: Deer leave rhododendrons alone.
The reality: Catawba rhododendron is rated as frequently damaged by deer, and the native species are browsed where deer pressure is high. Rabbits are a separate question, and the PJM Group is listed as resistant to their browsing.
The myth: Spots on the leaves mean the shrub is diseased and needs spraying.
The reality: Fungal spots of various colors turn up on rhododendron and azalea leaves through the year and are usually only cosmetic. The leaf troubles that matter announce themselves as stippling, notching, or whole branches browning.
What to expect in the first three years
Rhododendrons are slow to settle, and the work drops away over the first three seasons.
Year one is watering, and little else. A shrub set out in warm weather on sandy ground can want water twice a week through that first season, and young plants need a soak in every dry spell for several years after. Nothing goes into the hole with it, since rhododendrons are not fertilized at planting and are fed only once established.
The second year is spent underground. The root ball was cut open and pulled outward at planting so the roots would leave the shape of the pot behind, and those roots are shallow and fibrous, must never dry out, and resent cultivation, so the ground over them is left alone and watered. The watering rule that matters most covers anything set out inside the last two years: water whenever the week brings less than an inch of rain.
By the third year the plant stops being a project. Two to three years of establishment is the point where a well sited rhododendron needs very little maintenance and stays relatively pest free. What is left is a calendar: shaping in the weeks after bloom, because next season's blossoms form on old growth, and one deeper option held for an old leggy shrub, rejuvenation pruning, which restores the shape but goes in early spring and costs that year's flowers.
Flower photos
FAQ
Where is the best place to plant a rhododendron?
On the north or east side of a building, out of the wind and under a high cover of limbs that filters the light. Keep it clear of concrete foundations and walks, which create alkaline conditions at a pH of 6 or above, and away from the roots of walnut, butternut, pecan, and hickory.
Are azaleas or rhododendrons easier to grow?
Neither, on care: both want the same cool, moist, well-drained acid soil and the same partial sunlight.
Are rhododendrons native to the United States?
Some are. Most of the genus is native to eastern Asia and the Himalayan region, with smaller numbers in North America, Europe, and Australia, and the American species include Catawba rhododendron of the Appalachian ridges and rosebay, which reaches from Ontario and Nova Scotia south to Georgia.
Do you have to cut a rhododendron back every year?
No. They generally need minimal pruning, and taking out dead, diseased, or damaged branches is what keeps them healthy. Deadheading is the job that pays, because seed development takes a great deal of strength from the following year's flower buds.
Can an established rhododendron be moved?
Yes, and the shallow fibrous root system is what makes it practical, though those roots must never dry out. Move it on the planting calendar: early spring in cold areas with early fall the second choice, and fall in hot areas.
What should I use to make the soil more acidic?
Wettable sulfur or ferrous sulfate, or a fertilizer that acidifies as it feeds, ammonium sulfate among them.
Can I grow a rhododendron in a pot?
Yes, in a soil-less ericaceous mix, which has to be renewed on a schedule because it loses structure over time and starts browning the leaves through poor drainage.
Are rhododendron flowers dangerous to people?
The risk to people runs through honey: grayanotoxin sits in the pollen and nectar, and mad honey made by bees working these flowers has made people ill, though in humans the poisoning is rarely lethal.
When will winter damage show?
Not until spring. Scrape the outer bark of a doubtful branch, and green underneath means it is alive and likely to recover.
What are the tiny pale specks all over the leaves?
Lace bugs feeding underneath, and the confirmation is the fly-specked look their egg coverings and droppings leave on the back of the leaf.
Interesting tips
- A plant lost to dieback is worth replacing with one of the cultivars listed as resistant to it: 'Boursalt', 'Chionoides White', 'Cunningham's White', and 'English Roseum'.
- A branch killed by dieback holds its dead leaves until late summer, so a brown branch standing in an otherwise green shrub in July has already died, and it can come out that week.
Related Guides & Flowers
Azaleas
Azaleas belong to the same genus, and their own page covers the many-stemmed side of it, the one with thinner, softer, more pointed leaves.
Camellia
A full guide to camellia, the other broadleaf evergreen people weigh for a shaded corner.
Bigleaf hydrangea
A full guide to bigleaf hydrangea, the mophead shrub whose flower color follows soil chemistry.
Types of flowers
Garden flowers grouped by type, for whatever else the bed still needs.
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