In short
- It blooms on the current year's shoots — hard pruning in early spring does not sacrifice bloom but strengthens it.
- Flower color does not depend on soil pH; watering with aluminum sulfate changes nothing here.
- Very cold-hardy, down to zone 3a — even frost-killed shoots regrow from the base and bloom in the same season.
- It spreads by root suckers and runners, forming a dense clump over time.
- Globe-flowered cultivars such as 'Annabelle' bear sterile flowers with no pollen or nectar — for pollinators the lacecap forms matter, like 'Haas' Halo'.
Botanical data
- Family
- Hydrangeaceae (Hydrangeaceae)
- Height
- 1–1.8 m
- Width
- 1.2–1.8 m
- Habit
- Rounded
- Growth rate
- Fast
- Position
- Full sun, Partial shade
- Soil
- Humus-rich, Loamy
- pH reaction
- pH 5.5–7
- Moisture
- Moist, Moderate
- Bloom
- June–September
- Hardiness
- USDA 3a–9a
- Propagation
- From cuttings, By layering, By division, By runners
Characteristics
A deciduous shrub with many thin, upright shoots growing from the base, forming a rounded, dome-shaped mass 1–1.8 m tall and of similar width. It grows quickly and spreads by root suckers and runners, so over time a single shrub gives way to a dense clump — the old cultivar 'Grandiflora' can take over several square meters this way. The leaves are opposite, broadly egg-shaped to nearly round, sharply toothed, 5–15 cm long, matte green, and distinctly thinner and softer than the thick, glossy leaves of the bigleaf hydrangea; they turn yellow in autumn. The bark of older shoots peels in thin layers of varying shades — hence the American common name sevenbark, meaning "seven barks." The flower head is a corymb. In the wild form it is flat and made up almost entirely of small fertile flowers, surrounded by a ring of a few larger sterile flowers. In the globe-flowered cultivars — 'Annabelle', 'Incrediball', 'Grandiflora' — nearly all the flowers are sterile and form a dense ball 20–30 cm across. This difference determines the species' value to insects: pollen and nectar come only from fertile flowers, so the moderate nectar value applies to the wild form and the lacecap cultivars, not to the popular globe-flowered ones. The flowers open greenish, then turn white, and in late summer turn green again and dry on the shrub. Bloom lasts from June to September, and the flower heads form only on shoots grown in the current season — this is the key difference from the bigleaf hydrangea, and it dictates how the plant is pruned. Flower color responds neither to soil pH nor to aluminum availability, so it cannot be "switched" to blue. The species is exceptionally cold-hardy, withstanding zone 3a conditions, and tolerates a wider range of soil reaction than the bigleaf hydrangea — from acidic to neutral. In full sun it grows well only with consistently moist soil; in a dry, sunny spot the leaves wilt in the heat and lose quality for the rest of the season. The weak point of the large-flowered cultivars is their thin shoots, which bow after a downpour under the weight of water-soaked flower heads.
Popular cultivars
- ‘Annabelle’
- The most popular cultivar of the species, found in 1910 in the woods near the town of Anna in Illinois and introduced to cultivation in 1962. Globe-shaped flower heads, made up almost entirely of sterile flowers, 20–30 cm across. Shrub about 1–1.5 m. Fault: thin shoots that bow under water-soaked flower heads.
- ‘Incrediball’
- The trade name of the cultivar 'Abetwo'. A selection improving 'Annabelle's main fault: the shoots are much thicker and stiffer, so they do not flop even after a downpour. Flower heads up to 30 cm across, opening lime green and maturing to pure white. Shrub about 1.2–1.5 m.
- ‘Invincibelle Spirit II’
- The trade name of the cultivar 'NCHA2', bred at North Carolina State University. The first line of smooth hydrangeas with pink 'Annabelle'-type flowers; this second generation has stronger shoots and a more intense color than the original 'Invincibelle Spirit'. It repeats bloom until frost, and the flowers turn green with age.
- ‘Grandiflora’
- The oldest widely grown globe-flowered cultivar, known in America as Hills of Snow — found in Ohio in the late 19th century, in Europe from around 1907. Until the 1960s it dominated gardens, before 'Annabelle' displaced it. It spreads strongly by suckers. Note: the same cultivar name is borne by a panicle hydrangea cultivar — a different species.
- ‘Haas’ Halo’
- A lacecap cultivar, selected from seedlings from Joan Haas's garden in Pennsylvania. Very large, flat flower heads with hundreds of small fertile flowers and a ring of white sterile flowers at the edge; upright habit, stiff shoots. It won the five-year trial of native hydrangeas at the Mt. Cuba Center, among other things for high pollinator attendance.
Growing and care
Watering
It copes poorly with drying out — in drought the leaves wilt and lose quality even if the shrub survives. In full sun it needs consistently moist soil and mulch; in partial shade watering is far less troublesome.
Fertilizing
A moderate dose is enough — excess nitrogen lengthens and weakens the current year's shoots, so the heavy flower heads flop all the more easily. Flower color depends neither on fertilizing nor on soil reaction.
Planting
Fertile, humus-rich, and consistently slightly moist soil, with a reaction from acidic to neutral — unlike the bigleaf hydrangea it does not require acidifying the soil. It is worth mulching with bark, because drying of the topsoil layer is the main cultivation problem here.
Pruning
It blooms on the current year's growth, so hard pruning does not sacrifice bloom but strengthens it. A practical compromise: cut the shoots back to a permanent framework 40–60 cm tall rather than close to the ground — there are then more flower heads, they are smaller, and the shrub does not flop after rain.
Companion plants
Good companions
Dzieli upodobanie do półcienia i wilgotnej, próchnicznej gleby, a szerokie liście funkii zacieniają nasadę krzewu i ograniczają przesychanie.
Te same wymagania wodne i świetlne; pierzasta tekstura liści kontrastuje z ciężkimi, kulistymi kwiatostanami.
Niska obwódka na przedpolu krzewu, o zbliżonych wymaganiach glebowych — zakrywa gołe podstawy pędów pozostałe po wiosennym cięciu.
Bad companions
Lawenda wymaga suchej, przepuszczalnej i raczej zasadowej gleby — to przeciwieństwo stale wilgotnego stanowiska, bez którego hortensja krzewiasta traci liście w upały.
Gęsta mata korzeni przy powierzchni odbiera wodę, na której brak ten gatunek reaguje najszybciej ze wszystkich hortensji.
The evidence level indicates whether the relationship is backed by research, observation, or gardening tradition.
Diseases and pests
A white, powdery coating on leaves, shoots and buds. The leaves turn yellow, become deformed and drop prematurely. The coating develops on both sides of the leaf blade, is often clearly visible from above, and can easily be rubbed off with a finger — it is the superficial mycelium of an ectoparasite that draws nutrients from the epidermal cells by means of feeding structures (haustoria). At first single, round, white spots appear, which in time merge and cover the entire leaf surface and the young growth. Young leaves and the current season's shoots are the most susceptible; heavy infection causes them to pucker, stiffen and stop growing, and on buds it leads to deformation or death of the flowers. On the fruit of some species (gooseberry, apple, cucurbits) the coating leaves russeting or a corky skin. Towards the end of the season, small, dark fruiting bodies (chasmothecia, formerly called cleistothecia) — the pathogen's overwintering structures — appear within the coating. The disease affects many plants, including roses, oak, privet, lilac, cucurbits, cereals, grapevine, apple, gooseberry, phlox and coneflowers, with each of these hosts being infected by a specialised species of fungus.
Grey mouldA grey-brown, fluffy coating of mycelium on flowers, fruit, leaves and shoots, preceded by watery, rotting spots. It attacks weakened and damaged tissue and develops rapidly in humidity and cool conditions and where air circulation is poor. Common on strawberries, begonias, peonies and roses. The coating is characteristically 'mousy' — fluffy and ashy grey, and when the infected tissue is knocked a visible cloud of spores (conidia) puffs up, which is a diagnostic feature of this disease. Infection usually starts in the weakest tissues: fading petals, wounds, hail marks or pest feeding damage, as well as pollen residue settled on the petals. Small, brown, watery spots appear on the flowers and quickly engulf the whole bloom ('flower blight'); from the infected flower the fungus often grows down into the shoot, causing it to constrict and die back. On soft fruit (strawberry, raspberry, grapes, tomato) a watery, browning rot develops, covered with a thick layer of grey, fluffy felt — the berry softens and 'dissolves'. On seedlings and cuttings Botrytis causes basal stem rot and the sudden collapse of plants. In older, dying mycelium and on plant debris hard, black, irregular lumps form — sclerotia (resting bodies), by which the pathogen overwinters in the soil and in plant debris. The disease also shows up after harvest, as a rapidly advancing storage rot of fruit and vegetables.
Root rot (Phytophthora root rot)The disease develops in overly wet, poorly aerated substrate and attacks the roots and the base of the stem. The roots turn brown, rot and lose their bark, which slips off easily, and a dark, watery rot appears at the stem base. The above-ground part wilts, yellows and dies back despite moist soil – a characteristic symptom, because the damaged roots no longer take up water. Especially at risk are houseplants, succulents and cacti (from overwatering), and in the ground conifers, rhododendrons, ericaceous plants and strawberries on waterlogged sites. The process begins inconspicuously at the root hairs and root tips, which darken and disappear, after which the rot advances towards the framework roots and the root collar. Healthy, pale, active roots waste away — what remains is a dark, rusty-brown or blackish, watery mass, and the outer layer of the root bark slips off like a glove, exposing the discoloured central cylinder. In trees and shrubs, infection of the collar and the base of the trunk (stem-base rot) shows as a dark, often streaking patch, sometimes with oozing, and the bark there is moist and sunken; in conifers and rhododendrons, browning and dieback are seen from the bottom or from the centre of the crown while the soil is still moist. Characteristic is the nested (patchy) occurrence of symptoms in the bed — where water and the pathogen accumulate locally. It is worth distinguishing the two causal oomycete genera: Pythium mainly attacks seedlings and fine roots, causing damping-off and root rot in cool, wet substrate, whereas Phytophthora also attacks older, woody plants, framework roots and the collar, more strongly in warmer, water-saturated soil. Key for diagnosis is the contradiction of the symptoms: the plant wilts and looks 'dry', although the substrate is wet — this results from the destruction of the roots, not from a shortage of water.
AphidsSmall (1–3 mm) soft-bodied insects, green, black or pink, feeding in clusters on young shoots and the underside of leaves. They excrete sticky honeydew. The diagnostic feature that distinguishes aphids from other small sucking insects is the pair of siphunculi (tubular projections) at the rear of the abdomen together with a tail-like cauda; a single colony usually holds wingless forms (parthenogenetic females) alongside winged migrants that fly onto new plants in summer. In summer aphids reproduce parthenogenetically and viviparously over many generations, so the population builds up explosively; in autumn many species produce a sexual generation that lays overwintering eggs on the host, and host-alternating species move between a winter host (often a tree or shrub) and a summer host (a herbaceous plant). Feeding symptoms include curling, crinkling and distortion of young leaves, stunted growth, yellowing, and the shedding of buds and fruitlets. Abundant honeydew coats the leaves, and the sooty mould fungus that develops on it forms a black film which limits photosynthesis; the honeydew is fed on by ants, which in return defend the colony against predators. Aphids are also important vectors of plant viruses (mosaics among them), transmitting them from plant to plant even during a brief probing puncture.
Spider mitesTiny (0.3–0.5 mm) arachnids, hard to spot with the naked eye, feeding on the underside of leaves. Symptoms: fine, pale speckling (feeding punctures); over time the leaves turn grey, yellow and dry out. Under heavy infestation a fine webbing becomes visible. Warm, dry air favours their development — a common problem for houseplants in winter near radiators. Spider mites are not insects but mites of the family Tetranychidae — adults have 8 legs (larvae only 6). The most common in cultivation is the two-spotted spider mite (Tetranychus urticae): yellowish-green females carry two dark spots on the sides of the body, while overwintering females turn orange-red. In orchards the fruit tree red spider mite (Panonychus ulmi) is frequent, whose red eggs overwinter in clusters on the bark of the shoots. Feeding mechanism: the mites puncture the cells of the epidermis and mesophyll on the underside of the leaf and suck out their contents, and the emptied cells fill with air — hence the characteristic, silvery-grey, fine speckling visible from the upper side of the blade (resembling a mosaic discolouration). At high numbers the leaves turn brown, curl and drop prematurely, and the whole plant weakens and loses its growth. Dense webbing spun between the leaves and shoot tips appears only under an advanced attack and is a sign of the risk of the pest spreading to neighbouring plants. The development cycle is very fast in warmth and drought — at a temperature of 25–30°C a single generation can be completed in 1–2 weeks, hence the numerous generations per season and the rapid, exponential build-up of the population. Females lay spherical, transparent eggs on the underside of the leaves. Fertilised females overwinter (in bark crevices, leaf litter and plant debris) or — in the fruit tree red spider mite — the eggs on the bark. Early infestation is easy to miss; diagnosis is helped by shaking a leaf over a white sheet of paper (moving specks become visible) or examining the underside of the leaf with a magnifying glass.
Toxicity
| For whom | Level | Notes |
|---|---|---|
| Humans | Mild | Contains cyanogenic glycosides — eating the leaves and buds can cause gastrointestinal upset. |
| Dogs | Moderate | — |
| Cats | Moderate | — |
| Horses | Moderate | — |
History and origin
The species grows wild on moist and rocky woodland slopes of eastern North America, from New York State to Florida and west to Iowa, Missouri, Oklahoma, and Louisiana. It reached European gardens in the 18th century but for a long time remained something of a collector's plant — the wild form, with its small fertile flowers, was considered unremarkable compared with the bigleaf hydrangeas brought from Asia. The turning point came with selections whose flower heads were built almost entirely of enlarged sterile flowers. The first to become widespread was 'Grandiflora', known in America as Hills of Snow: found in Ohio in the late 19th century and distributed by the Burpee company, it appeared in Britain around 1907 and for half a century was the dominant cultivar of the species in gardens. The smooth hydrangea's real fame, however, came thanks to the cultivar 'Annabelle'. In 1910 Harriet Kirkpatrick noticed a shrub with unnaturally large flower heads in the woods of Union County, Illinois, near the town of Anna, and transplanted it to her home garden, from where the plant spread among neighbors over the years. Only half a century later did it reach J.C. McDaniel of the University of Illinois, who recognized it in 1960, described it, and in 1962 introduced it to cultivation under a name referring to the town of Anna. More recent breeding has worked mainly on the known faults of this cultivar: the floppiness of the shoots ('Incrediball' from Proven Winners), flower color (the pink 'Invincibelle Spirit', bred at North Carolina State University), and value to pollinating insects (the lacecap 'Haas' Halo', winner of a five-year trial at the Mt. Cuba Center in Delaware).
Uses
It works well in beds along the north or east wall of the house, at the edge of woodland, in informal flowering borders, and in low groups along a path. Because it regrows from the base each year, it suits any spot where a shrub must keep a predictable size — unlike the bigleaf hydrangea, which cannot be pruned hard without losing its bloom. It is planted in April–May or in September, at a spacing of 100–150 cm, in fertile, humus-rich, and consistently slightly moist soil; a bark mulch limits the drying out that this species tolerates worst of all hydrangeas. Pruning is done in early spring, before growth resumes. Cutting the shoots back close to the ground gives the lushest regrowth and the largest flower heads, but also the thinnest shoots — after a downpour the balls then flop to the ground. A practical compromise is to leave a permanent framework of older shoots 40–60 cm tall: the new growth rests on a stiffer base, there are more flower heads, they are smaller, and the shrub holds its shape without stakes. An alternative is to choose a cultivar with strong shoots, such as 'Incrediball' or 'Invincibelle Spirit II'. The dried flower heads are worth leaving over winter — they look good in frost — and removing only at the spring pruning. They also make good dried bouquets, cut once the sepals turn papery. In plantings it pairs well with hostas, ferns, coral bells, and false goat's beard, which like the same moist, humus-rich ground. It is not suited to dry, sunny beds or to a position under shallow-rooted trees. If the planting is also meant to work for insects, a lacecap form or other nectar plants must be set beside the globe-flowered cultivars.
Trivia
- The species epithet arborescens means "becoming tree-like," although the plant never forms a tree and remains a shrub — which is why Polish literature uses the names hortensja krzewiasta and hortensja drzewiasta side by side.
- The American common name sevenbark ("seven barks") comes from the fact that the bark of older shoots peels in thin layers of varying shades.
- Cultivars with globe-shaped flower heads are ornamental but almost useless to insects: the sterile flowers produce neither pollen nor nectar. In a five-year trial at the Mt. Cuba Center the top score went to the lacecap 'Haas' Halo' — precisely for the pollinator activity over its hundreds of small fertile flowers.
- Within its native range the species' leaves are food for the caterpillars of a moth in the family Sphingidae, known in English as the hydrangea sphinx.
Frequently asked questions
How does the smooth hydrangea differ from the bigleaf and panicle hydrangeas?
The smooth hydrangea (Hydrangea arborescens) has thin, soft leaves and globe-shaped flower heads, almost always white; it blooms on the current year's shoots, so it is pruned hard in spring. The bigleaf hydrangea (H. macrophylla) has thick, glossy leaves, blooms on the previous year's shoots, and it alone changes color depending on soil pH. The panicle hydrangea (H. paniculata) has cone-shaped panicles and also blooms on the current season's growth, but it is taller and less sensitive to drought. In practice, the smooth and panicle hydrangeas share a pruning method, and everything but the genus separates them from the bigleaf hydrangea.
How and when should you prune the smooth hydrangea 'Annabelle'?
In early spring, in March, before growth resumes. The shrub blooms only on the current year's growth, so hard pruning does not sacrifice bloom — on the contrary, it stimulates it. It is best to cut the shoots back not close to the ground, but to a permanent framework 40–60 cm tall: the flower heads are then more numerous and smaller, and the shoots stiff enough to hold them up after rain.
Why do 'Annabelle' flower heads flop after rain?
Because the balls made of sterile flowers soak up water and weigh many times more than when dry, and the cultivar naturally has thin shoots. Three things help: pruning to a 40–60 cm framework rather than to the ground, moderate nitrogen fertilizing (excess lengthens and weakens the shoots), and choosing a cultivar selected for stiff shoots, such as 'Incrediball' or 'Invincibelle Spirit II'.
Can you get blue flowers on the smooth hydrangea?
No. The mechanism that turns flowers blue works only in the bigleaf hydrangea and depends on aluminum availability at low pH. In the smooth hydrangea the flowers are white and pass naturally from green through white and back to green as they age; aluminum sulfate and acidifying fertilizers change nothing here. Pink color is given only by bred cultivars, for example 'Invincibelle Spirit II'.
Is the smooth hydrangea valuable to bees?
It depends on the cultivar, and more so than in most shrubs. The wild form and the lacecap cultivars have hundreds of small fertile flowers with pollen and nectar, readily visited by bumblebees, solitary bees, flies, and beetles. The popular globe-flowered cultivars — 'Annabelle', 'Incrediball', 'Grandiflora' — consist almost entirely of sterile flowers, which offer insects nothing. If pollinators matter to you, you have to deliberately reach for a lacecap form, such as 'Haas' Halo'.
Sources
- Plants of the World Online (POWO) — Hydrangea arborescensDatabase (GBIF, POWO…)
- Missouri Botanical Garden Plant Finder — Hydrangea arborescensInstitution / botanical garden
- NC Extension Gardener Plant Toolbox — Hydrangea arborescensInstitution / botanical garden
- Mt. Cuba Center — Wild Hydrangea for the Mid-Atlantic Region (raport z testów odmianowych)Institution / botanical garden
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