The Hydrangea E-Book

Maurice Foster

• Herausgeber: The Crowood Press
• Kategorie: Lebensstil
• Sprache: Englisch

Seitenzahl: 335

Veröffentlichungsjahr: 2023

Beschreibung

The genus Hydrangea goes far beyond the colourful and ubiquitous Hortensia of a million suburban front gardens; it offers a remarkable diversity and versatility, with attractive planting choices for gardens everywhere. From the tough mountain species that can take up to -30C in their stride, to the warm temperate lianas that can top twenty metres into a tree, different species are now being brought into play by breeders to provide new cultivars at a bewildering rate. This book picks up this challenge, helping readers choose the best hydrangeas for their particular gardens, no matter what the soil or situation.

Leseprobe

CONTENTS

Part I History and Origins

1 A ‘Potted’ History

2 Distribution and Classification of Species

Part II The Key Subsections

3 Americanae: The ‘Annabelle’ Phenomenon

Arborescens cultivars: ‘Annabelle’ and its relatives

Hydrangea quercifolia

4 Heteromallae: The Asian Stalwarts

Hydrangea heteromalla distribution and classification

Hydrangea paniculata: the universal crowd pleaser

5 Asperae: Endless Variability

Hydrangea aspera in the garden – characteristics and challenges

Hydrangeainvolucrata

6 Macrophyllae: Mainstay of the Summer Garden

Hortensias: the most popular Hydrangea of all

Hydrangea serrata – sophisticated summer chic

7 Petalanthae:Hydrangea scandens and Hydrangea chinensis

8 Calyptranthae: The Hardy Climbers

9 The Cornidia Section: Subtropical Lianas

Part III Practicalities

10 Uses and Good Practice in the Garden

11 Propagation

12 Pests and Diseases

13 Future Opportunities for Development

Postscript

Bibliography

Index

Acknowledgements

PART I HISTORY AND ORIGINS

CHAPTER 1

A ‘POTTED’ HISTORY

In 1736, John Bartram, son of a Quaker farmer migrant and known by some as the father of American botany, collected plants and seeds of Hydrangea arborescens from the Appalachian mountains down the eastern side of what was soon to become the United States. These finds were among many baskets of plants that he sent to a London merchant, Peter Collinson, keen to acquire new plants from the New World.

H. macrophylla ‘Le Cygne’ (Henri Cayeux, 1919).

The first introduction, H. arborescens, sent to England from North Carolina.

From the small white-flowered corymbs of these plants it is clear why, although the first hydrangea introduced to Europe was an American, it was not until the introduction of the first living plants from Japan and China 50 years later that interest in hydrangeas as a genus began to stir.

The impact of Hydrangea macrophylla from Japan

In 1789, a plant called Hydrangeahortensis was brought to Kew through the agency of Sir Joseph Banks from a cultivated plant in China, although originally from Japan.

The eighteenth century was a time of a significant exchange of plants between China and Japan, and this form of hydrangea gained attention for its showy globose flower head, typical of the now ubiquitous and varied cultivars of the Japanese maritime species H. macrophylla. Nearly two centuries later, the original import was christened ‘Joseph Banks’ by Michael Haworth-Booth (an English plantsman and nurseryman, and author of the first significant British book on hydrangeas, in 1950). This original cultivar is still grown today where conditions are mild: old plants can still be seen, for example, flourishing in cottage gardens in the soft maritime climate of the south Cornish coast of England. Indeed, Howarth-Booth discovered its origin when he found on the Isle of Wight that it had reverted freely to a wild form of H. macrophylla, whence ‘Joseph Banks’ had clearly arisen as a branch sport. He renamed this in 1950 as a new species, H. maritima, with the cultivar name of ‘Sea Foam’. It was later deemed illegitimate as a new species.

The first Japanese mophead (H. macrophylla) from Sir Joseph Banks.

H. macrophylla ‘Sea Foam’, considered the original source of ‘Joseph Banks’.

Two species of Japanese hydrangeas had previously been discovered by Carl Thunberg, a Swedish physician, botanist and representative of the Dutch East India Company in Japan, although he identified and described these two new plants as viburnums. Japan was closed to foreigners in the 1770s, but the Dutch East India Company was allowed to trade as a special concession by the Japanese, as unlike other organisations it had not sent unwelcome proselytising missionaries into Japanese society.

The authorities allowed Thunberg to live on an artificial island called Deshima in Nagasaki bay, and he was permitted one visit to the mainland each year to pay homage to the Emperor in Tokyo, but was not allowed to collect plants on these trips, so most of his information about the Japanese flora came from local foraging. He evidently kept goats, and his servants collected fodder on the mainland. Thunberg was able to examine this material, and is said to have discovered his two hydrangeas among it, described in his Flora Japonica in 1784.

If this account of the novel source for Thunberg’s new plants is overly imaginative, another possibility is that they were simply brought to him by Japanese friends who appreciated his passion for plants. In any event, he named the two plants Viburnum macrophyllum and Viburnum serratum, the latter with smaller leaves and more slender twigs. His mistaken identification was because of their striking similarity in flower to the European Viburnum opulus, and perhaps also to Viburnum plicatum, which he might also have seen in Japan. The genera have two things in common: a flat corymb of small, ‘real’ or fertile flowers surrounded by a ring of broad-petalled ray flowers, and forms with a globose head crowded with the showy ray flowers.

Carl Thunberg (1743–1828).

Title page of Thunberg’s Flora Japonica, 1784.

At the same time, a Chinese botanist introduced a Chinese garden variety of hydrangea to France. Philibert Commerson, a French botanist, was said to have named this plant Hortensia to commemorate a lady named Hortense, whose identity has never been definitively established. Jussieu, in his widely used 1789 Genera Plantarum, published this plant under the name Hortensia, the same year as Joseph Banks introduced his Chinese garden-originated plant to Kew; and when, in 1830, the French botanist Seringe transferred Thunberg’s two goat-fodder ‘viburnums’ to the genus Hydrangea, the two species names Thunberg had chosen when describing them as viburnums were preserved in the names Hydrangea macrophylla and Hydrangea serrata. (Both are now classified in the botanical subsection Macrophyllae: seeChapter 6). The names Hortensia and Macrophylla are used interchangeably to describe cultivars of H. macrophylla.

Viburnum opulus.

Thunberg’s ‘Viburnum’ (later Hydrangea) macrophyllum.

Thunberg’s ‘Viburnum’ (later Hydrangea) serratum.

A few decades later, in the mid-nineteenth century, another employee of the Dutch East India Company, Philipp von Siebold, a German physician and eye specialist, was also consequential in bringing more Japanese hydrangeas to Europe. Because of his specialist skill as an ophthalmologist, the authorities allowed him to live – and therefore be able to botanise – on the Japanese mainland. He introduced many other Japanese plants, some now well-known and dedicated to his name, such as Magnolia sieboldii, Malus sieboldii, Hosta sieboldii and Thuja sieboldii. Among the hydrangeas he introduced was Hydrangea paniculata, which is today, along with Hydrangea macrophylla, one of the most widely planted species.

In 1862, von Siebold also introduced ‘Otaksa’, a seminal cultivar of H. macrophylla in the history of the development of hortensias, quite different morphologically from the original Banks’ introduction, with smaller leaves, thinner shoots, more compact habit and less vigour. The name derives from Otaki-san, the reportedly beautiful Japanese ‘wife’ of von Siebold.

Wild form of H. paniculata with pink buds (bottom). Its cultivars are now among the most widely planted hydrangea in gardens. Same wild form close up (top).

Discovered carrying illegal maps of Japan, von Siebold was banished by the Japanese Government and returned to Europe. But he was determined to return, and was later able to go back to Japan, where he is now one of the best-known and respected historical botanists from the West. He then fully immersed himself in Japanese culture, setting up a school teaching both medicine and botany – at the time closely linked disciplines.

Siebold’s Japanese ‘wife’.

Hydrangea macrophylla ‘Otaksa’, named after Siebold’s Japanese ‘wife’.

A Russian botanist, Carl Maximowicz, who was allowed some access to the flora in Japan in 1860, also collected some hydrangeas there, among other plants: several of these, hybrids and wild forms of both H.macrophylla and H.serrata, eventually arrived in Europe.

Hydrangea macrophylla in paintings.

The origins of the seminal cultivars

However, the basis for the development of what has become the modern H. macrophylla industry was really only firmly established with the introduction of two further cultivars by Charles Maries, a horticulturalist employed by James Veitch to collect plants in Japan. Veitch was the biggest and most influential nursery firm in England in the late-nineteenth and early-twentieth centuries, and sponsored collectors to seek and collect plants all over the world. In 1879, Maries sent back H. macrophylla‘Mariesii’ and H. macrophylla‘Rosea’, both in fact hybrids of indeterminate origin, but with the limited knowledge of the genus at the time, regarded then as distinct species.

In 1881, a ‘lacecap’ type, in the floral style of the wild species, H. macrophylla‘Veitchii’, which Charles Maries is said to have brought in from Japan, was added to the mix (although this is now disputed as a Maries’ introduction). These were the plants generally available for breeding towards the end of the nineteenth century that enabled the emergence of Hydrangeamacrophylla as a decorative pot plant.

Hydrangea macrophylla ‘Mariesii’, halfway between a lacecap and a mophead cultivar.

Hydrangea macrophylla ‘Rosea’, often seen as an attractive blue: naming hydrangeas after their colours is always a risky business.

Hydrangea macrophylla ‘Vietchii’, still highly valued in gardens.

French initiative: potted plants of the first rank

In 1903, French nurseryman Victor Lemoine sowed seeds of H. mariesii, which yielded plants of very different character from the parent, showing it to be of hybrid origin. He named three, Mariesii perfecta, M. grandiflora and M. lilacina, and remarkably all are still available and widely grown today, 120 years later.

Hydrangea macrophylla ‘Mariesii Perfecta’.

Hydrangea macrophylla ‘Mariesii Grandiflora’.

Hydrangeamacrophylla ‘Mariesii Lilacina’.

Hydrangea macrophylla ‘Mme Emile Mouillere’ (left) and H. macrophylla ‘Générale Vicomtesse de Vibraye’ (right). Two seminal cultivars still popular today.

Hydrangea macrophylla ‘Merveille Sanguine’ with a reversion back to the original pink H.macrophylla ‘Merveille’.

Lemoine also spotted that among the showy sterile florets of plants with globose flower heads, there were a few fertile flowers. These yielded seeds, which he raised, and which in 1908 enabled him to launch a series of notable globose-flowered mophead novelties. At the same time, also in France, fellow nurseryman Emile Mouillere crossed these new seedlings with H. macrophylla ‘Rosea’ and introduced a whole new range, including two of the best mophead varieties for outdoor use today: the big white H. macrophylla ‘Madame Emile Mouillere’ and the butterfly blue H. macrophylla ‘Générale Vicomtesse de Vibraye’.

Henri Cayeux, another French breeder, introduced inter alia another fine variety, H. macrophylla ‘Merveille’, a deep-pink mophead, from which a branch sport known as ‘Merveille Sanguine’ arose, the darkest red mophead I have seen in cultivation.

Following the initiatives of Lemoine and Mouillere the scene was set for others to produce more varieties in France, Germany, Holland and Switzerland. The focus was entirely on producing colourful globose-headed flowers derived from H.macrophylla. These were selected for pot plant and conservatory use, rather than as garden plants, and many hydrangeas with a lacecap format that would today be considered as first-class garden plants were discarded in favour of these large-flowered globose mopheads. Like remontancy now in the rose industry, with breeders discarding many fine roses because they are not remontant but flower only once, this historic narrowing of selection still influences public appreciation of the genus Hydrangea today. The French found, too, that hydrangeas were easy to force into flower, which extended the flowering time-frame for pot plants, and thus expanded sales for that particular form of the genus.

British indifference

It remains something of a mystery why, as France became the main centre for this expanding and profitable activity in the early twentieth century, the neighbouring British played little or no part in it. It appears that in Britain, hydrangeas were simply not a horticultural priority in the early part of the twentieth century, and no British nursery appeared to have the foresight to capitalise on new developments and promote hydrangeas for profit, as happened in France. It may be that the market for pot-grown plants in Britain was not as developed as in Europe. In the case of hydrangeas, in the cold winters then prevailing on the Continent, the principal way of cultivating them there successfully was in pots, so they could be easily winter protected. Their garden use was of secondary importance.

A rare nineteenth-century Veitch catalogue, courtesy of Caradoc Doy.

H. macrophylla ‘King George’, one of the Chelsea Gold Award-winning hydrangeas of H. J. Jones in the mid-1920s.

Veitch, a major British force in horticulture at the turn of the century for other genera, strangely seems to have made little use of the new hydrangea plants introduced by Maries or the early European hybrid introductions; and even with the eventual advance of the hydrangea as a garden plant, Britain has continued to be something of an exception in this history of development in the genus, despite having both a climate and existing gardens with ideal conditions and opportunities to exploit its full range.

Not many, for example, have heard of British hydrangea breeder H. J. Jones, who was awarded gold medals for his hydrangea exhibits at Chelsea in the mid-1920s. Only one or two of his cultivars are in cultivation in Britain today, more perhaps to be found in France, and his name has disappeared into the hydrangea mists of time (seeChapter 6).

It is worth considering the possible causes of this relative indifference in the UK to a superb garden plant that provides showy colour exactly when other staples of English spring and early summer gardens are over. Different factors come to bear at different times.

Crucially, hydrangeas in Britain were probably not considered as suitable garden plants for outdoor use because they were initially assumed not to be hardy. The same assumption at first applied to other exotica from Asia, such as camellias. Both genera were initially simply not trusted outside in the unpredictable English climate, and were cosseted in greenhouses or conservatories as stove plants. Both were also seen as vulnerable to frost damage beyond repair: something only relatively recently understood as false.

This misperception about lack of hardiness was probably confirmed in the public mind, for example, when soft mophead hydrangeas, straight from the nursery greenhouse, forced into early flower and used as houseplant gifts for Christmas and winter birthdays, were planted in the garden in late winter or spring, after flowering, straight from a heated house. Their soft shoots were either immediately frozen or consumed by slugs, leaving little trace. This firmly reinforced the view that hydrangeas in damp British gardens were inevitably martyrs, if not to frost, then to slugs and snails. They also lacked fragrance.

Perhaps some influential horticulturalists also regarded the rather overblown flowers of the mophead forms as coarse and vulgar – a phenomenon precisely of continental fashion, thus out of place in the great individual gardens of the British shires, which prided themselves on being showcases for botanical collections and newly created garden hybrids in a natural, informal landscape. British gardens favoured plants grown for total landscape effect, where rhododendrons, camellias and magnolias took front and centre stage. They perhaps saw the mophead forms of H.macrophylla, then known mostly as pot plants with showy ‘artificial’ flowers, out of context in this setting.

Hydrangeaaspera ‘Hot Chocolate’ (left) and H. aspera ‘Rosemary Foster’ (right).

It may be that the Victorians whimsically saw the hydrangea in a negative light because of the then fashion for floral symbolism, equating it with vulgarity and vanity, a big showy flower lacking in real substance, with its colour changes a sign of unreliability, or change of heart. For example, when sent as flowers by men to women who had turned down their advances, the pale blue hydrangea was symbolically read as an emblem of the woman’s coldness.

Perhaps, too, the rather elongated foreign names, often not descriptive but celebrating European persons of little note in UK social circles, did not trip easily off an English tongue. What the French see as English parochialism could have played a part: a British inclination to mistrust any language other than English; to turn an English perception of the French on its head, ‘l’exception anglaise’.

There are probably other examples of a yawning (in both senses) British indifference to the genus. But there are signs that its garden value is being increasingly recognised, and a more informed appreciation of the hydrangea is steadily growing. The British gardening public is waking up to its merits.

This is partly the effect of a recent move to recognise other species and cultivars as having garden value as well as H.macrophylla, hitherto the only popularly known form of the genus. Having said that, over the last 50 years I have found it difficult to interest nurserymen in the UK in new varieties that are not H. macrophylla cultivars – in contrast to the interest shown in new H. serrata and H. aspera seedlings and hybrids (such as H. aspera ‘Hot Chocolate’ and H. aspera ‘Rosemary Foster’) by European nurserymen.

Until recently, although new cultivars of H. aspera and H. serrata demonstrate huge improvements in flower and foliage on what has gone before, present as first-class hardy flowering shrubs and have trialled successfully outdoors at White House Farm in Kent for years, their novelty value has appeared to count for little in the UK’s mass horticultural market.

Yet in Europe a wide range of hydrangeas beyond H. macrophylla are already valued highly as flowering shrubs for garden planting. In America, there have been competitive breeding programmes and an explosion of new varieties, with the hydrangea there being called the plant of the Millennium, characterised by vigorous commercial promotion that is conspicuously lacking in the UK. The present rapid expansion of the genus in the United States is arguably one of the more significant events currently happening in horticulture in that country. While the focus in the USA is still mostly on H. macrophylla cultivars, thanks inter alia to the promotion of ‘remontancy’ (seeChapter 6), other species are now also being brought into play.

As Christopher Lloyd wrote with typical impish humour in The Well-Tempered Garden in 2001:

‘The mopheaded hortensias… are regarded, in refined circles, as crude, blatant, obvious, coarse, vulgar. In that case I must have something of all those qualities myself. I do not like these hortensias at all times and in all places, but they have a tremendous luxuriance and vitality that one cannot help admiring. The wild types of hydrangea are known as lacecaps… but these are the only kinds worth growing according to the sensitive (but selfconscious) man of taste’.

Vive la diffèrence: lacecap (left) versus mophead (right) Hydrangea macrophylla forms.

Indifference among the British

Some 45 specialist plant societies in the UK are devoted to individual genera, such as carnations, dahlias, clematis and so on, reflecting a dedicated gardening nation. However, it is remarkable that even today, although there are hundreds of hydrangea species and cultivars listed as commercially available in the RHS Plant Finder, and every garden centre in the country sells a range of hydrangeas, no British plant society exists for hydrangeas.

The first European conference on hydrangeas was held in 2007 in Belgium, and it attracted more than 200 delegates from all over the world, many coming in numbers from as far afield as New Zealand, Japan and America. There were just four paid-up delegates from the UK (excluding lecturers and RHS exhibitors presenting trial results).

John Massey and Philip Baulk of Ashwood Nurseries memorably put on a magnificent Gold Medal display at the Chelsea Show in 2005 of more than 80 hydrangea cultivars, expertly brought forward in flower and superbly displayed in a variety of colour and form, and by any objective measure a contender for the award of best stand in the show. I remember looking on in some amazement as the BBC TV team walked straight past it, to a stand featuring grasses. In a whole week of nightly TV programmes it received scant, if any, coverage, suggesting that the producers took the view that this outstanding exhibit of hydrangeas was of minimal public interest.

John Massey’s Ashwood Nursery’s hydrangea stand at RHS Chelsea, 2005.

Haworth-Booth and the Hydrangea as a popular garden plant

There is one exception to this history of relative British indifference: the British horticulturalist Michael Haworth-Booth. In the mid-twentieth century, his writings and exhibits at RHS Vincent Square flower shows in London brought hydrangeas to the attention of a much wider audience. His book TheHydrangeas, the first dedicated title to be published on the subject in the UK, ran to five reprints: the first edition published in 1950, a further edition becoming a Garden Book Club selection and the last, fully revised, published as late as 1983.

This was essentially a horticultural work, instrumental in focusing on the hydrangea’s value as a plant for the average garden, rather than merely as a subject for pots – a crucial issue. It was not until after the war in the 1950s that the real value of the hydrangea as a garden plant really began to be appreciated in the UK, and this was due in no small part to Haworth-Booth’s knowledge and ability to articulate how to get the best out of them, in gardens large and small. Nonetheless, like all books published in the UK on the subject, the focus of his book was primarily on the Hortensia-style macrophyllas.

Haworth-Booth’s seminal books of 1950, 1976 and 1983, introducing the hydrangea to a wider British public.

My own interest in hydrangeas began when I worked briefly for Haworth-Booth as a casual labourer in the mid-1950s during the university long vacation. I have many of his plants in my collection and parts of my now mature garden model the effective long-term results of his planting methods. My interest in the genus picked up pace significantly in the 1990s, when I was finally able to expand beyond the Hortensia varieties and began collecting seed of wild hydrangea species in China in the 1990s, later visiting Japan, where H.serrata in particular is nationally treasured.

From Hortensia lacecaps in the 1970s to paniculatas today

The so-called ‘lacecap’ varieties – Haworth-Booth’s coinage for what the Swiss would later call the more prosaic German Teller (‘plate’) – have emerged as a significant feature of the hydrangea story in the late-twentieth century. This is a term for those flowers closer to the style of the wild species, with a flat or gently convex (‘cap’-shaped) rather than globose corymb, consisting of a disc of numerous tiny fertile flowers surrounded by a peripheral ring of relatively large and showy sterile ‘ray‘ flowers. Typical of these, and among the first to be deliberately developed, are the Swiss-raised Teller Series of 26 cultivars created, marketed and named after birds and distributed for propagation during the latter half of the last century (discussed in Chapter 6).

Three Teller cultivars: ‘Moewe’ (seagull) (left and top); ‘Rotschwanz’ (redstart) (centre) and ‘Pfau’ (peacock) (right) in neutral soil.

Initially this series was bred specifically for the pot-plant market as an alternative choice to the traditional globose mophead forms. But collections were eventually sent to large well-known gardens across Europe for garden trial, and most Teller cultivars subsequently turned out to be excellent hardy garden plants. Marketing a ‘series’, where crosses have yielded a range of similar saleable plants, is a modern phenomenon that is essentially a marketing tool, but has some value to the gardener seeking to make planting choices.

Growing the future

There is undoubtedly growing interest at present in the mass and range of colours that hydrangeas can bring to the summer garden – of which this book is evidence. Many big gardens open to the public that rely on the dramatic spring display of rhododendrons, camellias, cherries, lilacs and magnolias and so on, can become something of a green desert after Easter, except for formal rose beds and labour-intensive and expensive herbaceous borders. These gardens need summer footfall, and some are now recognising that hydrangeas can create a drawcard colour factor to bring in the paying customer, extend their season by a quantum amount, and are cheap and easy to maintain: a prominent and relatively trouble-free source of colour and interest.

Alongside the colourful hortensias, in recent years there have also been significant advances in developing new cultivars of both H. paniculata and H. arborescens, totally hardy and thus suitable for cold gardens anywhere, of any size. In particular, there has been a recent explosion in the numbers of H.paniculata introduced to the market, with a potentially bewildering range of similar choices for the gardener (I understand there are now some 140 named cultivars of H. paniculata in the UK national collection at Derby). Hydrangeaarborescens has produced ‘Annabelle’, perhaps the most widely planted hydrangea in today’s gardens.

Other new species, such as H. chinensis and H. scandens, are now being used to make new hybrids that extend the range of flower styles and growth habit: for example, the excellent white free-flowering H. ‘Runaway Bride’, plant of the year at Chelsea Flower Show 2018.

Hydrangea ‘Annabelle’, a mophead form of H. arborescens.

Over the last 30 years, H. serrata in its many forms has also gradually found its way into Western gardens. It is even more varied in form than its cousin H. macrophylla, but always smaller in stature and more understated and elegant in its appeal. The arrival of these hydrangeas as garden plants, very different to the hortensias, occurred mostly after World War II, and has accelerated in recent years; this is treated in further detail in Chapter 6.

At the moment the focus of marketing worldwide is still principally on the multicoloured hortensias (now being joined, to some extent, by the explosion of H. paniculata cultivars). This book aims to enlarge that picture: in particular, to show how versatile other types of hydrangea are as first-class flowering shrubs for the modern garden.

Hydrangea ‘Runaway Bride’, Plant of the Year at Chelsea 2018 – as a H. scandens a whisper of change?

Wild form of H. serrata in Japan, a species highly valued by the Japanese.

CHAPTER 2

DISTRIBUTION AND CLASSIFICATION OF SPECIES

The Geographical Divide

The two earliest introductions, H. arborescens from America and H. macrophylla from Japan, are representative of different species in what are two wholly disjunct, quite separate populations in the wild. The genus divides between the temperate areas of East Asia and eastern North America, with related species linking both areas.

Hydrangea aspera ‘Rosemary Foster’, a White House Farm hybrid.

This geographical division of related plant populations between America and Asia is not at all unusual. There are an estimated 65 similarly-related genera split between these two continents – Crataegus, Magnolia, Lindera or Spiraea, for example – reflecting the past close relationship between two now geographically separate floras. Elements of Hydrangeaceae and many other genera probably migrated overland from Eurasia when North America was contiguous with Europe and Asia; and survived and developed independently on the basis of their ability to adapt to changing climatic and geological conditions.

In addition to these two main areas of hydrangeas, there are also the evergreen liana-style hydrangeas that climb trees and cliffs in the subtropical and warm temperate climate of Central and South America. But at the moment these are of minor significance in our story, as with one or two exceptions, the majority are not hardy and thus not suitable for cultivation in cool temperate gardens. I say ‘at the moment’ as some of these plants are of great floral potential and could play a significant part in the development of future climbing hybrids of acceptable hardiness (seeChapter 13).

Map showing the Asian and American continents before and after separation.

The American hydrangea species are confined to the mountains in the east and south of the United States, mainly in the Appalachians, but the area of Asian hydrangea species’ distribution extends over a much wider area, along the Himalaya from Nepal to western, central and southern China, through to Japan, Korea, Taiwan, Sumatra and Java. In addition to the vitally important Japanese species, China has lately and unsurprisingly been recognised as a centre for all kinds of recently introduced hydrangea species and forms, particularly in its western provinces, where the influence of the monsoon and a happy combination of latitude and altitude provide an unrivalled favourable climate that sustains some of the richest and most diverse temperate woody plant populations in the world. Recent introductions of hydrangea species and forms since the early 1980s from China are likely to have an influence on further hybrid developments, so we are certain to see significant additions to the range of interesting and gardenworthy cultivars in the future.

But today, it is the variety and colour of Japanese hydrangeas spreading from this turn-of-century first flourish of interest that make up the vast majority of the plants in gardens everywhere. While there are many excellent and, I would argue, underappreciated and underexploited species in China, it is the three Japanese species, H. macrophylla, H. paniculata and H. serrata, that make up the bulk of popular cultivars grown in gardens across the world. Their colour, form and flower power make them the cornerstone of the substantial hydrangea nursery trade in Europe and, in recent years, the USA.

Classification and taxonomy: the McClintock monograph

Most horticulturalists agree that the most significant occurrence of the genus in the wild – in terms of both scale and horticultural potential – is in East Asia. But total numbers of species vary widely in the literature, depending on which authority you follow taxonomically. Elisabeth McClintock wrote the first comprehensive monograph of the genus in 1957, when she was Assistant Curator of Botany at the California Academy of Sciences. This has remained the most important work of reference since then, despite a degree of qualification by subsequent authorities. Her revision in 1957 was certainly needed, as previously many plants had been named as separate entities by a raft of individual explorers and botanists from different countries. This is perhaps unsurprising, as throughout their range, hydrangea species’ polymorphism – a multiplicity of forms within a species – is prevalent, so plants with relatively minor differences were often identified as separate individual species.

Floral variability within subsection Asperae: twelve forms of H. aspera cut from my garden, no two quite the same.

Hydrangeas fall into two main sections – Hydrangea and Cornidia. These are further subdivided into subsections, each with its own morphological character.

McClintock examined no less than 150 different taxa during her study, and was able to radically reduce the number of species to 23. She set this out in a letter to me: ‘You mentioned having seen or used my monograph; perhaps you noticed that I have only about 20 species in the genus and not the 60 or 70 listed by Haworth-Booth and others who have written about hydrangeas. In making a careful examination of the entire genus it is my opinion that the number of species in Hydrangea has been greatly exaggerated, and most of them I have listed in synonomy.’ She subsequently visited White House Farm and I enjoyed the great privilege of talking hydrangeas with her.

More recent revisions

For the most part, McClintock’s ‘lumping’ of species (the inclusion of many species under a single umbrella classification) has been regarded as justified, but in some cases it has been felt to be an oversimplification. For example, Bean, in his authoritative Trees and Shrubs Hardy in the British Isles (last edition 1973) was rather equivocal in his reaction to McClintock, admitting that while following her in lumping many familiar species together, he maintained others at species level, but only, as he put it, where there was ‘taxonomically some room for manoeuvre’. In other words, he accepted that a flexible approach was permissible if a reasonable case could be made, though admitting it was more for the convenience of gardeners than from any conviction that McClintock was wrong.

By contrast, in their Flora of China, Wei and Bartholomew (2001) characterise some 73 species in total, more than three times as many as McClintock, with 33 in China, of which 25 are endemic. But in the same work, the two authors failed to reach agreement, Bartholomew arguing that it should really be reduced to almost half that number: eighteen species in China, with nine endemic. This is typical of the conflict in taxonomic opinion that continues among botanists today.

The McClintock revision

Section 1: Hydrangea

Subsection 1: Americanae

(2 species)

1 Hydrangea arborescens

2 Hydrangea quercifolia

H. arborescens ssp. arborescens

H. arborescens ssp. discolor

H. arborescens ssp. radiata

Subsection 2: Asperae

(3 species)

1 Hydrangea sikokiana

2 Hydrangea involucrata

H. involucrata ssp. longifolia

3 Hydrangea aspera

H. aspera ssp. aspera

H. aspera ssp. strigose

H. aspera ssp. robusta

H. aspera ssp. sargentiana

Subsection 3: Calyptranthae

(1 species)

1 Hydrangea anomola

H. anomola ssp. anomola

H. anomola ssp. petiolaris

Subsection 4: Petalanthe

(2 species)

1 Hydrangea hirta

2 Hydrangea scandens

H. scandens ssp. scandens

H. scandens ssp. liukiuensis

H. scandens ssp. chinensis

H. scandens ssp. kwangtungensis

Subsection 5: Heteromallae

(2 species)

1 Hydrangea paniculata

2 Hydrangea heteromalla

Subsection 6: Macrophyllae

(1 species)

1 Hydrangea macrophylla

H. macrophylla ssp. macrophylla

H. macrophylla ssp. serrata

H. macrophylla ssp. stylosa

H. macrophylla ssp. chungii

Section 2: Cornidia

Subsection 1: Monosegia

(8 species)

Hydrangea seemannii

Hydrangea asterolasia

Hydrangea integrifolia

Hydrangea oerstedii

Hydrangea peruviana

Hydrangea diplastemona

Hydrangea preslii

Hydrangea steyermarkii

Subsection 2: Polysegia

(4 species)

Hydrangea serratifolia

Hydrangea farapotensis

Hydrangea jelskii

Hydrangea mathewsii

In 2015 and 2016, different taxonomic revisions of the genus have continued this tendency towards ‘lumping’ rather than ‘splitting’, with more recent proposed changes based solely on interpretation of molecular phylogenetics in the laboratory. This has radically shuffled the pack of species, coming up with relationships and classifications that now often leave out of account the morphology, habitats and ecological preferences of the plants.

That this laboratory revision work pays little heed to the needs of gardeners is hinted at by Bean’s comment about ‘taxonomic room for manoeuvre’. Its use value is unfortunately rarely questioned. I have argued elsewhere there should be a better balance between perceived botanical accuracy and empirical knowledge in practice in the field, in the effort to achieve long-term clarity and taxonomic security. Morphology – the science of form, analysing the structural physical characteristics of plants – was always the traditional method of establishing plant relationships, and is the basis of the gardener’s practical empirical plant knowledge and perceptions over generations. Botany itself began by seeking to discover close relationships between plants through morphological analysis and comparison, in order to promote a better understanding of their character and processes; but recently, DNA analysis has often been accepted unreflectively as gospel, as a kind of incontrovertible and definitive evidence of evolutionary relationships. This overturns many old accepted links, used and understood by generations of gardeners.

For example, in a 2015 analysis, botanists Samain and de Smet transferred into the genus Hydrangea eight genera within the family Hydrangeaceae that could not be more different or distinct in morphological terms. This radical move means a loss of definition and clarity for the gardener, and a consequent reduction in the use value of the name ‘Hydrangea’. It renders Hydrangea taxonomy a mile wide and an inch deep.

The taxonomic situation is thus confused at present, not to say anarchic, certainly for horticulturists, if not for evolutionary botanists. There is no law that every new laboratory diktat must be followed, and as McClintock is the major reference dealing with the genus as a whole, in this book I shall follow her taxonomy as my base for discussion and amendment; while also adopting the Bean approach of pick ‘n’ mix as best serving the interests of horticulturists keen to strike a balance between botanical accuracy and the need for empirical horticultural clarity.

In this context it is worth saying that this book is essentially for practical horticulturalists, landscapers and designers eager to know more about hydrangeas, and who may perhaps find some stimulus to plant them and get value from them in their gardens. While taxonomic shenanigans may feed the curiosity and interest of the investigative gardener, they are often of little practical horticultural help.

‘New’ Hydrangeas

The following previously designated as separate and well-defined genera within the family Hydrangeaceae, are now ‘sunk’ into the genus Hydrangea by Samain and de Smet following phylogenetic laboratory analysis. They are now hydrangeas, with proposed name changes to reflect this.

BroussaisiaA herbaceous perennial endemic to Hawai’i. Also known as kanawao in the vernacular.CardiandraFive species of herbaceous perennials and subshrubs. Not often seen as they need both shade and warmth to thrive in the garden.DecumariaTwo species of climbers, D. barbara in the USA and D. sinensis in China. Both have white inflorescences that consist only of small fertile flowers. The type is now called Hydrangea barbara.DeinantheTwo species of herbaceous perennials for the woodland: D. bifida from Japan and D. coerulea from China. The name derives from the Greek ‘deinos’ (strange) and ‘anthos’ (flower). Now called Hydrangeadeinanthe.DichroaAbout a dozen species, the best known of which is D. febrifuga, with blue flowers and fruits (now called Hydrangea febrifuga). Few are reliably hardy, but crosses with Hydrangeamacrophylla have yielded hardier lookalikes with blue flowers (Didrangea –seeChapter 13).PileostegiaPhotographed in China in the wild. Four species of this self-clinging evergreen climber, of which P. viburnoides is usually seen in cultivation. Flowers cream/white in terminal panicles. Shade tolerant on walls and into trees. Now called Hydrangea viburnoides.Platycrater argutaA low deciduous shrub to about 1m (3ft) that enjoys a cool position in light shade. Flowers are small and white with yellow stamens in a loose terminal cyme; the ray flowers are small 2cm (1in) flimsy discs of pale green. Now called Hydrangea platyarguta.SchizophragmaA genus of some ten species of self-clinging climbers, two of which are frequently seen in gardens: S. hydrangeoides from Japan, with two named cultivars, ‘Roseum’, with rose pink bracts, and ‘Moonlight’, with silvered, bluish foliage (both have been awarded the RHS AGM and are now called Hydrangea hydrangeoides); the second is S. integrifolium, a climber to 15m (45ft) also with an AGM, with floral bracts some 6–8cm (2–3in) long, forming large multi-flowered inflorescences.

Broussaisia.

Cardiandra.

Decumaria.

Deinanthe.

Dichroa.

Platycrater arguta.

Schizophragma.

Pileostegia.

The above, including the herbaceous perennials, are all now designated as hydrangeas (Samain and de Smet 2015).

Marie-Stephanie Samain defended this radical shift while admitting ‘Such a change may understandably cause consternation among those not familiar with molecular phylogenetic studies…’ (The Plant Review, December 2021).

A further interpretation by Ohba and Hakiyama in 2016 sought to rescue some of these sunk genera with a further revision of this revision, reclassifying yet again many of the sections and species, and reminding us that these molecular methods are themselves not definitive, but subject to rival interpretation among botanists.

PART II THE KEY SUBSECTIONS

CHAPTER 3

AMERICANAE: THE ‘ANNABELLE’ PHENOMENON

Driving south from Raleigh in North Carolina with the late Dr Clifford Parks, a friend and an expert on the local flora there as well as a world authority on the genus Camellia