Kindly sponsored by
Monique Gudgeon, Sculpture by the Lakes
Owen Johnson (2022)
Johnson, O. (2022), 'Forsythia' from the website Trees and Shrubs Online (treesandshrubsonline.
About 13 species of deciduous shrub (one a sterile wild hybrid of apparently prehistoric origin), erect, drooping or rarely prostrate, stoloniferous. Twigs lamellate (with chambered pith), or hollow but with solid pith at the nodes, often 4-angled and with raised lenticels. Buds with many rather loose scales. Leaves in opposite pairs, simple but rarely deeply 3-lobed to trifoliate, ovate to broad-lanceolate, serrate or distantly toothed or entire. Flowers opening before the leaves, bisexual, heterostylous, a proportion of individual plants with the stigma presented at the mouth of the tube, others with the stamens; flowers single or in clusters of 2–6 in each leaf axil, shortly-stalked. Calyx green, deeply 4-lobed, persistent. Corolla yellow, with a bell-shaped tube and four lobes longer than the tube, imbricate in bud. Stamens 2; style slender, stigma cleft into two parts. Fruit a capsule containing many slightly winged seeds lacking an endosperm. (Chang, Qiu & Green 1996; Bean 1981).
History, Taxonomy and Systematics
Within the Olive Family (Oleaceae) as currently understood, the Tribe Forsythieae H. Taylor ex L. Johnson includes just one other genus, Abeliophyllum (Ha et al. 2018). Abeliophyllum is monotypic: A. distichum, White Forsythia, is listed as Endangered and known wild only from seven sites in Korea (IUCN 2020), but remains commercially available in the west. Forsythia suspensa and F. × intermedia ‘Courtaneur’ have recently been experimentally pollinated by Abeliophyllum in Beijing (Shen et al. 2017), but the offspring seem unlikely to make horticulturally important plants.
The name Forsythia commemorates the Scottish botanist William Forsyth (1737–1804), who was Superintendent of the Royal Gardens at Kensington (Bean 1981); the description was published in the year of Forsyth’s death by Martin Vahl, Professor of Botany at Copenhagen University (DeWolf & Hebb 1971). The species Vahl knew was F. suspensa (Thunb.) Vahl, which has a wide distribution in central China and has long been cultivated in Japan; Carl Pehr Thunberg had studied it in Japan in 1775–6, describing it as Syringa suspensa in his Flora Japonica in 1784. Plants were in Europe by 1817, when P.J. van Melle catalogued it in the Leipzig garden of Christian August Breiter (DeWolf & Hebb 1971).
For years F. suspensa was the sole representative of the genus in western gardens, until a second Chinese species, F. viridissima Lindl., was introduced to England by Robert Fortune in 1844–5 (Bean 1981). In a development closely resembling that of the genus Abelia, the great majority of ornamental Forsythia clones in western gardens have come to derive from the hybrid between these two Chinese kinds, F. × intermedia Zab. This important hybrid was first experimentally bred by Thomas Meehan at Germanstown in Philadelphia in the 1860s (Bean 1981; Wikipedia 2021). Resultant clones tended to display hybrid vigour and to be more floriferous and adaptable than either of their parents, and indeed than any of the other Asian taxa that were subsequently introduced.
Since the wild Forsythia tend to resemble one another so closely, there has been little impetus to maintain these various taxa as ornamental plants; all except two of the thirteen described species are known to have been introduced to western gardens, but several of these now cling to existence as cultivated plants. Even in the wild, more than half the species have tiny, relic populations, with the four Korean endemics – F. koreana (Rehd.) Nakai, F. nakaii (Uyeki) T.B. Lee, F. ovata Nakai and F. saxatilis (Nakai) Nakai – seeming particularly scarce. Of these only the best-known and most studied, F. ovata, has so far been assessed (as Endangered) by the IUCN (IUCN 2020).
This otherwise Asian genus has one surviving European representative: F. europaea Deg. & Bald. was not discovered, in its mountain refuges in the Balkans, until 1897 (Bean 1981), and has never found a familiar place in gardens, though its extra hardiness has encouraged its use in breeding programmes in Poland, Canada and the north-eastern United States.
Recent phylogenetic analyses have reinforced the assumption that Forsythia is a close-knit group, originating in eastern China during the Miocene epoch around 20 million years ago and proliferating across Eurasia, whose microclimates were then much more widely suitable for temperate shrubs and trees. The first genetic study of the genus to be published (Kim 1999) interpreted its findings in a few counter-intuitive ways, suggesting that most F. × intermedia clones might not have arisen through hybridisation between F. suspensa and F. viridissima, and that F. europaea might have separated from its Chinese allies and migrated across Ice Age Eurasia within the last half-million years. These assumptions have since been refuted by more detailed analyses, nevertheless they continue to find their way into some treatments of the genus, including that in Wikipedia 2021. The most recent and thorough study (Ha et al. 2018) sampled the genes of ten of the wild species.
Like most members of the Olive family, forsythias bear perfect flowers, each containing both male and female parts, but they are one of several insect-pollinated plant genera which have developed heterostyly as a means of avoiding self-pollination. In about half the individuals of all forsythia species, the style is longer than the stamen filaments and the stigma is exposed, for pollination, at the narrow mouth of the corolla tube. The other half have styles shorter than the stamen filaments and it is the anthers that are exposed at the flower-mouth. As a neat example of convergent evolution, this dimorphic anatomy is very similar to that of the European Primrose Primula vulgaris, in which the long-styled flowers are traditionally called ‘pin-eyed’ and the short-styled ones ‘thrum-eyed’; the same terminology has been adopted for Forsythia. (In the Primrose the two morphologies are easier to spot, since a forsythia’s cleft stigmas can easily be mistaken for the paired stamens.) In forsythias, additionally, inhibitors exist that prevent germination by pollen from the wrong type of plant (pin on pin, thrum on thrum); rather neatly, each type further possesses enzymes that break down the inhibitors of the opposite type, thus ensuring pollination by the right type whilst making it is almost impossible for a ‘pin-eyed’ forsythia to produce seed with another ‘pin-eyed’ individual, and vice versa (Moweus 1950); as the great majority of garden forsythias are single clones, they cannot pollinate other examples of the same clone and tend to produce sterile seed-capsules. (Forsythias are occasionally misrepresented as invasive in cultivation, but this refers only to the way in which individual plants are able to persist and spread, very slowly, by layering their branches or suckering. Knowing whether a particular cultivar is supposed to be ‘pin-eyed’ or ‘thrum-eyed’ would help in determining whether stock is genuine or whether more than one clone is now cultivated under the same name, as does indeed seem to be the case with the popular F. × intermedia variants grown as ‘Beatrix Farrand’, but this information is not often recorded, and photographs of forsythia flowers are seldom detailed enough to show it).
During his experiments with forsythia breeding in Poland in the 1950s, Bolesław Suszka failed, as expected, to cross pin-eyed plants with other pin-eyed plants of the same species, but found that the physical and chemical inhibitory mechanisms broke down between species: he reported a 67% average success rate in crossing pin-eyed plants of one species with thrum-eyed plants of another (and vice-versa), but a 13% success in crossing like with like across the species barrier (Suszka 1959). Others have observed slight but consistent differences between the foliage of pin- and thrum-eyed plants within two forsythia species (F. koreana and F. saxatilis), the leaves of the pin-eyed plants being on average 2–5% longer and wider (Lee, Kim & Hong 1982).
Forsythias are often among the first garden flowers which people learn and remember from childhood; their brilliant yellow blossom on bare branches in early spring sets apart all the members of this genus from any other hardy shrubs. Nevertheless, it is probably tacitly assumed by many gardeners that there is only one kind of Forsythia (something partly true in suburban gardens across large parts of our area, and nowhere more so than in the UK, where the clone F. × intermedia ‘Lynwood Variety’ tends to have been planted to the near-exclusion of other sorts). The wild species differ only subtly in flower colour and timing and in foliage features – differences best observed when they are grown together in a single bed – while the history of forsythia breeding can largely be summed up as an as-yet-fruitless search for the one perfect version.
Many Forsythia clones have in fact been offered with variegated foliage or yellow summer leaves, but none has yet become particularly popular, partly because the earlier selections at least gained a reputation for instability, and partly because most gardeners probably feel that a forsythia’s job, when out of flower, is to remain as inconspicuous as possible. Among forsythias with interesting foliage, the sometimes deeply three-lobed or trifoliate leaves of F. suspensa could be cited, along with the peculiarly fresh green of F. × intermedia ‘Courtasol’ in summer and the deeply toothed, almost lobed leaves of F. ‘Tremonia’.
There are in fact numerous forsythias which tick the varied boxes required of a popular garden plant. Nearly all are quite easy to raise by cuttings from half-ripened wood (Bean 1981), easy to transplant, and tolerant of most conditions, though in dense shade and on very poor soils they will be shy to flower. Most dislike waterlogging, although one species, F. giraldiana Lingelsh., grows in flood-plains in its native central China (Chang, Qiu & Green 1996); as yet, it has contributed very few of its genes to garden hybrids. Forsythias also tend to cope well with urban pollution (Dirr 2009), and suffer remarkably little from pests and diseases in general. Several of forsythia’s relatives in the Oleaceae are susceptible to Ash Dieback disease (Hymenoscyphus fraxineus) including Fraxinus itself, but also Chionanthus and Phillyrea, but while garden forsythias have been tested for the disease they are not known to be susceptible (Forest Research 2018). Of less serious concern is Forsythia Gall, caused by the fungus Phomopsis, which produces jagged swellings along the twigs which are unsightly rather than lethal, and which can contribute to the generally rough, unkempt appearance of the plant in winter; the fungus can be controlled by cutting out and burning affected wood.
This shaggy appearance out of leaf is among the genus’ few demerits; studded with big, jagged buds and with raised lenticels like the bolts on a rusty iron girder bridge, they tend to form a criss-cross tangle, often with a few strong shoots jutting above the general canopy. Recent breeding programmes, especially in France, have concentrated on the production of lower and neater plants which will fit in today’s small gardens. Forsythias can be pruned to improve their habits, with the removal of a few older stems each year sometimes being recommended. But as they flower in spring from old wood – sometimes first-year growths, but more reliably on second-year and older stems – there is a rather narrow window for pruning, immediately after flowering, if next year’s crop of flowers is not to be sacrificed. For the same reason, forsythias are less than ideal as hedging plants, though their ease of production does sometimes lead garden centres to recommend them for this purpose. That most Forsythia flowers are almost scentless is perhaps another demerit – some people notice a faint waxy or plastic smell (Fragrant Earth 2021) – but one that is usually mitigated by the superb display. One exception is the mild, sweet aroma of the primrose-yellow flowers of F. giraldiana (Royal Botanic Garden Edinburgh 2021).
The flower-buds, conspicuous through winter in their opposite pairs, are particularly palatable to the Eurasian Bullfinch (Pyrrhula pyrrhula), though this colourful little bird has now grown too scarce to have much impact on the suburban forsythia display. One sentence in Bean’s Trees and Shrubs Hardy in the British Isles – which Trees and Shrubs Online aims in large part to supersede – stands to show how much we have lost, certainly in terms of native birdlife and possibly in the skill to shape simple prose with grace and economy: ‘The finest display of flowers comes from plants that have been sited where they can grow freely yet are not so remote from the comings and goings of daily life that the birds can destroy their flower-buds undisturbed’ (Bean 1981). In the later 20th century, it was recommended to ‘companion-plant’ forsythias with fruit trees, so that bullfinches would be distracted from eating the fruit buds (T. Johnson pers. comm.) though it has to be suggested that the consequences may simply have been to attract more birds into the neighbourhood.
All the forsythias are hardy plants, though hard frosts will kill flower buds before they kill the shoot itself, and the commonest clones reach their limits as ornaments when about 20–25°C of frost can be expected. F. europaea and F. ovata were recognised in the 20th century as slightly hardier than this, and much of the breeding work in North America and central Europe was concerned with combining these species’ genes with the flower power of F. × intermedia. The resultant hybrids can blossom reliably in climates as cold as USDA hardiness zone 3. In the much milder current conditions in southern England – equivalent to zone 9 – forsythias flourish just as well, though the common F. × intermedia clones may hold onto their green leaves until the New Year. (In colder, continental conditions, their autumn colour can be a good mix of yellows and purples). Monique Gudgeon, who has built a UK National Collection of Forsythias at her Dorset garden, Sculpture by the Lakes, and who has very kindly sponsored this treatment of the genus, even reports one young plant of F. × intermedia ‘Spring Glory’ in full flower in mid-December, 2021 (pers. comm.). (There is a single forsythia, F. ‘Klein’, which was distributed in the United States for its tendency to flower again there in autumn, but this display is often sparing). In climates any milder than England’s, numerous other shrubs such as certain species of Acacia, Azara and Sophora can be relied upon to combine an equally brilliant burst of early-spring sunshine with more enticing foliage or habits, but forsythias are so easy to cultivate that they are sometimes offered for sale even in Australia; the variegated F × intermedia ‘Fiesta’ originated as a sport of ‘Lynwood Variety’ at the Duncan and Davies nurseries in New Zealand.
Following Thomas Meehan’s efforts which gave rise to F. × intermedia in the 1860s, forsythia breeding got underway with earnest at the end of the 19th century, in Germany, when Späth of Berlin began to distribute several selections of F. × intermedia; of these, ‘Spectabilis’ was the showiest and quickly became the genus’ gold standard. In the 1930s Karl Sax and Haig Derman at the Arnold Arboretum, Massachusetts, experimented with the newly discovered method of inducing tetraploidy through the use of colchicine toxins; they went on to release a series of mutants of F. × intermedia, of which the plants now grown as ‘Beatrix Ferrand’ remain the most popular; Sax and Derman also attempted to breed hardier clones by crossing F. europaea with F. ovata. The Arboretum, which in the years from 1908 received the first seeds of several Forsythia species collected by its future Keeper, Ernest Wilson, maintains perhaps the world’s most extensive collection of the genus, and plans to make this nearly comprehensive (Arnold Arboretum 2018).
At the Warsaw University of Life Sciences (SGGW) in Poland in the 1950s, Bolesław Suszka successfully crossed all of the Forsythia species available to him (F. europaea, F. giraldiana, F. japonica Mak., F. ovata, F. suspensa and F. viridissima, along with a range of F. × intermedia clones) in the hope of combining the floral display of the latter with the hardiness of F. europaea and F. ovata, the only two species which would grow reliably across Poland at this time (Suszka 1959). Although documentary evidence tends to be lacking, most of today’s forsythias which are of Polish origin, such as ‘Fontanna’, ‘Kanarek’ and ‘Maluch’, seem likely to descend from Suszka’s work. Two new interspecific hybrids, F. × kobendzae and F. × variabilis, were described from Poland in 1960 by Włodzimierz Seneta.
In 1972 Luc Decourtye began a breeding programme at the Angers station of INRA (the Institut National de la Recherche Agronomique in France), with the primary aim of producing forsythias that were smaller and tidier than two of the most popular cultivars then available in Europe, the F. × intermedia clones ‘Lynwood Variety’ and ‘Spring Glory’ (Cadic 1988). Decourtye irradiated seedlings from these clones with gamma rays from Cobalt 60, and a secondary but unsuccessful aim was the hope of producing sports with red flowers as well as yellow (Rosati 2000). By 1988, 49 new varieties were growing at Angers (Cadic 1988); the named cultivars nearly all begin ‘Court…’, punning on the breeder’s name and the French word for ‘short’ (in this case, low and compact). Decourtye’s forsythias include the only two clones – apart from ‘Lynwood Variety’ itself – to hold the Royal Horticultural Society’s Award of Garden Merit, ‘Courtalyn’ and ‘Courtasol’. As the irradiated seedlings were bred by Decortye by open pollination, this group is often treated simply as a series of cultivars of the genus Forsythia, but, because all the potential parents seem to have been clones of F. × intermedia, it seems reasonable to list them as cultivars of that hybrid, as in this account.
In the orient, traditional forsythia breeding centred on the selection of foliage sports of F. koreana. Several have now been introduced to the west, via the United States, foremost among them probably being the showily reticulated ‘Kumson’.
The parentage of many other clones is unknown or unrecorded, making it difficult to decide what form their names should take. In this account, the cultivars are placed under the likeliest species (including the named garden hybrids, F. × intermedia, F. × kobendzae and F. × variabilis) but are treated simply as cultivars of Forsythia when information is lacking. The name ‘F. × interovata’ is sometimes used for complex crosses between F. × intermedia and F. ovata, but lacks a valid description.
Two Forsythia species may not yet have been introduced to cultivation in the west, and consequently receive no detailed treatment in this account. F. mira M.C. Chang was described in 1987 from roadsides and mountain slopes in the Shangang Xian, Shaanxi, China (Chang, Qiu & Green 1996), and is closest to F. suspensa (Gao et al. 2019). F. mira does appear on the species list of the Charles University Botanical Garden in Prague hosted by florius.cz (Florius 2021), but staff there have not confirmed its presence here and the list may be one of potential taxa rather than of taxa which are actually grown. F. togashii H. Hara was named in 1973 from the island of Shōdoshima in southern Japan and is closest to F. japonica. A presumed natural hybrid of F. koreana and F. saxatilis has also been reported in South Korea (Lee, Kim & Hong 1982), but has not been named.
The middle decades of the 20th century probably saw the height of Forsythia’s garden popularity. Since then, the very ease with which they can be grown and the year-on-year predictability of their floral display may have worked against them, leaving them as plants to be taken for granted rather than discussed. But they are long-lived shrubs and for a few weeks in spring, all across suburbia, they continue to leap from obscurity and remind us how justifiably abundant they remain. Breeders will, no doubt, continue in their quest to find a forsythia that stands above all others, and if some of the more obscure species are used in breeding programmes we may see new adaptations, but the general aesthetic seems unlikely to experience any dramatic change. Monique Gudgeon, who has generously sponsored the production of this text, is developing a Plant Heritage-recognised National Plant Collection at Sculpture by the Lakes, near Dorchester, Dorset, which will increasingly serve as an important reference for this neglected but important genus. Many of the images illustrating entries have been taken there.
Distinguishing even the different Forsythia species by visual clues alone can sometimes be all but impossible, and the key below intends to suggest differentiating features which are sometimes useful rather than to supply absolute guidance. A wild plant’s provenance will generally offer the most reliable clues to its identity, but it should be kept in mind that the overwhelming majority of cultivated plants are hybrids.
|1a||Leaves simple, ovate to lanceolate; pith regularly chambered||2|
|1b||Leaves sometimes 3-lobed to trifoliate; shoots hollow except at the nodes; habit sometimes trailing||Forsythia suspensa (and its hybrids, F. × intermedia, F. × kobendzae, F. × variabilis, with variously intermediate features)|
|2a||Twigs with appressed hairs into their second year||Forsythia nakaii|
|2b||Twigs soon glabrous||3|
|3a||Twigs greenish into their second year||Forsythia viridissima|
|3b||Twigs soon brown, purple or yellowish||4|
|4a||Mature leaves glabrous||5|
|4b||Mature leaves with some pubescence at least under the veins||7|
|5a||Leaf broad (ovate to rounded), variably serrated||Forsythia ovata|
|5b||Leaf rather narrower (broadly lanceolate to ovate), with a few teeth or entire||6|
|6a||Flowers bright yellow, not scented||Forsythia europaea, Forsythia koreana|
|6b||Chinese species with narrow leaves and flowers often soft yellow and slightly scented||Forsythia likiangensis, F. giraldiana|
|7a||Leaf broad (ovate), variably serrated||8|
|7b||Leaf ovate to broadly lanceolate, entire or with a few teeth||9|
|8a||Chinese species; a sterile triploid with an erect habit||Forsythia × mandshurica|
|8b||Korean species, sometimes prostrate in the wild||Forsythia saxatilis|
|9a||Japanese species; leaves ovate; flowers bright yellow, not scented||Forsythia japonica|
|9b||Chinese species; leaves lanceolate; flowers soft yellow, slightly scented||Forsythia giraldiana|