Kindly sponsored by a member of the International Dendrology Society.
Tom Christian & Peter Coles (2022)
Christian, T. & Coles, P. (2022), 'Morus' from the website Trees and Shrubs Online (treesandshrubsonline.
Deciduous trees and shrubs to c. 20 m, dioecious, occasionally monoecious, sap milky. Bole long or short (often short and low-branched in cultivation) deeply fissured. Crown dense, wide-spreading, lowest branches disposed to layering. Winter buds with 3–6 imbricate scales. Stipules free, sublateral, caducous. Leaves alternate, simple, entire to deeply palmately lobed; lobing frequent on young plants and vigorous shoots, the nature and degree of lobing varying even on the same shoot (leaves on older wood are more uniform); margin toothed; primary veins 3–5 from base, secondary veins pinnate. Male inflorescences axillary, spicate, many-flowered, shortly pedunculate; male flowers 4-merous (occasionally 5-merous in M. alba). Female inflorescences shortly spicate to capitate; female flowers sessile or sub-sessile, 4-merous, sepals of two sizes, style present or not, stigma 2-branched, ovary with 1–2 locules, superior. Fruit with enlarged, succulent calyx usually aggregated into a juicy syncarp which is edible in commonly cultivated taxa. Seed ± globose. (Wu, Zhou & Gilbert 2003; Cullen et al. 2011).
‘How many mulberries are there?’
‘How long is a piece of silk?’
When Linnaeus established the genus Morus in 1753 he included seven species. Only five of these are still considered to be true mulberries, the other two have been shown to belong to two closely related genera: the Paper Mulberry Broussonetia papyrifera, and Dyer’s Mulberry Maclura tinctoria. Since this time over 200 further names have been published in Morus (IPNI 2022) – a symptom of their long history of cultivation, cultural and economic significance, and the morphological plasticity that is well known in this family. Although a majority of these 200-plus names have long been consigned to synonomy, there is still no consensus as to just how many of the residual names merit recognition as species. The Kew resource Plants of the World Online suggests 18 (Plants of the World Online 2022); Flora of China says ‘about 16’ (Wu, Zhou & Gilbert 2003); and two recent studies have suggested there may be as few as 13 (Razdan & Dennis Thomas 2021) or even just 8 (Zeng et al. 2015).
In a genus (and family) plagued by taxonomic turmoil, and with acute difficulties regarding identification, there is a temptation to adopt the most simplistic solution, but it is perhaps telling that after nearly seven years Zeng et al.’s (2015) conclusions have not been widely adopted, while Razdan & Dennis Thomas’s (2021) views have not been around long enough to be adequately tested. Indeed, against a backdrop of so many names, and uncertainties regarding the truly natural distributions of several taxa, testing taxonomic hypotheses is a particularly difficult enterprise in this genus.
For the purposes of this work, attempting to mix-and-match taxonomies from multiple sources would very quickly result in total chaos, so we have opted to follow Plants of the World Online (2022) without deviation, not only with regard to taxonomy and nomenclature, but distributions too, even where these contradict local floras. This is an imperfect compromise; this treatment probably recognises more names than there are entities, but it has the benefits of being free to access, so others can examine the same source, and it considers the whole genus globally. Nevertheless, a further challenge in compiling the descriptions has been the extent to which different accounts appear to contradict one another with regard to certain, often key characters; in these instances we have followed the majority opinion.
Fortunately, those species that fall under the scope of Trees and Shrubs Online are, for the most part, taxonomically stable. This account will cover the following species: Morus alba L., M. cathayana Hemsl., M. celtidifolia Kunth., M. indica L., M. macroura Miq., M. microphylla Buckley, M. mongolica (Bureau) C.K.Schneid, M. nigra L., M. notabilis C.K.Schneid, and M. rubra L. It is possible that one or two more contentious species are grown on the very fringes of our area in Asia, probably only on a very local scale, so we excuse ourselves of these vexatious cases.
Other species recognised by Plants of the World Online (2022) but not included in our coverage here include: M. boninensis Koidz., M. insignis Bureau, M. koordersiana J.-F. Leroy, M. liboensis S.S. Chang, M. miyabeana Hotta, M. serrata Roxb., M. trilobata (S.S. Chang) Z.Y. Cao, and M. wittiorum Hand.-Mazz. The widespread African Morus mesozygia Stapf, not hardy in our area, is now treated as the single species in the distinct genus Afromorus, A. mesozygia (Stapf) E.M. Gardner.
Of all the mulberries, only M. alba, M. nigra and M. rubra are at all widely grown; M. cathayana, M. indica, M. macroura and M. mongolica are all gaining ground in western collections, albeit slowly, while hybrids of M. alba × rubra are increasingly grown for their fruit. M. alba remains by far the most important species on account of its value to the silk industry.
Sericulture – the production of silk – is an art that has been practiced for many thousands of years and one that still depends on the extensive cultivation of M. alba, which has consequently been introduced to all the tree-growing continents. For two and half millennia the people of ancient northern China kept sericulture a closely guarded secret, but knowledge of the process was gradually, inevitably exported. From the first half of the first Millennium CE sericulture’s migration, from its birthplace in ancient China, along the silk roads to the west, over the Himalayas to the south, and across the seas, has carried with it an incalculable number of fortunes made and lost, reputations built and demolished, communities enriched and ruined, and everywhere are its two indivisible keystones, the mulberry tree and the silkworm. The ‘worm’ is actually the larva of a moth, Bombyx mori, and while this moth, which is entirely dependent on its human partner, has long since disappeared from former sericultural centres, ancient, gnarled mulberry trees bear witness to history across several continents. This history is so rich that entire books have been dedicated to it. Peter Coles, the author of one recent publication on the subject, is a co-author of this work; the editors of Trees and Shrubs Online commend his excellent Mulberry (Coles 2019) to all those wishing to delve more deeply into this enigmatic topic than our dendrological focus permits us to here. Nevertheless, this is far from the final mention of sericulture in these articles, for it and the mulberries are so entwined that it is impossible to tell the story of one without discussing the other.
Throughout history, most mulberry introductions have been with sericulture in mind; any horticultural merit was usually secondary. Nevertheless, the distinctive, juicy fruits have long been extolled for their many benefits (this is probably why the Romans brought M. nigra to Britain) but the fruits of several species neither store nor travel, and therefore have never been of value except to those fortunate enough to live within walking distance of trees. Mulberries rarely take on a form that lends their timber great usefulness, except on a very local scale, and whilst in the Far East the timber of old-growth trees has fetched extravagant prices in the past, the few such trees that remain are usually in protected areas or are otherwise venerated (Coles 2019). For example, a massive specimen of Morus serrata (with a reported span of 25 m) may be found in the town of Josimath, Uttarakhand, India. Local folklore links the tree with the 8th century guru, Adi Shankaracharya, claiming it to be 1,200 years old. The tree is a popular destination for pilgrims and tourists, but ironically, a concrete viewing platform and fence erected to protect it may increase root compaction and do the opposite. The tree is one of several (not all Morus) throughout India known as kalpavriksha (कल्पवृक्ष) or ‘Tree of Plenty’ (Coles 2019).
Only two species – the Black (M. nigra) and White (M. alba) Mulberries – are at all common in western gardens, and whilst they make attractive and interesting subjects, their beauty is of a reserved, classical nature, not at all attention seeking. Perhaps it is for these very reasons that the mulberries have come to enjoy an esteemed position in gardens – writing of M. nigra, Bean captured perfectly their allure: ‘It is not much planted now, but nothing gives to a garden fortunate enough to possess it a greater sense of old-world charm and dignity than a rugged old mulberry standing on a lawn’ (Bean 1981). Despite its name, White Mulberry does not, necessarily, bear white fruits; in fact, it much more commonly bears pink, red or purple fruits. The easiest way to tell the Black and White Mulberries apart is by the foliage: the upper leaf surface of Black Mulberry is usually matt and always scabrous, with a dense covering of short hairs that lend the blade a rough, sandpaper-like texture; in White Mulberry, the upper leaf surface is glossy and smooth to the touch (Bean 1981). The tendency of the Black Mulberry to develop burrs and to lean heavily to one side make it look old even when it is still relatively young. This, along with the copious shade from its low, spreading branches, and its fruit, has long earned it a place in gardens and parks. White Mulberry, on the other hand, tends to grow taller and straighter, and has less of the picturesque charm of the Black, although it is often a veteran White Mulberry pollard that graces the gardens of farmhouses in Italy and France, as well as former sericulture sites in the southern states of the USA. In these cases, M. alba varieties that produce large black fruit earn it similar praise to that usually heaped on the Black Mulberry.
Bean would take comfort from the fact that during the past few years mulberries have enjoyed something of a resurgence, at least in Britain, as efforts have been made to locate and catalogue venerable specimens, an activity which never fails to inspire a wave of new planting. An award-winning project, Morus Londinium, has spearheaded these efforts, while a national collection has been established in the UK at Buckingham Palace by head gardener Mark Lane. An account of some of the varieties cultivated in the Royal gardens is given in a sumptuous and exclusive publication, The Queen’s Mulberries (Andrews, Feltwell & Lane 2012). Limited to 100 copies, this book provides botanical and horticultural accounts of several mulberry species and varieties, each accompanied by specially commissioned botanical illustrations by Alysia Hunt. The authors of this account of the genus for Trees and Shrubs Online are grateful to Mark Lane for providing access to a copy of this book for study, as well as access to the national collection itself and for his useful horticultural insights.
Much of Bean’s horticultural guidance remains valid, that mulberries grow best in a warm, well-drained, loamy soil, and some shelter is beneficial. Propagation by summer cuttings is straightforward and is the recommended method for M. nigra, while some species, notably M. alba, are best propagated in the autumn. So-called truncheons, lengths of branches up to c. 2 m long, are also said to root when treated as gigantic cuttings (Bean 1981) but several of the more characterful cultivars of M. alba must be grafted. Otherwise, the mulberries are generally noted for their resilience and seem well adapted to coping with difficult soils, seasonal droughts, as well as occasional innundation, at least when grown in areas with reliably warm summers (USDA 2022). Known pathogens associated with Morus species include Xylella fastidiosa (Delbianco et al. 2022), leaf spot diseases caused by species of Cercospora and Mycosphaerella, and a bacterial blight Pseudomonas syringae which can cause extensive unsightly dieback during the growing season and generally weaken plant health (USDA 2022). Pseudomonas in particular appears to be exacerbated by warm, humid conditions, and bad outbreaks may be associated with periods of high summer rainfall (moruslondinium.org); control includes practicing general good hygeine, ensuring good air movement around plants, and clearing up and burning infected leaves and any dead shoots which are removed.
Morus differs from its nearest relatives in the Moraceae by the juicy, succulent fruit produced from female flowers with four distinct sepals (cf. hairy fruit with tubular calyx in Broussonetia) and in its branches lacking spines (cf. branches with spines in Cudrania and Maclura) (Cullen et al. 2011).
The key below is based on the descriptions in the species entires and on observations of living plants in the UK. It must be stressed that it covers only those species treated here, not the entire genus, and that in attempting to identify living material vigorous extension growth should be ignored in favour of mature leaves on older growth.
|1a||Upper leaf surface prominently rough to the touch||2|
|1b||Upper leaf surface smooth, or slightly rough to the touch||4|
|2a||Branchlets densely pubescent even after 1 year||M. nigra|
|2b||Branchlets glabrous, or pubescent in their first year only||3|
|3a||Lower surface of mature leaves pubescent||M. rubra|
|3b||Lower surface of mature leaves pubescent along veins only||M. celtidifolia; M. microphylla|
|4a||Upper leaf surface more or less glabrous; smooth to the touch||5|
|4b||Upper leaf surface slightly rough to the touch||8 [but NB M. rubra may occasionally key out here]|
|5a||Leaf margin with teeth culminating in a distinct bristle||M. mongolica|
|5b||Leaf margin with teeth not bristle tipped||6|
|6a||Female flowers with prominent styles, clearly protruding||M. indica|
|6b||Female flowers with styles reduced, appearing absent||7|
|7a||Syncarp (fruit) typically 1–3 cm long at maturity||M. alba|
|7b||Syncarp (fruit) typically 5–13 cm long at maturity||M. macroura|
|8a||Lower surface of mature leaves densely pubescent; petiole pubescent||M. cathayana|
|8b||Lower surface of mature leaves glabrous, or pubescent along veins only; petiole glabrous||M. notabilis|