Parrotia subaequalis (H.T. Chang) R.M. Hao & H.T. Wei

Parrotia subaequalis is a very rare tree: the species-rich mountain forests to which it had naturally become confined now survive only as tiny fragments within the most populous and intensively cultivated part of China. Under Chairman Mao, ‘unproductive’ trees (such as Parrotia, were it to have been encountered) were systematically removed from these forests. The remaining specimens are vulnerable to disease and to insect pests, and seem to be losing out in competition with bamboos. Fourteen wild populations or sub-populations are known, totalling little more than a hundred mature trees; within eight of these populations in 2015 only 10% of the trees were producing any seed (Hammond 2020). For these reasons, the species is habitually claimed (even in scientific papers, e.g. Yan & Zhang 2022) to have been listed as Critically Endangered; its actual absence, as of 2023, from the IUCN Red List (IUCN 2023) says more about the rate at which the threats to plants can be expertly assessed than it does about this particular species’ likelihood of becoming extinct in the wild in the near future.

Despite or because of its great rarity, Parrotia subaequalis attracted little botanical interest through the 20th century. A fruiting specimen was collected in Jiangsu in 1935 (C. Chen 958), but lacked a botanical description until 1960, when Hung T. Chang made the assumption that it represented a new species of Hamamelis. In 1992 M.B. Deng, H.T. Wei and X.Q. Wang studied a flowering specimen and recognised that the absence of petals was a feature distinguishing the tree from that genus. Unfamiliar with the west Asian Parrotia persica, they described a new genus for the plant, Shaniodendron. Six years later a thorough study by R.T. Hao and H.T. Wei finally recognised the very close similarities to P. persica (Andrews 2008). Chang’s specific name subaequalis – ‘not quite equal’ – refers to the sometimes slightly oblique leaf-base – a feature that would have been distinctive within Hamamelis but which P. subaequalis in fact shares with P. persica; it should not be taken to mean the P. subaequalis is ‘not the equal’ of its more familiar western cousin.

This combination of rarity and obscurity meant that Parrotia subaequalis missed an introduction to western gardens during the heyday of Chinese plant exploration through the last two decades of the 20th century. The Harvard botanist Peter Del Tredici saw the species at Nanjing Botanical Garden in 1994, in the form of two very old-looking bonsai plants which had originally been collected in the Yixing Caves Scenic Area of Jiangsu Province; a cutting from one of these has since been established outdoors in the Botanical Garden (Li & Del Tredici 2008). The Japanese plant collector Mikinori Ogisu introduced the species from Jiangsu to Japan in 1998 – the year of its recognition as a Parrotia – and sent material to Roy Lancaster in England in 2001 and to Ozzie Johnson in Atlanta, USA, in 2004 (Andrews 2008; Dirr 2018). In 2000, four young scions were also sent from Nanjing Botanical Garden to the Belgian nurseryman Antoine Bultinck-Meuleman (Colin 2000), and most of the early plantings in Europe derive from these (Andrews 2008). A third introduction from China was made to the Arnold Arboretum in the United States just a few years later (Grimshaw & Bayton 2009).

The species’ threatened status should make Parrotia subaequalis a prime candidate for ex-situ conservation in western gardens, while the tree also shows real promise as an ornamental. The features distinguishing it from P. persica are certainly subtle, but include a generally narrower habit with slower growth, smaller and daintier foliage which turns a more consistent red or purple in autumn, a sometimes more showily peeling adult bark in which grey and silver tints predominate (although this may take many decades to develop), and, in a close-up study of the flowers, the way the sepals are fused to make a shallow saucer-shaped calyx (Hao, Wei & Liu 1996). Although neither species is spectacular in flower, the anthers of P. subaequalis are said to be a more brilliant red; however, these flowers only appear in every second or third spring, heralding ‘mast years’ when an entire population fruits together (Andrews 2008). In common with P. persica, this is an adaptable plant which can grow over limestone; in the United States saplings have withstood at least 32°C of frost with little damage (Avent 2021; Dirr 2018), albeit that the forests to which the species is now naturally confined are much milder than this. It also responds very well to the combined heat and humidity of the south-eastern United States (Stewart 2010). Like P. persica it is also accustomed to plenty of rainfall in the wild and high humidity through the growing season; P. persica is proving itself remarkably tolerant of severe summer droughts in Europe, but this is very seldom true of trees from central and southern China and the Chinese Parrotia’s limits in this respect have perhaps not yet been tested. The suckering and layering habit means that the species roots readily from summer cuttings treated with rooting hormone and placed under mist in a heated propagation unit (Li & Del Tredici 2008), so this need not be an expensive plant to buy.

Parrotia persica in its native forests is a conspicuously diverse species (Douglas & Sjöman 2021). Despite the tiny numbers of surviving specimens, the same could well stand true for P. subaequalis; both species represent the ultimate descendants of a widespread and successful Tertiary lineage. Because of the very small number of introductions to the west, and the tendency of garden examples to have been propagated from cuttings rather than from seed, the genetic diversity of P. subaequalis in cultivation is likely so far to be very limited, militating against the value of these plants for ex-situ conservation (Chris Lane grows five clones in his Witch Hazel Nursery (Kent, UK) for example, but notes that the significant majority of material in European collections is derived from the first introduction by Antoine Bultinck-Meuleman of Pépinière de Cedar – C. Lane pers. comm. 2024). Photographs, for example, show that the tree grown at Nanjing Botanical Gardens has more regularly dentate leaves than seems to be the case within the European or North American stock. However, there is already some variability in the autumn colours shown by the United States’ population of P. subaequalis: this can be bright red, or burgundy, or even royal purple in the case of a tree grown from the Ogisu introduction at Juniper Level Botanic Garden, North Carolina (Avent 2021). Remarkably, the first seedling to be raised by Ozzie Johnson from his tree in Atlanta, Georgia, appears to be evergreen in the local climate – but might instead turn out to be a new intergeneric hybrid (O. Johnson in Avent 2021). At Yew Dell Botanical Gardens in Kentucky, P. subaequalis is one of the first trees to start to colour and one of the last to lose its leaves, meaning that the fall display lasts a full two months (Cappiello 2023).

It is in the southern and eastern United States that the Chinese Parrotia’s promise as an ornamental tree seems most likely to be fulfilled. In northern Europe this remains for now more of a collector’s plant, growing slowly and sometimes disappointing in terms of autumn colour; a scion from the Belgian introduction, planted in 2003 in a shaded corner of Hergest Croft garden in Herefordshire, England – which is seldom a very sunny garden anyway – is particularly disappointing (R. Griffiths pers. comm.). The tree in Roy Lancaster’s own suburban Hampshire garden, grown from the Ogisu introduction, has done a little better and was a 4.5 m bush by 2022 (Tree Register 2023). Parrotia subaequalis was first offered commercially in the UK by Pan-Global Plants in 2011, as scions believed to derive from one of the Bultinck-Meuleman seedlings (Pan-Global Plants 2023; N. Macer pers. comm.).