Cornus alternifolia L.f.

Distinguished from all other dogwood species by their non-opposite leaves, Cornus alternifolia from eastern North America and its East Asian counterpart C. controversa both exhibit a distinctively tiered structure that has earned them both the common names Pagoda Dogwood and Wedding Cake Tree. This tabular branch habit, sometimes emphasised by judicious pruning, is particularly marked in younger trees, which can appear similar enough to perplex the unwary. Of the two, C. alternifolia is always less substantial, with smaller inflorescences, fewer-veined leaves, and a less arborescent habit. Cornus controversa can form a reasonably tall, airy tree, but cultivated C. alternifolia tends towards the shrubbier end of the spectrum, forming at best a small tree (occasionally to 9 m) with a characteristically flat-topped crown and short trunk.

Cornus alternifolia and its East Asian sister species are the only members of an early-diverging lineage of the blue-fruited dogwoods, recognised at subgeneric rank as Mesomora Raf., the two species having branched off from a common ancestor relatively recently (approx. 9 Ma) (Du et al. 2023). Their disjunct distribution is typical of lineages with a wide range during the Paleogene and Neogene (66–2.6 Ma) – fruit stones with the deep apical pits characteristic of the subgenus have been identified in Tertiary fossil beds in Europe dated to the Middle Miocene (c. 16–11.6 Ma) (Eyde 1988). The two species share a derived chromosome number (2n = 20, against the plesiomorphic 2n = 22 in the rest of the blue-fruited species), as well as many aspects of gross morphology and architecture (Clay & Nath 1971).

The species’ anomalous phyllotaxy and pagoda-like habit are closely related. It is worth emphasising that the epithet ‘alternifolia’ applies in a negative sense, reflecting an arrangement of leaves that contrasts with the opposite leaf-pairing characteristic of the rest of the genus, but that does not approach the regular alternation of leaves seen on the shoots of, say, Ilex or Fagus. It would perhaps be more accurate to say that the leaves of C. alternifolia are disposed in a subopposite pattern, often condensed into an irregular pseudo-whorl by a series of shortened internodes at branch tips. Applied to the main stem, this growth pattern – involving an extended first internode (hypopodium) that rockets up vertically before producing a cluster of buds that will produce horizontal (plagiotropic) shoots – is the basis of the species’ characteristic tiered architecture in which horizontal layers of foliage are separated by increments of clear stem. In vigorously growing plants, the secondary shoots sometimes extend during the season contemporaneously with the main axis without forming resting buds, a phenomenon known as ‘syllepsis’, common in tropical trees but rare as a constant feature of temperate woody species (Hallé, Oldeman & Tomlinson 1978). The resulting pagoda-like habit (Fagerlind’s model of tree growth) is seen in only a handful of non-tropical species. The two dogwoods differ slightly from the tropical Ur-model in that the main trunk is not a strict monopodium, the vertical axis being continued by a replacement bud if the terminal bud dies – Hallé, Oldeman and Tomlinson describe this as an ‘unstable monopodium’.

According to Aiton (1789), Cornus alternifolia was brought into cultivation by James Gordon of Mile End Nursery in 1760; its omission in the last edition of the Gardeners Dictionary (Miller 1768) is therefore rather surprising – perhaps the young trees may not yet have begun to exhibit their interesting architecture. Lamarck (1786) reported that the species (‘from Siberia’) had been cultivated in France at the Jardin du Roi for more than fifteen years – his geographical solecism stems from confusion with the alternate-leaved Rhamnus davurica (syn. Cornus citrifolia Weston), mistakenly placed in synonymy in the 1776 supplement to Diderot and d’Alembert’s dictionary (Robinet et al. 1776; see also Hess & Stearn 1979).

As might be expected of a species of woodland understorey and canopy gaps, Cornus alternifolia grows well in both sunny and woodland conditions. It does best in slightly acidic, reliably moist soils, and thrive in areas with cool summer temperatures (Cappiello & Shadow 2005). By contrast, in regions with hotter summers, afternoon shade may be necessary to ensure success; this is especially true of the commonly grown variegated or yellow-leaved cultivars.

Grown as untrained shrubs, plants require little maintenance. If a small tree is required then intervention will be required to select a leader early on (Brown & Kirkham 2004), or to progressively remove side growth on established plants to reveal a clean trunk. The latter method, which results in a stronger plant, may be found advantageous, since young trees can be slow to make headway at first. Mature root-balled specimens have been found to establish very successfully (Junker Nursery 2022).

Good seed germination can be achieved following three to five months of cold stratification, and a reasonable proportion (up to 60%) of softwood cuttings root if kept under fog and treated with a rooting hormone (6000 ppm K-IBA is recommended). Cultivars are typically grafted, seedling Cornus alternifolia or C. controversa being the most compatible rootstocks; cutting-raised C. alba are also widely used as understocks, but are more prone to suckering (Cappiello & Shadow 2005; Humphrey 2019). Spontaneous hybridisation with Cornus controversa has been documented where the two species are grown together in cultivation (Gawrońska et al. 2019).

The principal drawback of Cornus alternifolia is Golden Canker caused by Aurantioporthe corni (Wehm.) G.L. Beier & Blanchette (syn. Cryptodiaporthe corni). This ascomycete fungus coexists as an (often asymptomatic) endophyte of the species, manifesting as a pathogen only in certain circumstances (Beier et al. 2015). Aurantioporthe corni is present throughout the natural range of the host species, as well as in landscape plantings elsewhere in North America, but has not been identified as a pathogen outside the United States, and is not known to affect other species in the genus. The initial symptom of infection is the characterististic bright yellow or orange appearance of the (normally dark red) branchlets, and a raised appearance in the lenticels, followed by dieback beginning at branch tips before the canker advances into the main stem which is often killed back. Distinctive small orange fruiting bodies may be seen on the cankered wood.The infection is spread by spores, as well as by vegetative propagation from asymptomatic stock (Redlin & Rossman 1991; Beier et al. 2015). The fungus appears to become pathogenic only in plants that are low in vigour, so maintaining good general plant health and nutrition is the principal recommendation for minimising disease (Redlin & Rossman 1991).