Kindly sponsored by a member of the International Dendrology Society.
Julian Sutton (2022)
Sutton, J. (2022), 'Plagianthus' from the website Trees and Shrubs Online (treesandshrubsonline.
A genus of 2 deciduous to semi-evergreen species endemic to New Zealand, one a divaricating shrub, the other a normally heteroblastic small to medium tree. Leaves opposite, simple, petiolate, stipules deciduous. Flowers small, unisexual or hermaphrodite, in axillary or terminal cymes, or solitary; bracts if present small, distant from calyx; calyx 5-toothed; petals 5, small; staminal column divided at apex into many stamens; ovary with 1–5-locules, each with 1 ovule; styles 1–5, stigmatic on inner side. Fruit of 1–5 carpels, indehiscent or dehiscing irregularly. (Allan 1961; Cullen et al. 2011).
As currently understood, the genus Plagianthus is restricted to New Zealand. Of the two accepted species, P. divaricatus is a shrub widespread in coastal and estuarine habitats, with flowers solitary or in few-flowered inflorescences. The other, P. regius, is a forest tree, New Zealand’s tallest deciduous species, characterstic of moist lowland sites. Its flowers are in large compound cymes. Neither has made a big contribution to gardens in our area.
Plagianthus belongs to Tribe Malveae of the Malvaceae. It is a rare example of a properly woody genus in a group dominated by herbaceous and subshrubby plants (e.g. Malva (including the formerly segregated Lavatera) (Stevens 2021). It is closely related to Hoheria (q.v.), another woody New Zealand genus easily distinguished by its far larger, white-petalled flowers which are always hermaphrodite. A group comprising Plagianthus along with the Australian Asterotrichion and Gynatrix appears to be sister to Hoheria. In turn, this group including Hoheria is sister to Lawrencia, also Australian shrubs and subshrubs (Wagstaff & Tate 2011). The implication is that either the ancestors of Plagianthus and Hoheria migrated separately from Australia to New Zealand, or their common ancestor migrated to New Zealand, followed by a reverse migration by an ancestor of Asterotrichion and Gynatrix. Although the divergence between Hoheria and Plagianthus etc. may be very ancient, molecular data suggest that extant Plagianthus species diverged during the late Tertiary (Wagstaff, Molloy & Tate 2010).
Both species form a divaricating shrub at some stage, P. regius as a juvenile and P. divaricatus throughout its life. Divaricating shrubs make a dense tangle of widely branching, fine stems. Around 10% of the New Zealand woody flora is divaricate at least for part of the life cycle (Greenwood & Atkinson 1977). Some have suggested that this habit is an adaptation to avoid browsing by moas (Greenwood & Atkinson 1977), a group of large, flightless birds which became extinct in the 15th century following human colonization (Worthy & Holdaway 2002). Others invoke climatic adaptations, either as a protection from frost, wind or dry air (McGlone & Webb 1981) or providing self-shading to protect against cold-induced photoinhibition (Howell, Kelly & Turnbull 2002).
The generic name was among the first given to a New Zealand endemic, the type P. divaricatus being described from specimens collected on James Cook’s second voyage (Forster & Forster 1776). It derives from the Greek plagios (slanting, oblique) and anthos (flower), referring to the asymmetric petal bases. In the past, many related Australasian plants were included in the genus. Asterotrichion, Gynatrix, Lawrencia and – more surprisingly – the deciduous Hoheria species H. glabrata and H. lyallii were at one time all routinely treated as Plagianthus species.
Plagianthus seem to have no particular pest or disease problems. Freshly sown seed germinates readily (Cockayne 1911). Cuttings may be rooted semi-ripe in summer (Huxley, Griffiths & Levy 1992) or as softwood early in the year, with bottom heat (Burncoose Nurseries 2022).