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Saltmarsh Ribbonwood / Plagianthus divaricatus

Saltmarsh ribbonwood
Scientific classification
Kingdom:	Plantae
Clade:	Tracheophytes
Clade:	Angiosperms
Clade:	Eudicots
Clade:	Rosids
Order:	Malvales
Family:	Malvaceae
Genus:	Plagianthus
Species:	P. divaricatus
Binomial name
Plagianthus divaricatus J.R.Forst. & G.Forst.

Plagianthus divaricatus or saltmarsh ribbonwood is a plant that is endemic to New Zealand. The Māori name is makaka. It was first described by J. R. Forst and G. Forst in 1776.

Plagianthus divaricatus is an upright shrub with closely interwoven branches. The shrub is found in coastal environments in areas with salt swamp, sandy banks and throughout estuaries.

Description

Plagianthus divaricatus or salt-marsh ribbonwood is an endemic bushy shrub with thin twiggy intertwined stems that have small clusters of narrow leaves. Its branches are divaricate in form and it can reach up to 3 m tall (NZPCN). Its twigs have small star-shaped hairs when looked at under a magnifying lens (Wilson & Galloway 1993). The leaves are small and narrow, reaching 5-20 mm long and 0.5-2 mm wide. In spring it produces masses of small cream five petaled flowers. Its flowers are creamy white with purple tinged edges and small and drooping which. It has small white fruit, approximately 5 mm wide. It is known to hybridise with lowland ribbonwood (Plagianthus regius), which produces tall, bushy, deciduous shrubs. The leaves are grey-green, wider, and flatter than P.divaricatus (Allan 1961).

Range

Plagianthus divaricatus is endemic to New Zealand. The Plagianthus genus is also only found in New Zealand and the Chatham Islands with only two species, P. regius and P. divaricatus. Plagianthus divaricatus is present across New Zealand’s North and South Islands, as well as the Chatham Islands and Stewart Island. Found on both the west and east coasts in sheltered coastal and estuarine areas or inland off stony beaches. Likely to be down to the Dusky Sound in Fiordland (Wilson & Galloway 1993).

Habitat Preferences

Plagianthus divaricatus is found in coastal environments, such as in coastal wetlands, sandy banks and throughout estuaries. It is known to extend inland along estuarine waterways (Allan 1961, Singers & Rogers 2014). Also known to grow from stony beaches (Wilson & Galloway. 1993). It is predominantly a wetland plant but can occasionally be found in upland areas (NZPCN). Plagianthus divaricatus is commonly found growing with; Olearia solandri, Coprosma propinqua and Muehlenbeckia complexa (Singers & Rogers 2014).

Life-cycle

The flowers of Plagianthus divaricatus are creamy or whitish edged with purple. Only the male flowers are sweet smelling. The flowers are produced in early spring from September to October (Wagstaff & Tate 2011). Plagianthus divaricatus  is also known to germinate better in lower and higher saline conditions compared to other similar estuarine species (Partridge & Wilson 1987). It is also quite a cold tolerant species (Harris et al 1998), which could be due to its divaricating nature which Diels in 1897 proposed as an evolutionary adaptation of subtropical Genuses in response to the colder temperatures of the glacial periods (McGlone & Webb 1981).

Associated Fauna

It hosts many forms of animals, including many herbivorous and parasitic insects which feed upon it. Here is a list of them:

• Aceri plagianthi (Gall mite). Endemic to New Zealand. Only present on both P. regius & P.divaricatus. Causes similar deformities on the inflorescence as E. plaginus, as well as a white felty growth on the leaves.  
• Anzora unicolor (Grey-Winged Plant Hopper)
• Aphenochiton inconspicuous (Scale Insect)
• Aphis mellifera (Honeybee). Pollinates.
• Aspidiiotus nerii (Scale Insect)
• Batrachomorphus adventitiosus (Plant Hopper)
• Chloroclystis inductata (Moth). Caterpillars feed on the flowers.

• Eriophyes plaginus (Gall Mite). Endemic to New Zealand. Only present on both P. regius & P.divaricatus. Causes deformities on the inflorescence and induces ‘witches' broom’ galls.
• Graphania scutata (Moth). Caterpillars feed on the leaves
• Halormus velifer (Mirid Sucking Bug). Breeds on P.divaricatus.  
• Harmologa oblongana (Moth). Caterpillars feed on the leaves
• Hemiberlesia lataniae (Scale Insect)
• Hemiberlesia rapax (Scale Insect)
• Hoherius meinertzhageni (Beetle). Tunnels in dead wood.
• Leucaspis sp. (Tahuna Torea) (Scale Insect). Lives on stems.
• Liothula omnivora (Case Moth). Caterpillars feed on the leaves.
• Myzus persicae (Aphid)
• Paracoccus albatus (Mealybug)
• Peristoreus australis (Weevil). Larvae reared from male flowers.
• Pseudococcus hypergaeus (Mealybug)
• Pseudocoremia lactiflua (Moth). Caterpillars feed on the leaves.
• Romna scotti (Mirid Sucking Bug)
• Saissetia oleae (Scale Insect)
• Salicicola maskelli (Scale Insect).
• Scolypopa australis (Sucking Insect)
• Stigmella aigialeia (Moth). Caterpillars feed on the leaves.

(Landcare Research 2024)

One theory for the high numbers of divaricating plants in New Zealand, which  Plagianthus divaricatus is a part of, is that it is an ecological adaption as protection against moa browsing (Greenwood & Atkinson 1977). Therefore, sometime in the past one of the primary herbivores of P. divaricatus could have been moa resulting in the plant evolving this twiggy shrublike appearance.

The seed dispersal of P. divaricatus is currently understudied, but common genera that it is associated with such as Coprosma and Muehlenbeckia, even specifically C. propinqua and M. complexa, have their seeds dispersed by geckos (Whitaker 1987, Wotton et al 2016). As geckos are omnivores, they eat a wide variety of things and when fruits are available, they will regularly consume fruits, making them an important seed disperser (van Winkel et al 2018). Thus, it is likely that P. divaricatus is similarly distributed as it has small fleshy fruit like that of Coprosma and Muehlenbeckia genera, as well specifically being found in association with C. propinqua and M. complexa.

Other Info

Plagianthus divaricatus has a chromosome count of 2n = 42 (NZPCN).  

Plagianthus means Oblique or lop-sided flower and divaricatus means spreading or interlacing.