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Cyanothamnus rigens

Stiff Boronia

Family: Rutaceae

A compact rigid shrub that grows to a height of 40 cm.

It has a natural distribution from the Hunter Valley (near Singleton) and Kandos (Mt Coricudgy) area with disjunctions south to Lithgow and Katoomba, through Wollongong and the southern highlands through to Braidwood, with a few disjunct records around Bombala and Eden.

It grows on sandy soils including sandstone outcrop and sand dunes in heath and dry sclerophyll forest.

Boronia spp. (including Cyanothamnus) produce opposite leaves which can be simple or compound (sometimes on the same plant) and are usually aromatic (sometimes strongly-so). In this species, leaves are compound-trifoliolate, with leaflets to 6 mm long and 2 mm wide, hairless to hairy and with a thickish texture as well as glandular warts.

Boronia spp. (including Cyanothamnus) have complete flowers (bisexual and with all whorls present). The flowers are produced either solitarily or in groups, in the leaf axils, or, at the branch terminals. There are usually four sepals, four petals and generally eight stamens, surrounding one female part (carpel). Flowers are often pink to purple, which makes them easily identifiable in NSW bushland.

In this species, flowers are white to pale pink and are arranged singly in leaf axils on a pedicel to 3 mm long, occurring from July to August.

The fruit of Boronia / Cyanothamnus are described as a schizocarp-capsule; a capsule that splits into even segments, which each segment called a coccus. The fruit is mostly smooth, to 3.5 mm long and to 2 mm wide.

In the garden

Not much information is currently available about its cultivation. It is likely not often cultivated due to its difficulty in establishment or not being trialled sufficiently. It may become more well known in cultivation in the future.

Growing boronias can be a frustrating experience and they have a reputation for being difficult. Their attractiveness has led to substantial efforts to cultivate them. One tip is to try to grow forms that are local to your area, rather than attempting to grow those species from interstate. There are certainly some species that have proven easier to grow than others.

Most boronias have a short life span of two to three years in a garden situation but are a rewarding plant while healthy as they provide lovely fragrance and flowers in Spring

For them to grow at their best, select a position with dappled sunlight and especially protection from hot afternoon sun in summer, as well as from wind, which they dislike.

The soil must be well drained and have an even supply of moisture. If they dry out, they will surely die.

Planting on a slight slope is said to work well.

For a longer life, the best way to grow them is in a medium sized pot, say 30 cm in diameter where drainage and moisture can be controlled. A sheltered patio or courtyard that receives at least a few hours sunlight a day would be ideal

Fertilise after flowering.

The conventional wisdom is, think deeply about which species to plant and the location to plant it.

Propagation

Boronias can be propagated from cuttings but overall you will have limited success with most but not all species.

Other information

This species, until 2020, went by the name Boronia rigens.

This species has been moved to the closely related genus Cyanothamnus.

Most Cyanothamnus species would die in a fire and likely regenerate from the seedbank.

Cyanothamnus – cyano from Ancient Greek. kuáneos meaning “dark blue” and -thamnus (Θάμνος) meaning “bush” or “shrub” – referring to many species in the genus having blue flowers.
rigens – Latin for “frozen” or “stiff” referring to the rigid habit of this species

Not considered to be at risk in the wild.

Australian Native Plants Society Australia – Boronia profile page
http://anpsa.org.au/boronia9.html

Wikipedia – Cyanothamnus rigens and Boronia profile pages
https://en.wikipedia.org/wiki/Boronia
https://en.wikipedia.org/wiki/Cyanothamnus_rigens

Gardening with Angus Website – Boronia for Beginners
https://www.gardeningwithangus.com.au/boronias-for-beginners-2/

NSW Flora Online (PlantNET)
https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=sp&name=Boronia~rigens

By Jeff Howes, edited Dan Clarke