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Boronia thujona

Bronzy Boronia

Family: Rutaceae

Potentially a tall shrub, that grows to a height of 4 metres.

It is confined to eastern NSW, with some records form the north-west around Lithgow and Katoomba, then with many records between Sydney and Brooklyn, the Royal NP and spreading south to Wollongong, Bundanoon and, extending to west of Ulladulla. There is one disjunct record further south at Nadgee (close to the Victorian border).
It grows in wet and dry sclerophyll forest on sandstone, usually in shady situations.

Boronia spp. produce opposite leaves which can be simple or compound (sometimes on the same plant) and are usually aromatic (sometimes strongly-so).

In this species the leaves are aromatic, compound-pinnate with between 3 and 15 leaflets. Overall, the leaves are to 80 mm long and 70 mm wide with a petiole to 15 mm long; leaflets are to 30 mm long and to 6 mm wide, with the lower surface slightly paler, and sometimes with a bronze sheen.

Boronia 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 bright pink and are arranged in pairs or groups of up to 6 in leaf axils, each flower on a pedicel to 15 mm long, occurring from August to November.

The fruit of Boronia are described as a schizocarp-capsule; a capsule that splits into even segments, which each segment called a coccus. In this species, the fruit is smooth to 4 mm long and to 2.5 mm wide.

In the garden

Not a lot of information is available re the cultivation of this species. It is an attractive plant with its foliage and form and can also grow to several metres tall. Hence, it would be desirable in any garden.

It may be difficult to cultivate or may need more trialling. It may be more readily cultivated 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. Fertilise after flowering.

Propagation

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

Other information

Boronia is a genus of about 160 species of flowering plants in the citrus family Rutaceae. Most species are endemic to Australia and species can be found in all states. There are also some species in New Caledonia, which were previously placed in the genus Boronella.

Boronias are likely killed by fire and regenerate from the seedbank.

The common name, Bronzy Boronia, was first used by Jean Galbraith (1906-1999) in 1977. Galbraith was an Australian botanist, gardener, writer of children’s books and poet. The common name refers to a bronze sheen, sometimes present on the leaves.

Boronia – after Francesco Borone (1769-1794), an 18th century Italian botanist who assisted John Sibthorpe. Allegedly, he died at age 25, due to falling out a window whilst collecting plant specimens.
thujona – refers to the naturally occurring chemical Thujone, which is present in this species and many other plants including many members of the Lamiaceae (mint, sage and oregano), some daisies and some conifers. Thujone is a body-altering chemical that may cause convulsions but can act like a stimulant in small doses. It was isolated from this species by M.B Welch who formally published it with A.R Penfold.

Not considered at risk of extinction in the wild.

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

Wikipedia – Boronia thujona and Boronia profile pages
https://en.wikipedia.org/wiki/Boronia
https://en.wikipedia.org/wiki/Boronia_thujona

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

Wrigley, J.W. & Fagg, M.I. (2001). Australian Native Plants – Propagation, cultivation and use in landscaping. 4th edition. New Holland Publishers, Pty. Ltd. Australia.

By Jeff Howes, edited Dan Clarke