Common name: Brown boronia

Brown boronia is an ornamental and essential oil yielding shrub originating in the southwest of Western Australia, its native range covering a relatively small area, extending north-south from Perth to Albany and from the coast about 150 km (93 miles) inland. It occurs on low-lying sites, on the margins of swamps or wetlands or places subject to seasonal flooding.

It is typically 1 to 1.5 m (3 to 5 ft) tall, though on favourable sites, it may attain a height of up to 2 m (7 ft) and has a bushy appearance. 

Branching starts near the ground, with long, slender, ascending branches growing off the main stem and those from the bottom up extending wider than those immediately above, forming a domed canopy.

Leaves consist of four to five soft, dark green, needle-like leaflets arranged circularly at nodes spaced evenly along the branchlets. They remain on the plant in all seasons.

Flowers are bell-like with four waxy petals, outside chocolate-brown, less commonly burgundy or striped yellow and red, and inside lemon yellow. They bloom in perfusion in spring, usually in pendulous pairs at the leaf node and are intensely sweetly fragrant, with a citrusy aroma. 

Fertilised flowers develop into small four-celled capsules enclosing tiny brown or black seed. When mature, they burst open with force, scattering their seed a considerable distance from the plant.

Use

The flowers yield an essential oil with a long-lasting, fresh, fruity, spicy aroma underlaid with warm-woody floral notes, reminiscent of citrus, cinnamon (Cinnamomum verum), cut hay and tobacco leaf. Traded commercially as 'Boronia absolute', it is used in high-end perfumes such as 'Diorissimo' by Dior and 'Deci-dela' by Nina Ricci. It is also a highly regarded flavouring agent and is widely used to accentuate otherwise subtle fruit flavours, particularly blackcurrant, raspberry, strawberry, plum and peach, in alcoholic and non-alcoholic drinks, dairy desserts, baked goods, condiments, relishes and even meat products such as smoked salmon.

The essential oil is extracted from the flowers using a solvent, resulting in a dark green, waxy mass known technically as an 'essence concrete'. This mass is then washed with alcohol to produce the essential oil, a dark green, viscous fluid. This is a low-temperature extraction process that preserves the delicate fragrance of the flower, which might otherwise be altered if steam distillation or other high-temperature extraction process is used.

Depending on the variety, it takes around 170 to 330 kg (375 to 730 lbs) of flowers to produce 1 kg (2.2 lbs) of essence concrete, which converts to 0.5 kgs (1.1 lbs) of essential oil after alcohol washing.

Flower yields in Australian commercial plantations average 2000 kgs per hectare (1784 lbs per acre) per year, resulting in approximately 3 to 6 kgs of essential oil per hectare (2.7 to 5.4 lbs per acre).

Boronias are commonly grown in gardens and containers in Australia for their compact size, shapely form, eye-catching foliage, and sweetly fragrant flowers. They are also suitable for producing cut flowers used in floral arrangements. Flowering stems should be harvested when around half of the flowers have opened and have a strong fragrance, then stood in water with a preservative added and misted regularly. Cared for properly, they have a vase life of around six to ten days. Red boronia (Boronia heterophylla) is the main boronia species grown commercially for cut flowers.

Climate

Although native to a Mediterranean climate with dry summers, it adapts well to warm temperate climates with evenly distributed rainfall, generally areas with annual lows of 7 to 14 °C, annual highs of 17 to 25 °C, annual rainfall of 500 to 1100 mm and a dry season of 6 months or less, extending to 8 months with irrigation.

Growing

New plants are usually grown from cuttings, collected in summer when the plant is actively growing. After they are collected, the cuttings are placed in a container with a fast-draining potting mix, consisting mainly of bark and coarse sand, and regularly misted. They readily produce roots and start flowering within one to two years. 

Performs best on moist, free-draining sandy-loam and loamy-sand soils of an acid to neutral nature, generally with a pH of 4.0 to 7.0, and on sites with full to partial sun exposure.

Boronia has a shallow root system and needs protection for windy conditions and dry weather, using windbreaks and irrigation. 

Problem features

None known

Books

• Arctander, S. 1960, Perfume and flavor materials of natural origin, Elizabeth, New Jersey

• Byrne, T. & Bourke, M. & Salvin, S. 2004, The new crop industries handbook, Rural Industries Research and Development Corporation (RIRDC), Canberra, Australia

• Church, G. & Greenfield, P. 2002, Trees and shrubs for fragrance, David Bateman, Auckland, New Zealand

• Cribb, A. B. & Cribb, J. W. 1982, Useful wild plants in Australia, William Collins, Sydney

• Editors of Sunset Magazine 2012, The New Western Garden Book: The Ultimate Gardening Guide, 9th edition, Sunset Publishing Corporation, California

• Genders, R. 1978, Scented flora of the world, Robert Hale Publishing, London & St. Martin's Press, New York

• Groom, N. 1997, The new perfume handbook, 2nd ed., Blackie Academic & Professional, London

• Jones, R. 2001, Caring for cut flowers, 2nd ed, Landlinks Press, Victoria, Australia

• Khan, I. A. & Abourashed, E. A. 2010, Leung's encyclopedia of common natural ingredients : used in food, drugs and cosmetics, 3rd edition, Wiley Publishing, Hoboken, New Jersey

• Randall, R. P. 2002, A global compendium of weeds, R.G. and F.J. Richardson Press, Melbourne

• Seidemann, J. 2005, World spice plants: economic usage botany taxonomy, Springer-Verlag, Berlin