Boronias - genus Boronia, family Rutaceae

The genus Boronia contains some of the more well known native flowers of the Sydney region. They are popular due to the fragrant oils which come from the leaves and their attractive flowers.

They are evergreen shrubs found right across the whole of Australia, making up approximately one hundred species in the genus. Of these about forty or so are found throughout NSW, with the Sydney Sandstone region containing about seventeen species. In general the species of NSW seem to prefer higher altitude habitats, with the north and south coasts ranges containing perhaps the greatest diversity in the state.

Their environmental requirements in the majority of cases seem to be much more specific than many other native plants. And many seem to be found in areas of igneous rock substrates. They are generally found growing in sheltered locations of open forest or woodland and occasionally heathlands. Although as mentioned there are many with very specific requirements, from the Mallee to coastal dunes to almost alpine conditions.

The species of the Sydney region are found growing in dry sclerophyll forest and heath over sandstone. Exact identification of species can sometimes be difficult, although the well scented leaves easily distinguish Boronias from other plants.

Boronias in particular are identified by the pink through to white flowers with four petals and four sepals which are generally persistent. They also have eight stamens which are normally easily recognised. Leaves in the majority of cases are opposite. Although as with all plant identifications, exceptions to the rule do occur. For example more recently discovered orange Boronias.

Somewhat unusually identification of Boronias is easier by looking at the leaves, which show a more obvious differentiation than the flowers. In particular the ratio of leaf size to petal size is often an easy indicator. In many cases the leaf shape itself makes indentification easy, such as with B. microphylla. Often the relative sizes of the leaves compared to the flowers is quite marked between different species.

In this way many of the botanical names of the plants of this genus show the importance of leaf form in identification. Examples such B. pinnata and B. serrulata, in addition to the above mentioned B. microphylla are direct references to leaf form. Pinnate, serrated and small leafed, respectively.

Unidentified native moths on Boronia serrulata flowers. Somersby Plateau.

While some Boronias have become popular ornamental plants, both in Australia and abroad, such as B.pinnata, little is generally known of their ecology. Many Boronias seem to have ballistically dispersed dehiscent seed that is additionally adapted for distribution by ants. In a number of cases seeds have been recovered from ant nests. Flowering in the Sydney region of Boronias seems to peak in August through to October. Although there are others that probably for localised environmental reasons will flower at different times. Boronias are self incompatible, so cross fertilisation is required. Pollination is thought to be completed by beetles and moths. Whilst in the field I have only ever seen small native moths on the flowers of B. serrulata, it can be reasonably speculated that there is probably some linkage with the aromatic oils of Boronias and insect pollinators.

The tolerance to fire at present seems to be undocumented in any great detail, although it is suspected that regeneration occurs from the soil seedback. Thus if fire is too frequent the seedbank becomes depleted. In general seeds seem to need special treatment to break dormancy, in many cases what could be termed radical treatments such as acid or alkaline and other chemical scarification methods.

Exceptionally fire does not seem to play a major role in seed germination of many Boronias, although again this is getting into the realms of speculation. My own feeling is that dormancy might be broken by Autumn flooding and the related soil disturbance, particulary after summer fires.

There may even be a relationship with ant based dispersal. For example a number of studies and practical experimentation on the part of horticulturalists has indicated chemical scarification can help break seed dormancy. These often involve using strong alkalis or acids such as Sulphuric acid.

Like many native plants many are at risk. In the Sydney region itself the main concern seems to be loss of habitat. In more rural locations Boronias also seem to be attractive to feral animals such as goats. Unfortunately not only a number of species are at critical risk, but at least one, Boronia barkeriana subsp. gymnopetala is thought to be extinct. This was last seen in the Sydney Harbour region in the 1920's.

In recent years there has also been a number of new species found, often with extremely restricted distributions. In general though, the current state of knowledge of Boronias in the wild is extremely limited.

Boronias are found in many diverse environments as already mentioned, however they are not generalist when it comes to environmental conditions. Most have very specific habitat requirements.So while the genus is widespread across all sorts of environments within Australia, at a specific level they are highly adapted and many have unique requirements. That being said Boronias do seem to have some general requirements. They are all found in very well drained nutrient poor soils, and while they can take heat their root systems seem to need to be kept cool. In the wild the soils that they are generally found growing on are sandy and very shallow, often only a matter of inches in depth until bedrock is reached. Although there are always exceptions, species from Mallee landscapes come to mind, the shallow rock seems to allow them to survive in more lowland environments. For example the ambient air temperatures may reach into the 40's C, but the root systems may barely rise above the 20's. This is to a large degree speculation and anecdotal, as on the whole little is known of the ecology of the genus in any quantifiable way, however Boronias do seem to prefer cooler environments on the whole, with a large species diversity found along the mountain ranges. The underlying rock is also important, with species adapted and localised to particular substrates, for example sandstone, igneous and even serpentine substrates come into play. Again generally they are found on slopes rather than plains as such, and these environmental factors seem to correspond with what is known of their ecology.

Sydney Sandstone Bushland

Selected Species

Boronia ledifolia - Sydney Boronia	Boronia pinnata - Pinnate Boronia	Boronia serrulata - Native Rose
Boronia ledifolia, sometimes known as the Sydney Boronia. It is a very attractive native Australian plant flowering from late winter through spring in the southern hemisphere. It is a small shrub growing up to 1.5m high (5 feet). It can be very common in coastal sandstone environments around Sydney. From July through to October, the gorgeous pink flowers stand out against the white heaths, Epacris spp. and similar. The stem is rust coloured compared to Boronia pinnata.	Boronia pinnata, also known as the Pinnate Boronia. It is a pretty Australian native plant flowering from July through to October. It is a small shrub growing up to 1.5m high (5 feet). It can be very common in coastal sandstone environments. They are found in dry sclerophyll forest and heath on sandstone only the coast of NSW, Australia. Superficially the Pinnate Boronia tends to have wider petals that are less recurved than the Sydney Boronia.	Boronia serrulata, also known as the Native Rose. It is one of Sydney's more well known native plants and also one of its most spectacular when in full bloom. They are found in a restricted range on the sandstone heaths surrounding Sydney. It is a small shrub growing up to 1.5m high (5 feet). Unfortunately it is also now listed as endangered, probably due to a combination of land clearance and flower collecting combined with feral animals, which seem to be attracted to the attractive scents of Boronias in particular.

Horticulture Notes

While this can only be a brief guide, the information presented should be a useful start for all those wanting to grow Boronias in their garden. Boronias have traditionally had a bad reputation among gardeners as being extremely difficult to grow except for one or two species, however as with all gardening the trick is to provide the correct environment. The problem though is that this is often easier said than done with specialised plants such as these.

That being said, difficult does not mean impossible, many gardeners do have success with Boronias, however it comes down to the amount of effort the gardener wants to put in. For example, at the National Botanic Gardens in Canberra, Australia, Boronias are grown in beds that have slabs of sandstone laid underneath. Obviously that approach is not practical for everyone, however with some some creativity and fore thought, Boronias can be grown in home gardens. Although they will never really be in the category of "throw them in somewhere".

Additional Information

Unfortunately as mentioned many Boronia are endangered in one way or another, many have extremely localised distributions and this is combined with a general lack of information on ecology makes many quite vulnerable. So as always if you see them growing in the wild, don't pick the flowers. And if you are wanting to grow them, try and record what you do. If you get to the point where you are able to collect (your own) seeds, experiment. So little is known that very real contributions to understanding are still possible.