A guide to the genus Boronia.This is a little background on the genus Boronia, a very popular group of predominantly Australian wildflowers, known for their attractive flowers and perfume.All errors are entirely my own. Scott Douglass. |
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Boronias - genus Boronia, family RutaceaeThe 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.
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.
Selected Species
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. |
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