Boronia Family – Background
The large family of plants which includes the genus Boronia is distributed over many parts of the world. Botanically the family is known as the Rutaceae and it includes a number of commercially important plants such as the citrus group of fruit trees (oranges, lemons, lime, etc) and popular ornamental plants such as Diosma which is native to South Africa. Within Australia there are about 40 genera, many of which are cultivated. The most widely cultivated of these are the genera in the “Boronia group”.
Botanically the “Boronia group” is known as the Tribe Boronieae. Within this group are the well known Boronia itself, and a number of less well known genera. The table below lists the 19 genera in the Tribe and indicates the approximate number of species in each. All but one of the species in this group are endemic to Australia; the odd one out is Phebalium nudum, which occurs in New Zealand.
Table 1 – Genera in the Tribe Boronieae*
|Genus||No.of Species**||Genus||No.of Species**|
|* Information from the December 1990 issue of the newsletter of SGAP’s Boronia and Allied Genera Study Group, “Australian Plants” June 1999 and from “A Name Change for Most Eriostemons” (see the ‘Further Information’ tab).|
|** Approximate number only; some genera contain unnamed species and other genera are in need of botanical revision.|
While the Boroniae comprise the largetst group of Australian Rutaceae, several other genera are in cultivation. Principal among these is Citrus australasica, the Finger Lime, which is now commercially cultivated in several countries in addition to Australia. A number of cultivars of Finger Lime have been developed.
Characteristics of the Boroniae
Generally the Boronia group comprises plants of open forests and woodlands. They only rarely are to be found in rainforests or in arid areas. Overall the group is distributed throughout Australia but certain genera within the group may be restricted in their distribution (eg Correa is not found in Western Australia).The flowers are bisexual and usually have four or five petals (eg. four petals in Boronia and Zieria, five in Crowea, Eriostemon and Philotheca) but it is not unusual for some of the flowers on a particular plant to have an abnormal number of petals. In some cases (eg. most Correa, Nematolepis) the petals are fused into a bell-like tube while in others (Chorilaena, Diplolaena) the petals are small and the stamens are the conspicuous parts of the flowers, similar to the flowers of the well known but unrelated genera Callistemon and Melaleuca. The number of stamens either equals the number of petals or is twice the number of petals (eg. eight stamens in Boronia, four in Zieria). The fruits contain hard, waxy seeds which are expelled over a wide area when ripe.The following diagram of an Eriostemon flower shows some of the features which can be observed in the Boronia group as a whole.
|Based on Galbraith, J; “Australian Plants“, Vol.1, September 1960|
The Boronia group of plants are usually small to medium sized shrubs; none would reach even small tree proportions. A feature of most of the group is the presence of aromatic oils in the foliage and, in some cases, the flowers. When crushed or brushed against, the foliage gives off quite a strong aroma. In most cases this is an attractive feature but a few people find the very strong aroma of some Zieria species (for example) to be unpleasant. A number of the boronias have a very attractive perfume with the “Brown Boronia”, B.megastigma, being the most famous. The fragrance of other boronias such as B.serrulata (“Native Rose”) and B.florabunda is more subtle and not universally detectable.Table 2 lists some of the features of each genus.
Table 2 – Some Characteristics in the Tribe Boronieae
|Acradenia||5 petals, 10 stamens; leaves opposite petals|
|Asterolasia||5 petals, 10 stamens; calyx minute|
|Boronia||4 petals, 8 stamens; some species highly perfumed|
|Chorilaena||5 small petals, 10 stamens; flowers in clusters surrounded by large bracts; leaves lobed|
|Correa||4 petals, 8 stamens; petals fused into a bell-shaped tube|
|Crowea||5 petals, 10 stamens; hairy appendage on anthers|
|Diplolaena||5 small petals, 10 stamens; flowers in clusters surrounded by large bracts; leaves not lobed|
|Drummondita||5 petals, 10 stamens, only five of which are fertile; staminal filaments united into a tube|
|Eriostemon||5 petals, 10 stamens; distinctive “stellate” hairs on leaves and petals; leaves with 3 or more main veins|
|Geleznowia||5 petals, 10 stamens; flowers in clusters enclosed by large bracts|
|Leionema||5 petals, 10 stamens; formerly included in Phebalium|
|Microcybe||5 petals, 10 stamens; flowers in sessile (unstalked) terminal heads|
|Muiriantha||5 petals, 10 stamens; petals forming a bell shaped tube but not fused|
|Nematolepis||5 petals, 10 stamens; petals fused into a bell shaped tube|
|Phebalium||5 petals, 10 stamens; staminal filaments not hairy|
|Philotheca||5 petals, 10 fertile stamens; leaves with only one main vein|
|Rhadinothamnus||5 petals, 10 stamens; leaves closely clustered|
|Urocarpus||5 petals, 10 stamens; calyx inconspicuous|
|Zieria||4 petals, 4 stamens; foliage often very aromatic|
|* Typical characteristics. Some variations often occur, such as the number of petals.|
Boronia Family – Propagation
Propagation of the Boronia group of plants from seed is usually difficult and propagation by cuttings is the preferred method. This also enables plants with desirable characteristics of form or flower colour to be perpetuated. Some work has been carried out on propagation by grafting.
An excellent review of propagation methods for Boronia can be found in the article Propagating Boronia. The methods can usually be applied to related genera.
A frustrating feature of the Boronia group of plants for home propagators who try without success to germinate seed, is that seedlings will often come up in freshly graded road verges near natural bushland and even in gardens!
Seed of the group has a hard (usually black) seed coat and appears to have an inhibitor to germination designed to ensure that germination only occurs in nature under favourable conditions. Some research has been carried out into ways of overcoming this dormancy with a degree of success being achieved. (see the accompanying article Propagation – Germination of Seed). For example, it was found some years ago that placing seed of Eriostemon australasius in a muslin bag in running water for up to 2 weeks seemed to leach out the inhibitory agent, allowing germination to proceed. It is possible that such a method would be successful with other members of the Boronia group but it is of limited practical use to most home gardeners – although it has been suggested that leaching could be achieved by suspending the bag in the cistern of a flushing toilet….so far no one has admitted trying this!.
Another method that has been successful for at least some species is the use of smoke or “smoked water” as a pretreatment. This has been successful in the germination of species of Geleznowia and Philotheca and may have practical application for the home propagator. Further information on this procedure is available in the article Smoke Stimulates the Germination of Many Western Australian Plants (see the ‘Further Information’ tab above).
Many plants in the Boronia family are readily propagated by cuttings using hardened, current-season’s growth. Cuttings about 75-100 mm in length, taken in January in southern Australia would normally be suitable with the leaves carefully removed from the lower two-thirds. “Wounding” the lower stem by removing a sliver of bark and treating with a “root promoting” hormone both seem to improve the success rate.
Some research has been carried out on the grafting of members of the Boronia family, mainly by enthusiastic amateurs. Because of the difficulty of growing some desirable members of the family on their own roots, grafting onto hardy rootstocks offers the potential to expand the range where those plants can be successfully cultivated. Grafting is quite common with some exotic members of the family (eg grafted orange, lemon and lime varieties) and some success has been achieved with native species, notably finger lime (Citrus australasica).
There are two factors to be considered when selecting suitable rootstocks:
- Hardiness of the stock – the selected stock needs to be reliable in the area where the grafted plant is to be grown.
- Compatibility between stock and scion – the closer the relationship between the stock and scion, the better the chance of success. Stock and scion of the same species is ideal, stock and scion in the same genus is often successful and stock and scion in closely related genera may also be suitable if other options are not viable.
Those wishing to undertake grafting experiments might consider some of the following rootstock suggestions, but available data on compatibility is scarce and anyone working with these plants are encouraged to document their results, both successful and otherwise.
Table 3 – Possible Scion / Rootstock Combinations
|Boronia sp.||Boronia denticulata, B.clavata, Zieria smithii|
|Correa sp.||Correa alba, C.glabra, C.lawrenceana|
|Phebalium sp.||Phebalium squamulosum, Philotheca myoporoides (syn. Eriostemon myoporoides)|
|Eriostemon sp.||Philotheca myoporoides|
|Crowea sp.||Philotheca myoporoides|
Further details on general plant propagation can be found in the Plant Propagation section.
Germination of Seed of Rutaceae
John is a former leader of the Society’s Boronia and Allied Genera Study Group. The following article is reproduced, with minor amendments, from the June 1991 issue of the Group’s Newsletter.
Subsequent to this article being written some successful germination of at least some Rutaceae has been demonstrated through the use of smoke treatment (see ‘Further Information‘ below).
* * * * * * *
Many books detailing propagation techniques suggest that, in general, germinating seed of Australian Rutaceae is difficult, The seed coat contains one or more inhibitors (natural chemicals) which prevent the seed from germinating until the inhibitors are neutralised. The best way to overcome the inhibitors is still a matter of research. Various techniques have been tried, but reliable data is not yet available to us, and no specific inhibitor has been isolated for a range of species. It is known that some species of Boronia, Crowea and Eriostemon exhibit both chemical and physical dormancy.
Because of the way seed is shed as it matures, collecting sufficient for experimentation is difficult. As many desirable species are successfully propagated by cuttings, work on seed germination has not been seen as important. It is known that early collectors were able to germinate seed of some species, and more recently limited seed trials have been undertaken. There is obviously a lot more work to do on this and on the systematics of Boronia, including the study of seedling morphology.
In overcoming the inhibitor to germination, a number of methods have been tried.
- Leaching involves placing the seed in a muslin bag, (or an old stocking) and tying the bag securely in a source of running water for between 1 and 10 weeks. It is obviously not practical to use a running tap, so a small permanent stream or river where the bag can be left safely is the only answer. The action of running water carries away the chemicals which inhibit germination. The seed needs to be checked regularly, as germination may occur whilst the seed is in the bag.
- Soaking in still water is not recommended as the seed tends to rot.
- Nicking the seed coat (testa) before leaching may improve germination, and reduce the time needed for leaching,
- Correas have responded to leaching in warm water, (impractical), and using an alkaline solution, pH9, for leeching has proved successful with other genera.
- Other methods worth trying include nicking the testa near the radical before sowing, completely removing testa, scarifying the seed by rubbing lightly with sandpaper, hot water treatment similar to that employed for Acacia, washing the seeds in an acid solution for short periods, stratifying the seeds in a refrigerator for a period of weeks, and finally fire.It is not uncommon to visit a recently burned area and notice many seedlings germinating in the months following a fire. A number of years ago I chanced upon Gilgandra Flora Reserve, in central New South Wales, the spring following a fire, and Phebalium nottii had germinated in thousands, as had many other plants.
- A positive response has been noted with Boronia and Crowea when seed is stratified, then a fire set an top of the pot holding the seed. Use a terra cotta pot of course! Keep a small fire going for a few minutes using leaves, and when the ash cools, water the pot, and hope.
Should you be successful in germinating any seeds, the results would be of interest to all, so please keep records.
Seed of Western Australian species of Boronia have proved a lot easier to germinate than those of the eastern states, and without treatment in many cases. It was with interest that that I purchased a packet of commercially prepared seed of Boronia pinnata (an eastern species). The package gave 3 simple instructions for germination:
- Sow 5mm deep in seed tray. Seedlings appear 14 – 20 days,
- At 5cm high, transplant into tubes,
- Plant out at 15 – 20cm high.
I followed the first instruction, and waited. It is now 16 months, and still no results. After 4 months, I phoned the company to ascertain how they arrived at the recommended instructions. I had assumed that some sort of pre-packaging treatment must have taken place. After the usual runaround, and speaking to a number of personnel, no one was able to offer any solution. However the packets are still being sold.
Despite the difficulties, a number of seedlings appear in gardens from time to time, Many growers have had correas pop up, and when I visited an acquaintance at Shepparton in north-east Victoria, he showed me a number of Crowea exalata seedlings which had germinated over a number of months. It’s obvious that someone up there knows what to do!
When it comes to collecting seed, close watch needs to be kept on maturing fruit, as one warm day could be all that is needed for the fruit to disperse the seed to all parts of the garden. If you notice fruit setting, and the odd seed already dispersed it might be time to collect the rest. Seed collected just before maturity will be fine. Place the fruits in a paper bag in a warm dry place for a couple of days, and the seed should be released.
When and if it does, go to it! Try whatever method you like, and record your results. Best of luck.
- Blombery, A., 1977 Australian Native Plants, Angus and Robertson, Publishers.
- Elliot,W.R. and Jones, D.L., 1980, Encyclopaedia of Australian Plants, Lothian Publishing.
Boronia Family – Cultivation
Like most groups of plants, some members of the “Boronia Group” have proved to be easy to grow in cultivation over a wide range of climates, others grow well in some districts and not in others while others have proved to be a cause of frustration, generally. The genus Boronia, itself, is the source of much of the frustration because of the many attractive species it contains which have not adapted well to cultivation.
As a general rule, members of the group require the following combination of conditions:
- Excellent drainage – they will not tolerate waterlogging
- Assured moisture – but freely draining
- Protection from direct summer sun – dappled shade is ideal
- Good light – not dense shade
- Light soils (eg sandy loams)
- Suitable climate
The last item is probably the most difficult to accommodate. Generally, it is pointless trying to grow species native to Mediterranean climates (dry summer/wet winter) in tropical and sub tropical areas. This means that plants such as the brown boronia (B.megastigma), which is renowned for its beautiful perfume, are not long term propositions in humid climates. They can, however, be grown for 1 or 2 seasons as small pot plants and then replaced.
There are always exceptions to any general rule and B.denticulata, a western Australian species is reasonably hardy in humid areas.
A number of horticultural forms have been brought into cultivation, particularly in Boronia, Correa, Crowea and Philotheca. These have generally resulted from collections of unusual forms from the wild or from chance hybridisation between garden-grown plants. There has been little or no deliberate hybridization in this group of plants. Table 4 lists some of the more reliable or readily available species and cultivars but the list is not meant to be comprehensive and availability will vary from district to district.
More details for many of the plants listed in the table (including photos) can be seen by searching in the Plant Profiles section.
Table 4 – The Boronia Group; 20 of the Best!
|Plant||Common Name||Size HxW (m)||Flower Colour||Comments|
|Asterolasia hexapetala||None||White||Very attractive shrub with large flowers; not commonly cultivated.|
|Boronia denticulata||None||Pale pink||Attractive and easily grown in many areas.|
|Boronia megastigma||Brown boronia||Brown and yellow||Magnificent perfume; difficult to grow in humid area. Several cultivars available; eg “Chandleri” (burgundy), “Harlequin” (striped red/yellow). “Lutea” (yellow)|
|Boronia mollis||Soft boronia||Deep pink||The cultivar “Lorne Pride” is particularly good.|
|Boronia pinnata||Pinnate boronia||Mid pink||Large flowers and attractive foliage; can be difficult to establish. White form exists.|
|Chorilaena quercifolia||None||Green or red||Attractive, bushy shrub with “oak” shaped foliage; very unusual flowers in which the stamens are prominent.|
|Correa baeuerlenii||Chef’s cap correa||Yellow||Rare in nature but easily cultivated in many areas. Attractive glossy foliage and flowers with and unusually shaped calyx.|
|Correa “Dusky Bells”||None||Red||One of the most reliable and attractive.|
|Correa reflexa||Native fuchsia||Red/yellow||Widespread in nature and very variable; can be difficult to establish and maintain. Selection of local forms is advisable where possible.|
|Crowea exalata||Small crowea||Deep pink||Very attractive bush which can be a little difficult to establish. A white form is in cultivation.|
|Crowea “Poorinda Ecstasy”||None||Mid pink||The best Crowea for the garden; hardy and reliable in many areas.|
|Diplolaena microcephala||Lesser diplolaena||Orange||Best in drier climates; flowers similar to Chorilaena.|
|Eriostemon australasius||Pink wax flower||Pale to mid pink||Very spectacular but difficult to propagate and to establish in the garden; worth the effort!|
|Leionema dentatum||None||Creamy yellow||Beautiful shrub for well drained positions; formerly Phebalium dentatum.|
|Phebalium glandulosum||Desert phebalium||Yellow||Spectacular small shrub; requires excellent drainage and sunny position.|
|Phebalium squamulosum||Forest phebalium||Cream to yellow||Variable plant with several forms in cultivation. Most forms attractive and reliable.|
|Philotheca myoporoides||Long leaf wax flower||White||Formerly Eriostemon myoporoides. The commonly cultivated form is a very hardy shrub with an excellent spring display of flowers.|
|Philotheca nodiflora||None||Mauve||Formerly Eriostemon nodiflorus. Colourful small shrub for non-humid climates.|
|Zieria prostrata||None||Pale pink||Rare in nature; available as “Carpet Star”. Good groundcover but not a vigorous grower.|
|Zieria smithii||None||White or pink||Not spectacular but very hardy – may self sow|
The plants in this group are generally adapted to nutrient-deficient soils are are not demanding as far as fertilizing is concerned. They do respond to applications of slow release fertilizer applied after flowering. If desired, the plant can be pruned back by about one third after flowering to promote a bushy habit of growth.
Plants are sometimes attacked by scale insects which can be controlled by physical removal (for small plants) or by use of white oil.
Rutaceae Study Groups
The Boronia and Allied Genera Study Group and the Correa Study Group were set up with the aim of studying the cultivation and propagation of genera within the Boronia family (tribe Boronieae of the family Rutaceae), including Boronia, Correa, Diplolaena, Leionema, Phebalium and Zieria, so that more species could be seen in gardens. The former group is now closed but the still active Correa Study Group concentrates specifically on that genus.
Both groups have produced numerous informative newsletters – further information, including access to the newsletter archives, can be found at the links below.
Boronia Family – Further Information
Most books dealing with Australian native plants will contain useful information on the botany and horticulture of plants in the Rutaceae. Some of the most detailed references are listed below.
- Elliot, W. R and Jones D (1980-1997), The Encyclopaedia of Australian Plants, all volumes, Lothian Publishing Company Pty Ltd, Melbourne.
- Hitchcock, M (2010), Correas – Australian Plants for Waterwise Gardens, Rosenberg Publishing, Kenthurst, New South Wales.
- Johnson, K and Burchett, M (1996), Native Australian Plants – Horticulture and Uses, University of NSW Press.
- Wrigley, J and Fagg, M (1996 – 4th ed), Australian Native Plants, Collins Publishers Australia.
- Bayly, M (1999), A Name Change for Most Eriostemons, in “Native Plants for New South Wales” (Newsletter of the NSW Region of SGAP), January 1999.
Several issues of the Society’s journal “Australian Plants” are particularly useful for those interested in Boronia and its allies.
- Vol.6 No.48 September 1971; Growing Boronia and Correa.
- Vol 8, No.65 December 1975; Extensive details of the Rutaceae generally; The genus Acradenia.
- Vol 8, No.66 March 1976; Cultivation of Boronia and Phebalium.
- Vol 13, No.104 September 1985; Ecological studies of Boronia serrulata.
- Vol 13 No.105 December 1985; Propagation and cultivation of Boronia serrulata.
- Vol 14, No.116 September 1988; Outline of characteristics of Crowea.
- Vol 15, No.118 March 1989; Cultivation of Phebalium.
- Vol 19, No.150 March 1997; Commercial cultivation of Boronia.
- Vol 20, No.159 June 1999; Philotheca, Phebalium, Leionema; name changes.
- Vol 21, No.169 December 2001; Boronia and its relatives; Correa cultivars; Correa ‘Federation Belle’.
- Vol 22, No.174 March 2003; Full issue on Correa species and cultivars including descriptions, propagation, cultivation and numerous photos and diagrams.
- Vol 23, No.182 March 2005; “Boronia:- Not just pretty in pink – Part 1″
- Vol 23, No.183 June 2005; “Boronia:- Not just pretty in pink – Part 2″
- Attractive Correas
- Correa – Wild Fuchsia
- Growing Native Plants – a series of plant profiles by the Australian National Botanic Gardens; includes a number of Rutaceae.
- A Name Change for Most Eriostemons
- Growing Finger Limes
- Poor Relations – Phebalium/Leionema/Nematolepis, often overlooked members of the Boronia family.
- Propagating Boronia – although more than 20 years old, this in-depth review is still remarkably up to date
- Smoke Stimulates the Germination of Many Western Australian Plants, by K.Dixon and S.Roche – contains useful information on research into many plant genera including some of the Rutaceae.
- The Adaptable and Rewarding Crowea
- The Use of Correas in Garden Design – Careful selection can produce a colourful display of these bird-attracting plants