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The Boronia Family – Background

Introduction

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 is 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**
Acradenia
2
Asterolasia
5
Boronia
95
Chorilaena
1
Correa
11
Crowea
3
Diplolaena
6
Drummondita
4
Eriostemon
2
Geleznowia
1
Leionema
22
Microcybe
3
Muiriantha
1
Nematolepis
7
Phebalium
25
Philotheca
36
Rhadinothamnus
3
Urocarpus
5
Zieria
25

 

* 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 on the Boronia Family page).
** Approximate number only; some genera contain unnamed species and other genera are in need of botanical revision.

Characteristics

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

Genus Characteristics*
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.

 

The Boronia Family – Propagation

Introduction

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.

Seed

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. See the Propagation-Germination of Seed tab on the Boronia Family page.) Some research has been carried out into ways of overcoming this dormancy with a degree of success being achieved. (See Propagation-Germination of Seed tab on the Boronia Family page.) 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 on the Boronia Family page) and from the Regen 2000 web site.

Cuttings

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.

Grafting

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. However, as far as is known, little commercial grafting of native species is being carried out at present.

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

Species Possible Stock
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

 

General Propagation

Further details on general plant propagation can be found at the Society’s Plant Propagation Pages.

Germination of Seed of Rutaceae
John Knight
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 article under the “Further Information” tab).

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.

Ruler
“It is not uncommon to…….notice many seedlings germinating in the months following a fire.”
Ruler
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.

References
Blombery, A., 1977 Australian Native Plants, Angus and Robertson, Publishers.
Elliot,W.R. and Jones, D.L., 1980, Encyclopaedia of Australian Plants, Lothian Publishing.

The 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.


Table 4 – The Boronia Group; 20 of the Best!

Plant Common
Name
Size
HxW (m)
Flower
Colour
Comments
Asterolasia hexapetala None
1.5 x 1.5
White Very attractive shrub with large flowers; not commonly cultivated.
Boronia denticulata None
1.0 x 1.0
Pale pink Attractive and easily grown in many areas.
Boronia megastigma Brown boronia
0.5 x 0.5
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
1.0 x 1.0
Deep pink The cultivar “Lorne Pride” is particularly good.
Boronia pinnata Pinnate boronia
1.0 x 1.0
Mid pink Large flowers and attractive foliage; can be difficult to establish. White form exists.
Chorilaena quercifolia None
1.5 x 1.5
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
1.0 x 1.0
Yellow Rare in nature but easily cultivated in many areas. Attractive glossy foliage and flowers with and unusually shaped calyx.
Correa “Dusky Bells” None
0.5 x 1.0
Red One of the most reliable and attractive.
Correa reflexa Native fuchsia
0.5-1.5 x 1.0
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
0.8 x 1.0
Deep pink Very attractive bush which can be a little difficult to establish. A white form is in cultivation.
Crowea “Poorinda Ecstasy” None
1.0 x 1.0
Mid pink The best Crowea for the garden; hardy and reliable in many areas.
Diplolaena microcephala Lesser diplolaena
1.0 x 1.0
Orange Best in drier climates; flowers similar to Chorilaena.
Eriostemon australasius Pink wax flower
1.0 x 1.0
Pale to mid pink Very spectacular but difficult to propagate and to establish in the garden; worth the effort!
Leionema dentatum None
2.5 x 2.0
Creamy yellow Beautiful shrub for well drained positions; formerly Phebalium dentatum.
Phebalium glandulosum Desert phebalium
1.0 x 1.0
Yellow Spectacular small shrub; requires excellent drainage and sunny position.
Phebalium squamulosum Forest phebalium
1.0-2.0 x 1.0-2.0
Cream to yellow Variable plant with several forms in cultivation. Most forms attractive and reliable.
Philotheca myoporoides Long leaf wax flower
2.0 x 2.0
White Formerly Eriostemon myoporoides. The commonly cultivated form is a very hardy shrub with an excellent spring display of flowers.
Philotheca nodiflora None
0.5 x 0.5
Mauve Formerly Eriostemon nodiflorus. Colourful small shrub for non-humid climates.
Zieria prostrata None
0.1 x 1.0
Pale pink Rare in nature; available as “Carpet Star”. Good groundcover but not a vigorous grower.
Zieria smithii None
2.0 x1.5
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.

The Boronia Family – Further Information

Most books dealing with Australian native plants will contain useful information on the botany and horticulture of plants in the “Boronia group”. Some of the most detailed references are listed below.

Books:

  • 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.

Journals:

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″

Internet: