July 13, 2001, Newsletter Issue #38: The Bi-sexual Boronia!

Tip of the Week

Baronia
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.

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 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.

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.

Seed
A frustrating feature of the Boronia group of plants for home, for 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. 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" 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, no 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.

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.
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.

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