Cultivation and Propagation of Boronia and its Relatives

John Knight

Rue, from the Greek word "rueo", to set free, refers to the value of this herb as a medicine. Rutin, its essential principle is used among other things for the treatment of high blood pressure.

The large family Rutaceae worldwide contains about 150 Genera, and over 1800 species. It includes the Citrus trees such as Citrus limon from India and the well known garden shrub from South Africa, Diosma vulgaris.

Within this family is the Tribe Boronieae, with about 250 species. This tribe is almost exclusively Australian. By far the largest of the Australian tribes within the Rutaceae, Boronieae is represented in all states, recorded from many diverse habitats, and displays a wide ecological tolerance.

It contains most of the cultivated genera of Australian Rutaceae. Many plants have desirable ornamental qualities, be it attractive foliage or habit of growth, fragrance of both flowers and foliage, or spectacular floral display.

Some plants in this group have proved their adaptability in cultivation over many years, being long lived, attractive garden plants in a variety of situations. Others however, such as the desirable heart breaker, Boronia megastigma, the Brown Boronia, continually tests the patience of even experienced growers.

Members of the Boronia and Allied Genera Study Group are working to overcome some of the problems associated with growing these desirable plants, and to bring more species into cultivation. Information on both successes and failures is being recorded in the hope that the right formula for cultivation can be ascertained. Whilst Boronia is by far the largest genus in the study, other genera such as Asterolasia, Crowea, Diplolaena, Eriostemon, Phebalium, Philotheca, and the under-rated Zieria are also included. We also include Correa, but work with the Correa Study Group rather than duplicate any work they may be undertaking.

Asterolasia is a small genus of about 5 species and is fairly uncommon in cultivation. This is Asterolasia hexapetala, a shrub of 1-2 metres in height. Select the thumbnail image or plant name for a higher resolution image (24k).

Introduction

At Karwarra Australian Plant Garden on the eastern slopes of Mt Dandenong near Melbourne, some 85 species and a number of cultivars/forms within the tribe Boronieae are being grown. Included there are 34 species and 12 forms or varieties of Boronia. Many of these plants are over 10 years old, some as much as 15 and, in the main, still thriving and healthy.

Boronia invokes passionate responses from growers. "One year wonders" I hear many say. The record at Karwarra, and of Study Group members suggest that this is not so. But it is no surprise that those bought from nurseries or other outlets as delightful bunches of potted flowers, often fail to perform in the home garden. Why?

Let me state first of all that the generalisation above mostly refers to Boronia megastigma, as it this one which begs us all to take 1 or 2 home to enjoy its exquisite aroma.

In the vast majority of cases, it appears that our lovely Boronia is sentenced to death from the day the cuttings are taken. As a rule these plants have a relatively weak root system, but one which is obviously suited to their natural environment. Plants are often found growing in light soils, and with overhead protection, or in heathland where crowding affords protection from the hot sun. It is rare, even with desert species, to find plants growing in isolated, unprotected situations. Seedling roots develop quickly, and penetrate the light soils and crevices to astounding depths.

Collection of Boronia seed, or that of other genera in the Tribe, is difficult as it is usually dispersed quickly once ripe. One warm day is all that is needed for near ripe seed to be thrown wide and far as the fruits explode. Collection of unripened seed provides no joy in my experience, as it refuses to ripen once picked. If you are lucky enough to be around when the fruits begin to burst, collection of nearly mature fruits is possible. Even when seed is available, germination as with many Australian genera, is difficult, and no reliable method has been established although some success has been reported for some species by leaching inhibitors in running water. Those species from Western Australia are sometimes easier to germinate, but results are often unsatisfactory.

It takes about 18 months for a nursery to produce a bushy, full of flower specimen. In this time the pot is filled tightly with roots. An innocent buyer, so taken by the perfume and/or attractive floral display on such a bushy little plant, proudly takes the plant home, lovingly places it in the chosen position and watches sullenly as life slips, often quickly, from its limbs.

Should boronias be seen as a bunch of flowers to be enjoyed as a potted specimen on the kitchen window sill, and then thrown away? I hope not. But plants are generally only available when in flower, for obvious commercial reasons. I must stress that there is nothing unethical about this, for growers have a right to be compensated for their endeavours. However, the treatment of plants in a nursery situation is far removed from that experienced in a tough home garden environment.

Because boronias are recognised as Australian plants, it seems all Australian plants are tarred with the one brush, that is, short lived, not hardy, unreliable, and therefore not horticulturally desirable. Surely this is totally unreasonable.

Thankfully, not all Boronia species are difficult to cultivate. Many of the other genera also contain desirable and relatively easy to grow species.

Phebalium is a genus of about 49 species. Most have yellow or cream flowers as shown by Phebalium squamulosum. Select the thumbnail image or plant name for a higher resolution image (40k).





Phebalium nottii is unusual in the genus in having deep pink flowers. It grows naturally in northern NSW and southern Queensland. Select the thumbnail image or plant name for a higher resolution image (36k).

Cultivation

It might be a good idea to avoid species such as Boronia megastigma until a little success and confidence is gained. Grow firstly some of the more adaptable species. Think seriously about the aspect. boronias and their relatives in general have fairly specific, straightforward requirements.

1. Good Drainage. The number one key to success in growing a wide range of Australian plants is to ensure that the soil is well drained. Heavy soils should be well worked, and incorporating some gypsum, compost and coarse sand will assist in keeping the soil open. This allows the roots to penetrate deeper, away from the surface, and moisture will also travel away from the surface, protecting plants from collar rot.

It may be more practical to raise the garden bed level by 150 to 300mm, to ensure drainage is unimpeded, but don't make the mistake of just putting extra soil on top of existing ground. Make sure that the new soil is incorporated into the natural ground so that roots can travel through to the sub-soil.

In light soils, adding compost through the top 150 - 300mm of soil will assist in retaining precious water in the warmer months, and build up the number of worms and soil microbes so necessary in achieving healthy soils.

2. Consistent Moisture. The emphasis is on "consistent". It is no good letting the soil dry out, causing stress to the plants, then wetting and drying out. This will almost certainly guarantee failure with most plants. If the soil has been worked properly, waterlogging should not be a problem. Beware though of watering during hot humid weather as many species are susceptible to root rotting fungi which develop quickly in moist warm soil.

3. Protection of roots from vast soil temperature fluctuations. This can be achieved by planting in an easterly or north easterly aspect, or below open canopied Eucalypts or other plants which will provide dappled shade at soil level. Protection can be afforded by planting closely to other shrubs so that the effects of strong winds can be reduced. Mulching more open areas can reduce the amount of heat reaching the soil drastically, but be careful to keep organic mulches away from the stems to avoid collar rot, and damage from soil organisms which feed on the mulch material. These organisms have trouble distinguishing our living plant stem.

Some growers prefer mulch of gravel, or pieces of rock close to the plant.

4. Pruning. "You don't prune Australian plants", was a lamentable generalisation from the '60's and '70's. Unfortunately many people seem to think this is true, to the detriment of their plants. If pruning Australian plants worries you, try thinking of them as plants, and forget the "Australian" bit. Maybe then we can overcome our reluctance to prune.

5.Fertilising. Members of the Boronieae tribe are not gross feeders, and none appreciate great dollops of citrus food even if they are related to the lemon tree. A light feeding with slow release Osmocote or similar each Autumn should be all that is required. Avoid feeding in early Spring because this can result in a flush of growth at the expense of flowers, and likewise avoid Summer feeds as the extra growth would need to be carried through the hottest time of the year, putting extra stress on the root system.

If all this seems a bit too hard for "just a Boronia" think of what effort you have put in for some other special plant, be it Australian or exotic. Are not all plants special? Plants treated well respond with kind.

For those who have given up, or whose conditions are such that none of the above seems practical, most of the Rutaceae make excellent container plants. Given an open free draining potting mix and light fertilising, they can be grown to quite a size. Watering and pruning are of course necessary.

An advantage of containerised plants is that they can be moved to take advantage of winter sun or summer shelter, and brought close by to enjoy during flowering.

To build up confidence with these plants, try first some of the easier to grow species, and have another go at that blessed (cursed) Boronia megastigma maybe next year.

If you really must have a Brown Boronia this year, pot your purchase up immediately to the next size pot to give it a little room to move, enjoy the flowers, then take some cuttings so that you will have some smaller, better balanced plants to try in the garden.

The flowers of, Boronia megastigma "Harlequin" are more attractive than and just as fragrant as those of the common "Brown Boronia". Unfortunately the plant retains the cultivation problems of the more common form. Select the thumbnail image or plant name for a higher resolution image (32k).

Propagation

There are different mixes for setting cuttings, different types of hormone powders, gels and liquids, and different times to take cuttings. Simple advice is to use the method with which you have reasonable success.

For those who have not yet tried, could I suggest that the well tested mix of 3 parts sharp coarse sand and 1 part finely sieved peat moss is the ideal way to start. For our own use we need just a few cuttings to start with, and I use 100mm square punnets which hold between 15 and 20 cuttings comfortably.

The use of hormones to promote root initiation is optional, but most growers feel they get better results with hormones. The most common among Study Group members is a powder suitable for semi hardwood cuttings, about 2000 ppm. I have trialled mixes up to 8000 ppm with varied results, but as most of our material is on the soft side, a lower concentration is most suitable. It is difficult to say that you definitely get better results by using hormones, but the general consensus is that you do get a better root structure.

Some means of maintaining humidity is necessary to prevent the cuttings drying out, and to provide a means for the pieces to maintain life without a root system. Simple structures such as upturned drink bottles, plastic bags held on a wire frame, small plastic frames, or even glass houses all prove suitable. The important thing is to ensure that the container does not get too hot, and so cook the plants. The smaller the container, the hotter it will get, so things like bottles or plastic bags need to be well sheltered. Larger structures can be more exposed to the sun, but need protection too during the hotter months.

Let's do some cuttings

Members of the Study Group from all over Australia have relayed information pertinent to their area, and a few generalisations can be made.

They all recommend cuttings of firm new growth produced after flowering. This can mean any time from October in warmer areas to January or later in cooler climates. A good rule to try is to let the plants grow about 150mm in the Spring growing season, prune the plant by one third as recommended earlier, and use as cuttings the tips which are neither too soft so that they would quickly wilt, nor too woody so that striking would be too slow. Tips are too soft when, if bent down they remain limp. Wood is too hard when, if bent, it snaps or fractures. Somewhere between these 2 extremes is the ideal cutting.

Cuttings taken from February to April produce excellent results. Pruning after flowering will produce a flush of new growth which will be ideal propagating material. Cuttings taken at this time will usually strike within 6 weeks or so, and be potted well before the cold weather and short days cause plants to hibernate.

Listed below are plants which are fairly easy to strike.

Those species with hairy foliage, which can rot quickly under mist conditions, are better grown in a dry area of the propagating house if mist is used. One idea worth trying is to set the cuttings in the propagation house and cover with a plastic bottle; a mini glasshouse within the propagation house. I use opaque orange juice bottles with the bottom removed with some success for plants like Boronia mollis and Diplolaena species.

Boronia microphylla is an attractive small shrub found in coastal and mountain habitats in southern Queensland and New South Wales. Select the thumbnail image or plant name for a higher resolution image (29k).

Some of the difficult plants like Boronia edwardsii, inornata, ledifolia and purdeiana, Eriostemon australasius, Geleznowia verrucosa, Phebalium coxii and Philotheca salsolifolia respond also to this treatment.

The bottle needs to be removed periodically to inspect the cuttings: too dry, too wet, and to remove any rotting leaves before fungal problems set in. Washing the bottle regularly (weekly) in a solution of Biogram or similar should prevent fungal problems.

Some members have trialled grafting as a means of propagating difficult species, using Correa spp. as the stock. Early results are encouraging, but Correa stems are often too large for the slender Boronia scion to get a complete union. I have just begun trials using Boronia clavata as a stock for B.megastigma, which may be successful. B.clavata is very adaptable in a range of soils and withstands winter wet very well.

Australian Plants online - December 1996
The Society for Growing Australian Plants