Growing boronia in Western Australia

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This web page outlines the commercial cultivation of Boronia megastigma (brown boronia) and B. heterophylla (pink).

Boronias are renowned for their spectacular floral displays and unusual scent. Boronia megastigma has been harvested for many years from natural bush stands in Western Australia for cutflowers and essential oil production.

Interest in this species for essential oil production has declined due to the widespread production and use of synthetic counterparts for boronia oil, however niche markets remain.

Boronia heterophylla has been intensively cultivated in Western Australia for several years for cutflower production for export to markets in Asia, Europe and Canada.

A ban on Rutaceous plants prevents boronia flowers from entering the United States. Major obstacles to expansion of exports are the short flowering season and limited colour range.

Brown boronia (Boronia megastigma) is used for oil production as well as being picked for cut flowers
Brown boronia (Boronia megastigma) is used for essential oil as well as cutflowers

Site selection and preparation

Boronia generally prefers slightly acid (pH 5.5 to 6.5 in water) and well-drained soils. A cool period is required to initiate flowering. Strong winds can damage plants, so avoid windy sites or provide suitable windbreaks.

Test potential sites for the presence of Phytophthora and nematodes. Phytophthora affects a wide range of native plants and is almost impossible to eradicate. If sampling reveals the presence of this pathogen, find an alternative site.

If root knot nematode (Meloidogyne sp.) is found, choose a clean site or treat the planting area with a soil fumigant such as metham sodium or a nematicide.

Soil analysis before planting to determine the residual phosphorus level is suggested. Soils with a very high residual phosphorus level should be avoided. Less than 40 parts per million is ideal.

Land preparation

Prepare the ground at least three months before planting by removing large tree roots and cross-ripping the site. Most sites will benefit from deep ripping but allow the ground to settle before planting, especially on duplex soils.

To minimise the risk of introducing or spreading Phytophthora, clean all soil from machinery before use.

Species selection

The Boronia genus contains 95 species. B. heterophylla and B. megastigma are the main species grown for cutflower production. B. megastigma is the only one grown for essential oil production.

Selection programs in the 1990s produced a range of cutflower cultivars that were registered under Plant Breeder's Rights legislation, but only Purple Jared remains registered today.

When choosing the species and number of plants to be grown, consult exporters, nurserymen and experienced growers to determine likely returns, market trends and  demand.

Most plants are bought as rooted cuttings from specialised and reputable nurseries. Allow at least four months between ordering and delivery. The best time for taking cuttings is November to May.

Planting

Boronia plants are difficult to cultivate and attention to detail is important to maximise survival. Even with the best management, an annual loss rate of at least 5% is not uncommon.

Spring is the preferred time for planting. The plants should be disease-free, no more than 100mm high and propagated in 120mm tree tubes or similar and not root bound. Remove them carefully from the potsand place in hole with minimum disturbance when planting.

Sand blasting and strong winds can damage young plants and a suitable windbreak should be in place well before planting. Young plants can be protected with artificial windbreaks, ‘grow cones’ or a cereal crop.

The planting layout depends on irrigation design, bed system and species. Single rows 1.5m apart in groups of six, 1m apart in a row (leaving a workway of 3-4m where required, or every six single rows) provide a reasonable plant density of more than 5000 plants per hectare.

Double rows with 0.7m between plants in a staggered pattern in beds 1.2m wide with 0.6m between the double rows on 3–4m centres are sometimes used, giving 4700 and 3500 plants per hectare.

Most growers plant in blocks with a roadway between blocks to allow for boom spraying and machinery access during harvesting.

In Tasmania, higher densities up to 19 500 plants per hectare are used for B. megastigma. Aligning plant rows north to south is preferred.

Mulching

Boronia plants generally grow best when the soil area surrounding them is kept weed-free, moist and cool. This can be achieved through artificial or organic mulches, such as plastic mulch, weed mat or wood chips.

No research has been done on the effect of the different mulches on boronia production. However, a mulch of composted straw spread around the drip zone of plants gives some control of Phytophthora by encouraging the growth of organisms antagonistic to the pathogen.

Boronia does not like warm moist soils and losses from root rots can occur.

Fertiliser

Boronia responds to applied fertiliser, especially nitrogen. For spring flowering species, vegetative growth occurs from mid-spring to autumn with a peak in summer. Most nitrogen is applied early in the growing season, as late applications of fertiliser can reduce yield.

Many growers stop or significantly reduce fertiliser applications after January. For young plants or immediately after planting, some growers use a short-term slow release complete fertiliser.

Research in Tasmania under field conditions found that applying nitrogen at up to 100kg N/ha as calcium nitrate in October gave best results on B. megastigma. At this high rate, the plants were susceptible to wind damage and some problems with mechanical harvesting were noted.

A split application in October and January is suggested to reduce the risk of nitrogen loss in October due to leaching by heavy rains.

For B. megastigma, nitrate forms of nitrogen appear to give better results than urea or slow release IBDU. A suggested fertiliser program based on Tasmanian and Perth data is provided in Table 1.

Table 1 Nutrients used in commercial production of boronia

Nutrient

B. megastigma1
  (kg/ha/y)

B. heterophylla2
(kg/ha/y)

Suggested program3
(kg/ha/y)

NITROGEN (N)

50

90

50–75

PHOSPHORUS (P)

79

10

10

POTASSIUM (K)

100

130

100

SULPHUR (S)

37

-

-

TRACE ELEMENTS

     

ZINC (Zn)

5

0.004

 

COPPER (Cu)

3.2

0.011

Apply a proprietary trace element mix at label rates and monitor NPK and trace element levels with an annual leaf test

IRON (Fe)

-

0.021

BORON (B)

0.5

0.006

MANGANESE (Mn)

-

0.001

1 Tasmanian results (adapted from Roberts and Menary 1994).

2 P Watkins, personal communication.

3 Suggested generic program for boronia. It may be advantageous to alternate calcium and magnesium in the mix, depending on other fertilisers' formulation.

Trace elements should be applied in areas that are deficient in micro-nutrients. The correct rates have not been determined but some growers apply a proprietary trace element mix at label rates annually. Fertiliser can be applied as a solid, as a liquid through the trickle irrigation, or as a combination of the two.

If applying NPK as a solid fertiliser, do not place it too close to the plant as damage and even plant death can occur.

Irrigation

Irrigation or rainfall is essential for successful establishment and plant growth. The total dissolved salts (TDS) of irrigation water should not exceed 150mS/m. Most growers use drip irrigation - the type of dripper depends on soil type and ground slope. Boronia requires from 2 to 10 litres per plant each day, applied as a split irrigation at higher levels.

The best replacement of evaporation is still to be determined, but 100% replacement is suggested. Irrigation may be required at least daily during hot weather on sandy soils but only every two or three days on heavier soils.

Using soil moisture monitoring equipment to schedule irrigation is recommended. Boronia plants do not generally recover from severe water stress. Waterlogging can result in plant death from soil-borne diseases.

Weed control

Mulching generally provides good weed control. If hand weeding is required, do not disturb the soil around the base of the plant as root damage and disturbance can increase the risk of root diseases.

Grasses can be controlled with a systemic herbicide such as Fusilade®. Broad-leaved weeds are more difficult to control but good results are usually achieved by spraying with a hooded wand using a non-selective herbicide like Basta or glyphosate. No selective broad-leaved weed herbicides are registered for boronia. Weedmat or mulch can be used to reduce weed levels.

Pests and diseases

A number of pests attack boronia, including nematodes, black beetle, stem borers, grasshoppers, Rutherglen bug, scale and psyllids. Nematodes and black beetle are the most devastating pests in Western Australia and are best controlled by a pre-plant application of suitable insecticide.

Wind damage and stem-chewing insects may not directly kill plants but their damage allows various secondary soil-inhabiting fungi to enter the plant and often results in death after the initial damage.

Insects that do not cause major problems on boronia can still cause flowers to be rejected if found in shipments to most importing countries. Monitoring and field control of insects is essential, coupled with some postharvest treatment, as disinfestation after harvest does not usually provide a high enough level of kill.

The most important disease of boronia in Western Australia is caused by the fungi Phytophthora spp., commonly known as ‘dieback’. Dieback affects the root system and can kill plants at any age. Plants may die suddenly or become chlorotic (pale) and grow poorly.

Boronias are also susceptible to Pythium spp. Plants affected with Phytophthora or Pythium may wilt or die during warm weather. Phosphite is used to control both these diseases.

Another species that can devastate plants is boronia rust (Puccinia boroniae) which affects many species and tends to occur during autumn and late spring.

Botrytis can infect cultivated material. Warm humid conditions favour the incidence of this disease. Damage is not often seen in the field, but serious losses can occur in packed material. Control starts in the field with regular applications of chemicals such as Rovral®, which must be applied fortnightly from bud initiation until harvest is complete. During wet weather, more frequent spraying may be required.

If growing B. megastigma for the essential oil market, check with the processor before spraying, as some pesticides may give undesirable residues in the oil product.

Contact your local agronomist to discuss chemical rates and application times for disease control.

Contact information

Kevin Seaton
+61 (0)8 9368 3244
Page last updated: Friday, 9 December 2016 - 9:58am

Authors

Aileen Reid
Kevin Seaton