Robert Tcolton And Gjohn Murtagh

NSW Agriculture, Orange, Australia; Agricultural Water Management, Goonellabah, Australia

INTRODUCTION

In its natural habitat, the tea tree plant grows as a small 5-8 metre tree. In commercial plantations however, it is grown as an intensive row crop. It is planted at high densities to maximise leaf and oil production per hectare and is harvested every six to eighteen months by cutting the whole plant close to ground level Plantations are currently established using wild seed, with some screening for chemotype, but there is considerable plant to plant variation. The variation will be less with the new varieties under development as discussed in Chapter 7.

SITE REQUIREMENTS

Tea trees are hardy perennial plants, adapted to a wide range of soil types. They are drought tolerant and can survive flooding and fire (Murtagh 1996a). However, while the trees are very adaptable, the production of consistent high yields of oil requires fairly specific climate and soil conditions.

Tea tree grows naturally on damp soil within humid, sub-tropical areas of the north coast of New South Wales (NSW) (Anderson 1956). Sites which mimic these conditions have proven to be highly productive for commercial plantations as have warmer districts in Queensland. However, other coastal areas with poorer soils and lower rainfall supplemented with irrigation are also suitable sites for productive plantations.

CLIMATE

As a native of wet sub-tropical areas, tea tree responds to warm temperatures and a continuous supply of moisture. In the northern rivers area of NSW where most tea tree oil is produced, annual average rainfall generally ranges from 1000-1600mm. Rainfall has a strong summer incidence with the driest conditions occurring during July-September (Bureau of Meteorology 1988).

Although tea tree needs ample moisture to grow, it can survive in very dry conditions. In extreme drought, it will shed its leaves and will re-shoot after rain (Small 1981a).

High relative humidity appears to be important in achieving high oil yields, but the effect has not been fully quantified. In a glasshouse study, Murtagh and Lowe (1997) found that oil concentration was significantly greater when relative humidity was high (81% min) than when it was low (40%). Oil concentration in the leaf is known to fluctuate seasonally as well as from day to day (Murtagh and Etherington 1990). The reasons for this are complex and are not fully understood but relative humidity levels seem to be a factor in the generally lower levels of oil in the leaf during the dry winter-spring period. (Drinnan 1997b).

The peak temperature for tea tree growth exceeds 30°C (Curtis 1996). In this controlled environment experiment growth increased with increasing temperature up to the highest level tested of 35°C day/30°C night temperature. This represents an unusual tolerance to high temperatures. In north Queensland, annual growth rates in plantations are about 50% higher than in northern NSW (Drinnan 1997b). In the northern rivers area, winter temperatures restrict growth so that crops are dormant or near dormant for 3-4 months. Sustained growth starts in September when mean air temperature reaches 17°C and finishes about the end of May (15.5°C) (Murtagh 1996b). Allowing for differences between soil and air temperatures in spring and autumn, 16°C is considered the lower limit for sustained growth.

Temperature affects tree growth rates and it also appears to affect the concentration of oil in the leaf. In NSW, oil concentration typically increases from 40-50mg/g of dry leaf in September to 60-90mg/g in January. It begins to decline during March and falls most rapidly during winter. The lower September figure occurs further away from the coast where frosts are common (this volume, Chapter 6). However, warmer night temperatures such as those experienced in the tropics do not adversely affect oil concentration as was once feared (Southwell and Wilson 1993).

Frost will severely damage young growth but the height of older trees offers a degree of protection and damage is normally minimal (Colton and Murtagh 1990).

Plantations are currently located along the sub-tropical and tropical coastal strip from Port Macquarie to far north Queensland. This area provides a long warm to hot growing season, moderate to high rainfall sometimes supplemented with irrigation and high to very high humidity. In this environment, well managed plantations produce average oil yields of 150-250kg/ha/yr with the higher yields more common in the more tropical areas.

Experience with tea tree in cooler, drier inland areas is very limited. Trees will undoubtedly grow and produce oil in many areas but yields will be lower and the economics more questionable.

Trees grow best in sheltered areas where they are protected from the drying effects of wind.

NATURAL HAZARDS

Several studies have shown that the genus Melaleuca is very tolerant of flooding and waterlogging (Barlow 1988; van der Mozel et al. 1991). Established M. alternifolia plants can survive total immersion for short periods, up to seven days, without plant loss. Partial immersion for periods of 1-3 months will not kill established plants but will reduce their rate of growth (Colton and Murtagh 1990).

Gomes and Kozlowski (1980) found that seedlings of M. quinquenervia were very tolerant of flooding for up to 90 days. Experience with M. alternifolia is that seedlings will not survive flooding when they are completely immersed.

Tea tree can tolerate and recover from bushfires with new shoots emerging from charred stems, similar to the way eucalypts recover after fire.

SOILS

In its natural environment, tea tree is found on alluvial soils in swampy areas and along drainage lines of the coastal river flood plains, but it will grow well on a range of soils provided there is adequate moisture (Weiss 1997). However, it is less well suited to shallow light sandy soils, heavy clay soils and acid sulfate soils.

Trees grow best on deep sandy loams or friable loams which retain deeper moisture over the dry season or have irrigation available. Light, sandy soils, having little organic matter, do not retain their moisture or nutrients and require regular irrigation over the dry season to maintain growth. Lighter soils are much less risky if they have a permanent watertable within reach of the tree roots to provide a buffer against dry spells. These sites are more likely to be found on a valley floor than on ridges and hillsides.

Heavy clay soils will grow tea tree, but growth is often impeded by the high bulk density and lack of oxygen in these soils. Adequate surface drainage to allow water to get away after heavy rain or flooding is essential on heavy textured soils and those in low lying situations. Poor drainage can limit access to plantations during the wet season, preventing timely harvesting and management operations. Soil compaction is also a common problem following the use of harvesting and other heavy equipment during wet periods.

Good moisture holding capacity is an important consideration where production relies on rainfall and some compromise is often necessary between the heavier, moisture retentive soils and the lighter, high growth soils which may tend to dry out.

Acid sulfate soils are found in the wetlands of many of the coastal rivers. These soils contain iron pyrite or iron sulphide. While they are covered by water and thus insulated from the atmosphere, this pyritic layer is innocuous. When the land is drained and oxygen reaches the iron sulphide, it is oxidised to become sulphuric acid (Creagh 1993). The combination of sulphuric acid and aluminium which is also released is very deadly to both vegetation and most soil and water life forms. It is better to avoid shallow acid sulfate soils than to consider managing and treating them.

Tea tree grows naturally on acid soils pH 4.5-5.5 (measured in water) but also grows well on soils with neutral reaction (Weiss 1997). It will not thrive on alkaline soils. It is intolerant of saline soils, particularly if they are also prone to waterlogging (van der Moezel et al. 1991).

Plants grown on fertile soils will carry a much heavier canopy than those where soil fertility is low (Colton and Murtagh 1990). Fertiliser can substitute for low fertility on some soil types (Drinnan 1997b).

CHARACTERISTICS OF MAJOR GROWING AREAS

The tropical and sub-tropical east coast of Australia experiences high rainfall and humidity in summer followed by dry winters and springs. Towards the north, temperatures are higher, particularly in winter, and rainfall becomes increasingly summer dominant. There is a much more pronounced dry season in winter and spring. These differences can be seen in Tables 1-3.

Rainfall in the northern rivers area of NSW is significantly higher than in the Mareeba-Dimbulah area of north Queensland, and is much less summer dominant (Table 1).

Daily maximum temperatures for the northern rivers are similar to those at Mareeba in January, but January minima and winter maxima and minima are much lower at the southern centres (Table 2).

Table 1 Rainfall incidence and seasonal distribution

December-May

June-November

Year

mm

mm

mm

Mareeba

825

86

914

Dimbulah

632

96

718

Broadwater

947

534

1481

Casino

731

377

1108

Grafton

691

383

1074

Table 2 Mid summer and mid winter temperatures

January

M

Mean daily

Mean daily

Mean daily Mean daily

maximum (°C)

minimum (°Q

maximum (°Q minimum (°C)

Mareeba

31.2

20.8

25.3 11.2

Broadwater

28,2

19.5

19.2 7.4

Casino

31.4

18.7

21,3 6.3

Bellingcn

29.5

17.9

19.9 4.6

Table 3 Relative humidity

February

October

9.00 am

3.00 pm

9.00 am

3.00 pm

%

%

%

%

Mareeba

79

66

58

44

Casino

81

58

68

50

Grafton

79

61

67

51

February relative humidity levels are similar in the northern rivers and at Mareeba, but October levels tend to be lower in Mareeba. Humidity is highest in February-March and lowest in the September to November period in both areas (Table 3).

The Mareeba-Dimbulah area is on the edge of the Atherton Tableland, west of Cairns at an elevation of 400-500 metres. Being west of the coastal ranges, rainfall is considerably lower than on the coast and falls away sharply from east to west. Although the area is located in the tropics, its altitude can lead to light frosts, with an occasional heavy frost. Most tea tree is grown on light, sandy, tobacco-growing soils which need regular fertiliser applications and irrigation. These soils were selected because tea tree is grown as a substitute for tobacco, and do not necessarily represent the best soil type for tea tree.

In the northern rivers area, tea tree is grown largely within its natural habitat on the coastal river flood plains. Soils are more likely to be medium to heavy textured and summer waterlogging often occurs. Light frosts occur in most winters.

PLANT PROPAGATION AND ESTABLISHMENT Nursery Seedling Production

Since tea tree seed is particularly small and seedlings have a poor competitive ability, plantations are established from seedlings raised in nurseries or seed beds (Plate 6) (Murtagh 1991 a). Seed (Plate 14) is collected from high yielding individual trees (Plate 2, 13) (List et al. 1996) which have been screened for chemotype and are known to yield well. Seed with higher yield and oil concentrations is expected to be available from the tea tree breeding program from 1998.

A single productive tree will yield 50-400 grams of seed and a gram of seed will usually contain 8000-12000 viable seeds (Colton and Murtagh 1990). Tea trees flower (Plate 3) in spring and seed is normally mature after fifteen to eighteen months. After maturity, seed remains viable in the capsule and can be collected at any time of the year. If kept in a warm dry place after collection, capsules open and release the seed.

Small plantations (a few hectares) are usually established with seedlings purchased from a commercial nursery. Larger plantations find it more economical and convenient to set up their own nursery and produce seedlings to suit their own planting schedule.

Seedlings can be grown either in cell type containers or as open rooted seedlings. Containerised seedlings are easier to grow, are well adapted to mechanised transplanting and suffer less transplanting shock than open rooted seedlings. They can be planted over most of the year and will suffer fewer losses even when weather conditions are less than ideal provided irrigation is available. Open rooted seedlings are much cheaper to produce and can be quite reliable for those with appropriate skills and experience.

Seedlings usually spend 6-8 weeks in a plastic tunnel and a further 6-8 weeks hardening off under shade cloth. They are then usually 15cm high and ready to plant out.

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