Medicinal and Aromatic Plants—Industrial Profiles


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ROBERT T.COLTON AND G.JOHN MURTAGH
76
Single or multi-row heavy duty forage harvesters are commonly used. They must be
robust enough to handle the woody stems, have a high throughput capacity and be operated
by a powerful tractor, given the bulk of material commonly handled (Colton and Murtagh
1990).
Field bins are usually tipping trailer bins with a capacity of up to 3 tonnes. They can also
be cartridges which are transported to the distillery on a trailer and lowered onto a fixed
base. Once distillation is complete spent leaf is tipped out and the bins are returned to the
field for refilling.
Coppicing
Tea tree has a strong coppicing ability (
Plate 9
) (Small 1981; Colton and Murtagh
1990) and vigorous coppicing is vital for rapid regrowth. Harvest initiates coppice
regrowth, with new shoots beginning to appear within a few weeks. Blake (1983) found
that coppicing vigour is influenced by a range of factors including month of harvest.
He quoted a number of species which gave the best coppicing after cutting in winter or
early spring, but this was not a universal finding. While Murtagh (1996b) observed few
differences in coppicing vigour following different months of harvest, anecdotal evidence
in NSW supports slower early growth following late autumn harvests, probably due to
the onset of winter dormancy.
Major plant losses have been recorded where plants were harvested under high water
and temperature regimes in the glasshouse (Murtagh and Lowe 1997). Following harvest,
guttation water accumulated on cut stems and provided favourable conditions over several
days for pathogen invasion. Cut surfaces subsequently blackened and plants died. Drinnan
(1997a) reports similar experiences following field harvesting during the wet season in
north Queensland. Losses only occurred when summer harvests occurred on very wet soil
and plants continued to pump water through the cut surface for some time after harvest.
Plants develop new shoots as normal but symptoms begin to appear 3–4 weeks after harvest
and plants die. Plant surfaces above and below ground become blackened as plants die.
Queensland Department of Primary Industry pathologists have identified the soil borne
pathogen as charcoal root disease (Macrophomina spp). Plants from some seed sources
appear more susceptible than others. Losses, which have been as high as 60%, have been
greater from first harvests in new plantations but have also occurred following coppice
harvests.
REFERENCES
Anderson, R.H. (1956) The Trees of New South Wales (Government Printer. Sydney).
Barlow, B.A. (1988) Patterns of differentiation in tropical species of Melaleuca L. (Myrtaceae).
Proceedings of the Ecological Society of Australia, V15, 239–247.
Blake, T.J. (1983) Coppice systems for short rotation intensive forestry: the influence of cultural,
seasonal and plant factors. Australian Forest Research, 13, 279–291.
Bolton, K.G.E. and Greenway, M. (1995) Growth characteristics and leaf phosphorus
concentrations of three Melaleuca species sand cultured in different effluent concentrations.
Proc. National Conference on Wetlands for Water Quality Control , Townsville, September
1995.
Copyright © 1999 OPA (Overseas Publishers Association) N.V. Published by license under the Harwood Academic Publishers imprint,
part of The Gordon and Breach Publishing Group.

CULTIVATION
77
Bureau of Meteorology (1988) Meteorological Summary, July 1988. In Climatic Averages Australia.
Australian Government Publishing Service, Canberra.
Clarke, B. (1995) NSW Agriculture. Unpublished results.
Clarke, B. (1997) NSW Agriculture. Personal communication.
Colton, R.T. and Murtagh, G.J. (1990) Tea-tree oil—plantation production. NSW Agriculture and
Fisheries, Agfact P6.4.6.
Creagh, C. (1993) Working together with acid sulphate soils. Ecos 77 CSIRO1993.
Cromer, R.N. (1996) Silviculture of eucalypt plantations in Australia. In P.M.Attiwil and M.A.Adams
(eds.), Nutrition of Eucalypts, CSIRO, Australia, pp. 259–273.
Curtis, A. (1996) Growth and Essential Oil Production of Australian Tea Tree (Melaleuca alternifolia
(Maiden and Betche) Cheel), Master of Agricultural Science Thesis, The University of
Queensland.
Diatloff, E. (1990) Effects of applied nitrogen fertiliser on the chemical composition of the essential
oil of three Leptospermum spp. Australian Journal of Experimental Agriculture, 30, 681–685.
Drinnan, J.E. (1997a) Queensland Department of Primary Industries. Personal communication.
Drinnan, J.E. (1997b) Development of the North Queensland Tea Tree Industry. Final report for project
DAQ-184A. Rural Industries Research and Development Corporation.
Drinnan, J.E., Virtue, J.G., Murtagh, G.M. and Lowe, R.F. (1997) Queensland Department of Primary
Industries—NSW Agriculture. Personal communication.
Gomes, A.R.S. and Kozlowski, T.T. (1980) Response of Melaleuca quenquenervia seedlings to flooding.
Physiologia Plantarum, 49, 373–377.
Grove, T.S., Thomson, B.D. and Malazcjuk, N. (1996) Nutritional physiology of Eucalypts: Uptake,
Distribution and Utilisation. In P.M.Attiwil and M.A.Adams (eds.), Nutrition of Eucalypts, CSIRO,
Australia, pp. 77–108.
Judd, T.S., Attiwil, P.M. and Adams, M.A. (1996) Nutrient concentrations in Eucalyptus: a synthesis
in relation to differences between taxa, sites and components. In P.M.Attiwil and M.A.Adams
(eds.), Nutrition of Eucalypts, CSIRO, Australia, pp. 123–153.
Khan, A.G. (1993) Occurrence and importance of mycorrhizae in aquatic trees of NSW, Australia.
Mycorrhizae, 3, 31–38.
List, S.E., Brown, P.H., Low, C.S. and Walsh, K.B. (1996) A micropropagation protocol for Melaleuca
alternifolia (tea tree). Australian Journal of Experimental Agriculture, 36, 755–760.
Mengel, K. and Kirkby, E.A. (1987) Principles of Plant Nutrition. International Potash Institute:
Wonblaufen-Bern, Switzerland. 4th Edn. Ch. 2, pp. 25–112.
Murtagh, G.J. (1991a) Tea tree oil. In New Crops (eds. R.J.Jessop and R.L.Wright), Inland Press,
Melbourne, pp. 166–174.
Murtagh, G.J. (1991b) Irrigation as a management tool for production of tea tree oil. Final report for
project DAN-19A. Rural Industries Research and Development Corporation.
Murtagh, G.J. (1996a) Challenges facing Australian Agriculture—Tea tree oil. Proc. Aust. Institute of
Valuers and Land Economists Rural Conf. Kooralbyn, July 1996.
Murtagh, G.J. (1996b) Month of harvest and yield components of tea tree. I. Biomass. Australian
Journal of Agricultural Research, 47, 801–815.
Murtagh, G.J. (1996c) The irrigation response by tea tree and implications for effluent reuse. In
P.J.Polglase and W.M.Tunningley (eds.), Some Application of Wastes in Australia and New Zealand:
Research and Practice, CSIRO Forestry and Forest Products, Canberra, pp. 137–141.
Murtagh, G.J. and Etherington, R.J. (1990) Variation in oil concentration and economic return from
tea tree (Melaleuca alternifolia Cheel) oil. Australian Journal of Experimental Agriculture, 30,
675–679.
Murtagh, G.J. and Lowe, R.F. (1997) NSW Agriculture. Unpublished results.
Murtagh, G.J. and Smith, G.R. (1996) Month of harvest and yield components of tea tree. II Oil
concentration, composition and yield. Australian Journal of Agricultural Research, 47, 817–827.
Peak, C.M. (1982) Essential oil production from Melaleuca alternifolia. Wollongbar Agricultural
Research Centre, Biennial Report, p. 53.
Reilly, T. (1988) Investment in tea tree plantations. Reports of the 1st Tea Tree Oil Seminar, Lismore,
November 1988, pp. 42–47.
Copyright © 1999 OPA (Overseas Publishers Association) N.V. Published by license under the Harwood Academic Publishers imprint,
part of The Gordon and Breach Publishing Group.

ROBERT T.COLTON AND G.JOHN MURTAGH
78
Reuter, D.J. and Robinson, J.B. (eds.) (1986) ‘Plant Analysis’ Inkata Press, Melbourne.
Small, B.E.J. (1981a) Department of Agriculture, NSW. Personal communication.
Small, B.E.J. (1981b) Effects of plant spacing and season on growth of Melaleuca alternifolia and
yield of tea tree oil. Australian Journal of Experimental Agriculture, 21, 439–442.
Small, B.E.J. (1986) Tea tree oil. Department of Agriculture, NSW Agfact P6.2.1, 2nd edition.
Southwell, I.A. and Wilson, R.W. (1993) The Potential for tea tree oil production in northern Australia.
Acta Horticulture, 331, 223–227.
Van der Moezel, P.G., Pearse-Pinto, G.V.N. and Bell, D.T. (1991) Screening for salt and waterlogging
tolerance in Eucalyptus and Melaleuca species. Forest Ecology and Management, 40, 27–37.
Virtue, J.G., Murtagh, G.M. and Lowe, R.F. (1997) NSW Agriculture. Personal communication.
Virtue, J.G. (1997) Weed interference in the annual regrowth cycle of plantation tea tree (Melaleuca
alternifolia). PhD Thesis. The University of Sydney.
Weiss, E.A. (1997) Essential Oil Crops, CAB International, Oxford, pp. 302–311.
Copyright © 1999 OPA (Overseas Publishers Association) N.V. Published by license under the Harwood Academic Publishers imprint,
part of The Gordon and Breach Publishing Group.

Plate 6 Cell grown Melaleuca alternifolia seedlings in the nursery (R.Colton)
Plate 7 A four-row planter adds fertiliser and water to the seedlings as they are planted (R.Colton)
Copyright © 1999 OPA (Overseas Publishers Association) N.V. Published by license under the Harwood Academic Publishers imprint,
part of The Gordon and Breach Publishing Group.

Plate 8 Herbicides keep seedlings weed free for up to 12 weeks (R.Colton)
Plate 9 Trees coppice vigorously after harvest (R.Colton)
Copyright © 1999 OPA (Overseas Publishers Association) N.V. Published by license under the Harwood Academic Publishers imprint,
part of The Gordon and Breach Publishing Group.

81
4. WEED MANAGEMENT IN TEA TREE PLANTATIONS
JOHN G.VIRTUE
Animal & Plant Control Commission, Primary Industries, SA, Australia
INTRODUCTION
Weed management integrates preventative and control techniques to minimise crop yield
loss due to weed interference. Preventative techniques include quarantine to avoid new
weeds, and providing conditions unsuitable for weed establishment. Control techniques
include chemical, physical and biological methods of weed destruction, and manipulation
of crop competitiveness. Weed interference slows crop growth by competition for resources
of sunlight, soil water and soil nutrients, and by production of growth-inhibiting chemicals.
Weed interference reduces tea tree yield during both the initial establishment phase and in
the annual regrowth cycles. This chapter describes the nature of the weed problem in tea
tree plantations and the effect of weeds on tea tree yield. An understanding of the mechanisms
of weed interference in tea tree plantations then provides the basis for discussing effective
and sustainable weed management techniques. The chapter is based on research conducted
on the north coast of New South Wales (NSW), Australia, where tea tree was first established
in plantations.
WEEDS IN TEA TREE PLANTATIONS
A simple definition for a weed is an unwanted plant. In cropping systems weeds are primarily
considered as plants which interfere with crop growth and consequently reduce crop yield.
Table 1
 lists those weeds which were regionally widespread in tea tree plantations in a 1992–
3 survey. The predominant weeds were herbaceous annuals and perennials, and included
broadleaf, grass and sedge weeds. Annual weeds go through a rapid succession of germination,
growth, flowering, seed set and death. Annual weeds readily colonise bare ground, as occurs
after a soil cultivation. Perennial weeds have a life cycle that alternates between growth and
flowering phases over many years. Perennial weeds are often favoured by agricultural systems
which have minimal soil disturbance. Some perennials reproduce vegetatively from root
fragments or bulbs, and soil cultivation aids their spread within a field.
Environmental conditions in a typical tea tree plantation favour strong weed growth. In
a 1992 survey of 28 Australian plantations (Virtue 1997) the majority were located on the
upper north coast of NSW. The subtropical climate is characterised by relatively high annual
rainfall (ranging from 1,000–1,400mm), wettest in February—March (late summer) and
driest in August—September (early spring). Winters are relatively mild and summers warm
and humid. Such a climate allows both winter and summer-growing annual weeds, and
favours warm-season perennial grass weeds. Most plantations in the survey were established
Copyright © 1999 OPA (Overseas Publishers Association) N.V. Published by license under the Harwood Academic Publishers imprint,
part of The Gordon and Breach Publishing Group.

JOHN G.VIRTUE
82
on alluvial soils and were prone to flooding and waterlogging. The high fertility and
moisture levels of such soils favour vigorous growth of herbaceous weeds.
THE EFFECTS OF WEEDS IN TEA TREE PLANTATIONS
In a 1992 survey (Virtue 1997), approximately 80% of tea tree growers with plantations
larger than ten hectares considered weeds to be a major limit to production. The main
effect of weeds in tea tree plantations is a reduction in tree growth due to weed
interference, and thus a decrease in oil production. Other indirect effects of oil
contamination and harvest inefficiency are minimal. Weeds do have some beneficial
effects on soil health.
Weed Interference and Tea Tree Oil Yield
Weed interference encompasses the two mechanisms by which weeds reduce crop
yields; competition and allelopathy. Weed competition is the use of sunlight, soil
water and/or soil nutrients at the expense of the crop. Allelopathy is the production
by weeds of chemicals which can inhibit crop growth. Tea tree oil yield quantity
(kgha
-1
) is the product of leaf biomass and leaf oil concentration. Yield quality is
determined by the oil’s chemical composition. Experiments have shown weed
interference reduces leaf biomass yield of both seedling and regrowth tea tree. No
effects of weed interference on leaf oil concentration and chemical composition have
been detected.
Weed interference strongly reduces growth of seedling tea tree. McMillan and Cook
(1995) observed a 97% reduction in tree mass where annual grasses were uncontrolled
in the period between 31 and 180 days after planting (
Figure 1
). Even in the short
period of 31 to 55 days after planting, annual grasses reduced tree mass by 56%. Such
high yield reductions due to herbaceous weed interference are typical for tree seedlings
(
Table 2
). Sustained weed interference can even kill tree seedlings, leading to replanting
costs.
Table 1 Weeds which were widespread over north-east NSW tea tree plantations in a 1992–3 field
survey of 25 plantations (McMillan and Cook 1995)
Copyright © 1999 OPA (Overseas Publishers Association) N.V. Published by license under the Harwood Academic Publishers imprint,
part of The Gordon and Breach Publishing Group.

WEED MANAGEMENT 83
Leaf yield reductions due to weed interference in the annual regrowth cycle are much
less than at tea tree seedling establishment, but integrated over successive cycles they are a
substantial long-term cost. Measured reductions in mature leaf biomass yield due to weeds
have ranged from 9 to 44% on a per tree basis (
Table 3
). The reductions are due to slowed
tea tree growth, and no significant effect of weed interference on the rate of tea tree leaf fall
has been detected. The magnitude of leaf yield loss increases with weed biomass, which in
turn is affected by seasonal conditions (temperature and soil moisture), tree density and
weed species. Tree density is important in calculating economic loss on a per hectare basis.
For example consider two tree densities, 15,000 and 30,000 trees ha
-1
, each with a mature
harvest oil yield of 8g tree
-1
. If weed interference reduces yield per tree by 40% for the
lower density and 25% for the higher density then the economic costs are $2,160 and $2,700
respectively (assuming an oil price of $Aus45kg
-1
). Thus the greater potential yield of high
density plantations means that weeds can cause greater economic losses. However if weed
management costs per hectare are equivalent at low and high density, then the profits from
weed control are greater at the higher tree density.
Indirect Effects
Indirect effects of weeds on tea tree yield are likely to be minor. Native pests and diseases of
tea tree are unlikely to use exotic herbaceous weeds as an alternate host plant in their life
Figure 1 Decline in tea tree seedling biomass with increasing periods of weed interference from
planting (from McMillan and Cook 1995)
Copyright © 1999 OPA (Overseas Publishers Association) N.V. Published by license under the Harwood Academic Publishers imprint,
part of The Gordon and Breach Publishing Group.

JOHN G.VIRTUE
84
Table 2 Percentage yield reductions of tree seedlings due to high herbaceous weed interference.
Reductions are relative to low weed interference or weed-free treatments
Table 3 Measured reductions in mature regrowth leaf yield of
tea tree due to weed interference
cycle. Weed biomass in harvested material should be minimal and thus not waste space in
distillation bins. Tea trees are unlikely to be harvested until they have formed a canopy,
shading then causing weed senescence. Where a plantation has wide row spacing then
harvesters are directed along the tree rows, avoiding weeds between rows. Any weed essential
oils collected during distillation would be very dilute in the tea tree oil.
Benefits of Weeds
Weeds have some advantages. They form a groundcover that protects the soil from erosion
and insulates roots from high summer temperatures. Decaying weeds add to soil organic
matter, improving soil structure and promoting biological nutrient cycling. Weeds act as a
reservoir for nutrients, reducing losses due to leaching below the root zone. Weeds can also
act as food sources for beneficial insect predators and parasites.
Copyright © 1999 OPA (Overseas Publishers Association) N.V. Published by license under the Harwood Academic Publishers imprint,
part of The Gordon and Breach Publishing Group.

WEED MANAGEMENT
85
MECHANISMS OF WEED INTERFERENCE
An understanding of the mechanisms, relative importance and timing of weed interference
in seedling and regrowth tea tree is important for informed weed management decisions.
Planting to First Harvest
Newly planted tea tree seedlings have a very restricted root system of only 2.5cm diameter,
and a shoot height of 10–15cm. Their small size makes them poor competitors against
rapidly growing weeds. A range of weed interference mechanisms is possible, with early
shading and moisture competition being of most importance.
Shading
Plants require light for photosynthesis, to produce chemical energy for growth. Tea tree
shoot growth declines quickly with increased shading. Weed shading is most likely in the
first three months after planting, when tea tree seedlings and weeds are of similar height.
This potential shading period is longer for autumn plantings as falling temperatures slow
tea tree growth (Murtagh 1996) whilst cool season annual weeds actively grow. Older tea
tree seedlings of approximately 0.5m in height are still at risk of shading by erect, broadleaf
weeds and climbing weeds.
Moisture Competition
Plant roots extract water from the soil for use in photosynthesis, nutrient transport, chemical
processes and plant turgidity. Tea tree will be prone to weed competition for water where
the root systems interact and soil water is insufficient to meet both tree and weed needs.
This is particularly likely in soils without a high water table, with low waterholding capacities
(e.g. sandy loams) and/or which receive low rainfall. The majority of tree and weed roots
occur in the surface 30cm of soil (Bowen 1985), so competition between these roots can
occur. At this soil depth herbaceous weeds can have root densities 10–100 times greater
than trees (Bowen 1985), giving the weeds a competitive advantage for water uptake. Thus
young tea tree seedlings with their shallow root systems are very susceptible to moisture
competition, and water stress results. Such a mechanism of weed interference is very
important for tree seedlings and extreme water stress can cause deaths (Nambiar and Zed
1980; Sands and Nambiar 1984). Moisture competition decreases as trees age and larger
root systems access soil water at depth.
Nutrient Competition
Plant roots extract soil nutrients for growth and chemical processes. Major soil nutrients
are nitrogen, phosphorus and potassium. Competition for soil nutrients occurs near the
soil surface where nutrients are concentrated (Nambiar and Sands 1993). As discussed
above, dense weed root systems have a competitive advantage in this soil layer. This
remains regardless of tea tree age. Competition between weeds and tea tree is strongest
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part of The Gordon and Breach Publishing Group.

JOHN G.VIRTUE
86
for nitrogen as it is the most mobile soil nutrient in both wet and dry soils (Nambiar and
Sands 1993).
Interactions
Interactions between competition for light, water and nutrients will occur. Weed
competition for surface soil moisture will also cause competition for nitrogen. Reduced
tea tree seedling shoot growth due to moisture competition increases the likelihood of
weed shading.
Allelopathy
Many plants release chemicals which are inhibitory to the growth of neighbouring plants.
Such chemicals can slow water and nutrient uptake and inhibit photosynthesis.
Allelochemical-producing weeds found in tea tree plantations include couch (Cynodon
dactylon), Farmer’s friend (Bidens pilosa), barnyard grass (Echinochloa crus-galli) and
summer grass (Digitaria sanguinalis) (Putnam and Weston 1986). Allelopathy has not been
investigated in tea tree plantations, however young tree seedlings would be most vulnerable
due to their small root systems.
Annual Regrowth Cycles
After the first harvest tea tree begins its first regrowth cycle. This is achieved by tea tree’s
strong coppicing ability from cut stumps. Shoot regrowth then reaches harvestable maturity
approximately every 12 months. Harvesting to near ground level is a major disturbance to
the normal root/shoot balance in tea tree. Changes in root/shoot relations in the annual tea
tree regrowth cycle are illustrated in 
Figure 2
 for a typical summer harvest on the north
coast of NSW.
Prior to harvest tea tree has a normal root/shoot balance, which represents an equilibrium
between carbon uptake by leaves and nutrient (and water) uptake by roots (Cannell 1985).
A typical myrtaceous root system consists of a lateral network of fine roots in the topsoil,
and a taproot and several lateral sinker roots which reach deep into the subsoil (Lamont
1978). The shoot system of regrowth tea tree consists of five or less main stems to a height
of 1.5–2.5m. Where tree rows are spaced approximately 1m or less apart canopies are merged.
The removal of all shoots at harvest suddenly gives a very high root/shoot ratio. New
shoots grow from buds in the process of coppicing. This early regrowth is vigorous. Such
vigour is attributed to the high root/shoot ratio enabling abundant uptake of soil water and
nutrients, the utilisation of stem and root nutrient reserves, and a hormone balance favouring
shoot growth (Blake 1982). The larger the tree prior to harvest then the greater this vigour.
This vigour decreases with cooler temperatures, and average daily temperatures below
approximately 16°C (Murtagh 1996) will strongly limit shoot emergence and growth. Thus
on the north coast of NSW tea tree effectively has a winter dormancy.
The rapid coppice shoot growth hastens the return to the normal root/shoot balance. This
return is also achieved by fine root death, due to a deficiency of photosynthate. In spring,
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