Medicinal and Aromatic Plants—Industrial Profiles


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part of The Gordon and Breach Publishing Group.

WEED MANAGEMENT
87
Figure 2 The annual regrowth cycle in tea tree plantations with mechanisms of weed competition
indicated. A summer harvest is assumed
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.

A.J.CAMPBELL AND C.D.A.MADDOX
88
tea tree has regained its normal root/shoot balance, with increased demands on the root
system for moisture and nutrients. Rising temperatures enable major shoot growth flushes
in mid spring and mid summer. These are accompanied by new fine root growth. Tea tree
root systems will not grow much larger with successive harvests as trees are only growing
back to their original pre-harvest size.
The changes in root/shoot relations throughout the regrowth cycle influence the
mechanisms and timing of weed interference. Harvest removes shading of the ground and
so encourages weed germination and growth. The changes in root and shoot relations during
the tea tree regrowth cycle present opportunities for different mechanisms of weed
competition (
Figure 2
). Competition for soil nitrogen is the major mechanism of weed
interference in the regrowth cycle of tea tree. Competition for light, moisture and other
nutrients are of lesser importance. Allelopathy has not been investigated in the regrowth
cycle and is not discussed.
Shading
Whilst early tea tree regrowth is of similar height to newly planted seedlings, the former has
much more vigorous height growth. Shading is only likely early in the regrowth cycle
where fast growing, climbing or upright weed species are present, and/or where the cool
temperatures of a late autumn harvest slows tea tree regrowth.
Moisture Competition
Many tea tree plantations on the north coast of NSW are established on floodplains which
have a high soil water table. Weed competition reducing surface soil moisture has not
increased tea tree water stress in such plantations (Virtue 1997). It appears that the
established root system of regrowth tea tree utilises soil water at depth to avoid moisture
competition. A similar conclusion was reached by Sands and Nambiar (1984) and Woods
et al. (1992) in young Pinus radiata plantations. Where a high soil water table is absent
(e.g. hillslopes, drought years) then tea tree would compete with weeds for surface moisture
from rainfall events. Such competition would be most intense during late regrowth, with
the return to the normal root/shoot balance and with a larger leaf area increasing tree
demands for water.
Nutrient Competition
Competition for soil nitrogen in the latter half of the tea tree regrowth cycle is a major
mechanism of weed interference. Nitrogen supply is an important determinant of tea tree
shoot growth, and weeds reduce both leaf nitrogen concentration and total shoot nitrogen
uptake (Virtue 1997). The weed root density advantage in the nutrient-rich surface soil has
been discussed previously. Nitrogen competition is greater in late regrowth as trees have
lost the early advantage of the high root/shoot ratio. Competition for potassium and
phosphorus has been occasionally observed in regrowth tea tree (Virtue 1997) but is of
minor importance compared to nitrogen.
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.

INSECT PESTS OF TEA TREE
89
WEED MANAGEMENT IN TEA TREE PLANTATIONS
Weed management in tea tree plantations should include both preventative and control
techniques. It is important to prevent the movement of new weeds into and within the
plantation, and to provide conditions which are unsuitable for weed establishment. Direct
weed control techniques such as herbicides are the main consideration in weed management,
but control can also be achieved by increasing the competitiveness of tea tree.
Preventative Techniques
Preventing movement of weeds is much cheaper than the long term control costs of allowing
their entry and spread within a plantation. Making conditions unsuitable for establishment
of certain weed species also avoids control costs.
Quarantine
The movement of stock and machinery between plantations can spread new weeds. Stock to be
used for grazing in plantations should be quarantined for two weeks. This ensures that the majority
of weed seeds in the gut are defecated in a single area. Sheep should be recently shorn to limit
externally-attached seeds of burr weeds. Mud attached to farm machinery can spread weed seed,
bulbs and tubers. Such mud should be washed off before use in a new plantation.
Patch Management
New weeds tend to occur in discrete patches. Patches of those species that are strongly
competitive, such as perennial grasses, tall broadleaves and vigorous climbers, should be
mapped. One can then readily return to a patch for concerted direct control effort against
the weed species. In addition, measures can be taken to limit spread of a patch and creation
of new patches. This may involve fencing out stock, avoiding cultivation and/or restricting
flood irrigation, depending on how the weed is dispersed.
Unsuitable Conditions for Weed Establishment
Long periods of soil waterlogging favours germination and establishment of sedges and
rushes. These are difficult weeds to control. Laser-levelling prior to planting will allow a
plantation to drain faster after rainfall. This reduces the incidence of these species and
allows earlier machinery access to control all newly-germinating weeds.
Direct Control Techniques
The existing weed population needs to be managed to avoid periods of strong weed
interference. Given the high susceptibility of seedling tea tree to many mechanisms of weed
interference, it is vital to maximise weed kill throughout seedling establishment. In regrowth
tea tree, weed control should particularly aim to avoid tree shading during early regrowth
and competition for nitrogen during late regrowth. Good weed kill is especially important
prior to and during late regrowth, as the greatest rate of leaf yield loss occurs at low weed
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.

A.J.CAMPBELL AND C.D.A.MADDOX
90
levels (Figure 3). Whilst a high degree of weed kill is not essential during early regrowth,
access to weeds is easiest and weeds are smallest at this time. Thus it is an opportune time
to particularly control perennial and long-season annual weeds to reduce later competition.
The various direct weed control techniques differ in their ability to kill weeds and in their
impacts on soil health.
Herbicides
Appropriately selected and applied herbicides are the most favourable direct weed control
technique in tea tree in terms of weed kill, leaf yield and relative cost. Various herbicides
have been screened for their safe use in tea tree (McMillan and Cook 1995). 
Table 4
summarises herbicides which can be used in NSW plantations. Specific recommendations
regarding application rates and comments for use are given in Storrie et al. (1997). There
are two general classes of herbicides used in tea tree plantations; pre-emergents and post-
emergents. Pre-emergent herbicides are applied to the soil surface and kill germinating
weeds. They are particularly suited to establishment of tea tree seedlings, and are best applied
just prior to planting. It is best to select pre-emergent herbicide mixtures with broad spectrum
action against a wide range of weed species. Post emergent herbicides are applied to already
established weeds. As with pre-emergents there are some which are selective to certain
families of weeds, such as grasses. Other post-emergents have a broad-spectrum action and
need to be directed away from tea tree leaves to avoid damage. Care is required in selecting
Figure 3 An example response curve of tea tree leaf yield to increasing weed biomass at mature
harvest
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.

INSECT PESTS OF TEA TREE
91
and applying herbicides to minimise effects on operator health, impacts on off-target
organisms, and residues in any farm products. Populations of herbicide resistant weeds can
develop if a single herbicide is relied upon as a sole means of control for that weed species.
Herbicides are grouped according to their mode of action, and it is important to regularly
switch between groups, to minimise development of resistance.
Soil Cultivation
Soil cultivation is a physical disturbance of the soil which controls weeds by uprooting,
severing the root-shoot connection and/or weed burial. Cultivation is the most effective
non-chemical technique to kill weeds in tea tree plantations. Small seedling weeds are
killed more quickly, efficiently and cheaply than large weeds. The major limit to the
effectiveness of cultivation is soil waterlogging. This may prevent machinery access and
weed kill by desiccation is reduced. Other factors limiting cultivation’s effectiveness are
soil disturbance promoting new weed germination, selection for perennial weeds which
reproduce vegetatively, and difficulty cultivating within tree rows and in closely-spaced
hedges. Hand-hoeing should only be used for spot-weeding due to the high labour cost.
Aside from reducing weed interference, cultivation also favours tea tree growth in the short
term through improved soil penetrability for root growth, increased rainfall infiltration and
increased mineralisation of nutrients from soil aeration. In the long-term there may be negative
impacts on tea tree growth through damage to soil and trees. Excessive cultivations can damage
soil structure so that rainfall infiltration and soil aeration is reduced. Cultivating wet soil can
Table 4 Herbicides for use in tea tree plantations in NSW. See Storrie et al. (1997) for details on
rates and application methods
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.

A.J.CAMPBELL AND C.D.A.MADDOX
92
result in a compaction layer just below the tilled layer which then restricts root growth (Tisdall
and Huett 1987). Cultivation lowers organic matter levels, causing a decline in soil fauna
populations, reducing nutrient cycling and soil structural repair (Douglas 1987). Regular,
deep cultivations will sever thick lateral roots of trees and eventually result in a deeper root
system (Tamasi 1986), below the highest concentration of nutrients at the soil surface.
Mowing
Mowing is the cutting of weeds to near-ground level. Mowing reduces weed shading and
seed set, but is ineffective for killing many weeds. In tea tree plantations mowing has given
equivalent or lower yields than leaving weeds uncontrolled (Virtue 1997). Weeds with
growing points below the cutting height regrow. Perennial grasses are particularly favoured
by mowing and become increasingly dominant. The high root density of these grasses (Bowen
1985) and the regular promotion of vigorous regrowth with each mowing favours strong
competition with tea tree for soil nitrogen.
Mowing does have some benefits. Soil organic matter levels are increased to improve
nutrient cycling and soil structure. Maintenance of a groundcover also assists trafficability
on wet soils, protects soil from erosion and keeps soil cool in summer.
The orchard weed control technique of a herbicide strip along the tree row and mowing
in the inter-row is not appropriate for tea tree plantations. The mown sod improves machinery
accessibility, but research has found that a strip width of 0.5m either side of the tree row has
been insufficient to avoid weed competition for soil nitrogen (Virtue 1997). For the average
tea tree row spacing of 1.4m for single row configurations (Virtue 1997), wider herbicide
strips will overlap leaving no mown inter-row.
Grazing
Grazing animals can be used to control weeds. They require good management for effective
weed control and to maintain animal health. Grazing has not been experimentally compared
with other weed control techniques in tea tree, but one would expect only a marginal yield
improvement over mowing. In the short term and with high stocking density weed biomass
would be significantly reduced, increasing tea tree yield compared to no weed control. In
the long term weeds which are non-palatable and/or recover well from grazing will increase
and compete with tea tree. The attraction of grazing for weed control is the potential to
minimise costs through sale of the animals. Sheep have become increasingly used in tea
tree plantations for weed control. Compared to cattle they are less damaging to trees, graze
closer to the ground and are less selective eaters. However they are poorly suited to the
warm, humid and often waterlogged tea tree plantation environment, and health problems
that may arise include worms, clostridial diseases, footrot, foot abscess, fleece rot, fly strike
and lice.
Mulches
A mulch is a material that covers the soil surface. For weed control a mulch must stop
weeds germinating and growing, by starving them of light. Mulches can be thin,
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.

INSECT PESTS OF TEA TREE
93
impenetrable barriers such as woven plastic or paper. Alternatively mulches can be
thick, loose layers such as spent tea tree litter or straw. Complete mulching of the soil
surface provides excellent weed control but the cost of materials and application is
very expensive. Post-distillation tea tree litter is a ready supply of mulch but given a
retail purchase price of $Aus12–15 per cubic metre for landscaping, a 0.1 m thickness
x 1.0m width of mulch is worth $12,000–15,000 per hectare, before application costs.
The mulch may only last several years, may have an allelopathic effect on the growing
tea tree, and may not stop growth of perennial weeds. A thin layer of well-composted
tea tree litter would be cheaper per hectare and still provide the benefits of organic
mulches; protection from high summer temperatures, reduced soil erosion, greater
rainfall infiltration and reduced soil evaporation, increased soil organic matter and
nutrient addition (Firth 1991).
Legume Crops
Growing a legume grain crop in the year prior to planting enables early control of grass
weeds with selective herbicides, and increases soil nitrogen for the seedling tea tree. Perennial
cover crops are often used in orchards to provide a year-round groundcover to protect the
soil and suppress weeds. Requirements of a perennial legume cover crop for densely planted
tea tree are difficult to meet. The crop must establish quickly, persist under shaded conditions,
remain prostrate and not shade tea tree, be shallow rooted to minimise moisture competition
yet survive periods of low soil moisture, produce its own nitrogen requirements, and be
tolerant of acidic, waterlogged soils. No species meets all of these requirements. Maku
lotus, Lotus pedunculatis, is well-suited to acidic soils (Firth 1992) but twines up trees
causing shading. Osciola white clover, Trifolium repens, has greater waterlogging tolerance
than other cultivars but white clovers are shade intolerant (Firth 1992). Tropical legumes
are strongly competitive with tree crops in warmer months (Firth 1992). The most appropriate
role for annual and perennial legumes in tea tree plantations is to provide nutritious feed
where grazing is the main weed control technique.
Summary
All weed control techniques have advantages and disadvantages in their use regarding extent
of weed kill, relative cost, effect on soil health, tree damage, skills required and suitability
in wet conditions. Weed kill should be maximised at minimum cost for greatest profit, but
with long-term soil health in mind. A single technique cannot be solely relied upon as it will
inevitably select for weeds that are tolerant to the technique. Rather, different techniques
should be integrated within and between years, considering the stage of the regrowth cycle,
weather conditions and the risk of yield loss due to weed competition.
Improving Tea Tree’s Competitiveness
Plantation management decisions affecting tea tree growth have implications for tree
competitiveness against weeds.
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.

A.J.CAMPBELL AND C.D.A.MADDOX
94
Harvest Time
Harvesting in late spring-summer provides warm temperatures for rapid coppice shoot
emergence and growth. Tea trees are less likely to be shaded by weeds, will form a
canopy sooner in the regrowth cycle to then shade weeds, and will suffer less fine root
death.
Pre-harvest Tree Size
Larger tea trees at harvest result in faster regrowth due to a higher root/shoot ratio. The aim
in the establishment phase of tea tree should be to grow a large, deep root system for vigorous
regrowth in subsequent harvest cycles.
Plantation Configuration and Density
Choice of plantation layout is very important for weed management. Closely-spaced single
row plantations have several advantages. They provide a high tree density to maximise
yield per hectare, enabling increased expenditure on weed management per hectare. Close-
spacings increase tea tree’s competitiveness with canopy closure shading weeds. Single
rows provide greater access to trees for cultivation and herbicide application than in hedge
rows. Row crop equipment developed for other crops (e.g. vegetables, cotton) can be readily
used. Tree spacing of 0.8–1.0m between rows by 0.3–0.4m within rows will give a tree
density of approximately 30,000 trees ha
-1
. Straight rows are essential for accurate weed
control.
Nitrogen Nutrition
Nitrogen supply is a major determinant of tea tree leaf yield, and weed competition for soil
nitrogen is a major mechanism of weed competition (Virtue 1997). Tea tree leaf nitrogen
concentration needs to be greater than 1.5% dry mass (averaged over whole trees) to maximise
yield (Virtue 1997). Nitrogen must be supplied to trees and not weeds. Research is required
into tea tree uptake of nitrogen as no responses to fertiliser have been achieved (Virtue
1997). Foliar application of urea to trees (e.g. Coker et al. 1987) may provide a means of
providing nitrogen whilst maintaining a weed groundcover for soil health.
High Water Table
To reduce moisture competition in regrowth tea tree, plantations should be established where
there is a permanent and relatively high water table accessible.
Insect Management
Defoliation by insects during early regrowth has similar effects to cool temperatures, slowing
shoot growth and increasing tree root death. Trees are set back for the rest of the regrowth
cycle and will be more prone to late weed competition.
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.

INSECT PESTS OF TEA TREE
95
CONCLUSION
Herbaceous weeds are a major limit to leaf yield in tea tree plantations. Weed interference,
if not controlled, will reduce the mature leaf yield of regrowth tea tree by an average of
around 30%, and the growth of seedling tea tree by 60–90%. Leaf oil concentration and
chemical composition are not significantly affected by weeds. Weeds compete for moisture,
nutrients and light in seedling tea tree, but mainly for nitrogen in regrowth tea tree. This is
because of the established root system of regrowth tea tree, which enables rapid early
regrowth above weeds and access to water deep in the soil. Weed management is critical at
seedling establishment, and weed kill should be maximised until first harvest. In subsequent
regrowth cycles, weed control should be concentrated in the latter half of cycles, when
weed competition is strongest. Weed management should aim to prevent weed establishment
in plantations, integrate various direct weed control techniques, and promote tea tree’s
competitiveness against weeds.
REFERENCES
Blake, T.J. (1982) Coppice systems for short-rotation intensive forestry: The influence of cultural,
seasonal and plant factors. Australian Forestry Research, 13, 279–291.
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(eds.), Attributes of trees as crop plants, Institute of Terrestrial Ecology, Titus Wilson & Son Ltd,
United Kingdom, pp. 303–315.
Britt, J.R., Zutter, B.R., Mitchell, R.J., Gjerstad, D.H. and Dickson, J.F. (1990) Influence of herbaceous
interference on growth and biomass partitioning in planted loblolly pine (Pinus taeda). Weed Science,
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Cannell, M.G.R. (1985) Dry matter partitioning in tree crops. In M.G.R.Cannell and J.E.Jackson (eds.),
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Kingdom, pp. 160–193.
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Ellis, R.C., Webb, D.P., Graley, A.M. and Rout, A.F. (1985) The effect of weed competition and nitrogen
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Firth, D.J. (1991) The role of mulch and organic manures for sustainable horticulture in subtropical
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Firth, D.J. (1992) Covercrops for subtropical orchards. Agfact, H6.3.10. NSW Agriculture.
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McMillan, M. and Cook, T. (1995) Final Report; DAN 74A—Herbicides for weed control in tea tree.
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Copyright © 1999 OPA (Overseas Publishers Association) N.V. Published by license under the Harwood Academic Publishers imprint,

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