Plantation management decisions affecting tea tree growth have implications for tree competitiveness against weeds.
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.
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.
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 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.
To reduce moisture competition in regrowth tea tree, plantations should be established where there is a permanent and relatively high water table accessible.
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.
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.
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