Ever wonder what fuels the impressive size and power of the awesome Pacific Island rugby players? Look no further than taro. Taro is a tropical starchy root crop which is a staple food in many subsistence communities, particularly in the Pacific islands. And while the rugby team members most likely have a broad selection of meals from which to choose, for many people living in these areas taro is an essential part of their diet.
Often called “the potato of the tropics,” its role is so significant that taro is celebrated in festivals and on coins in many Pacific Island cultures. And it is no wonder: taro makes up almost 20% of peoples’ daily calorific intake in some areas, compared to only 3-5% represented by potatoes for people in the US and Europe. Economically, it is an important source of export revenue, mainly to supply ex-patriot islanders living in Australia, New Zealand and the west coast of the US.
As a tropical crop, protecting taro from weeds is critical. Weeds thrive in such hot and humid climates, robbing yield. Hand hoeing crops is one option, but, not only is this time consuming and labour intensive, but it means other opportunities, particularly for education in poorer communities, may be missed. Using the right choice of herbicide provides an effective alternative.
Paraquat is an essential tool for taro farmers
“Paraquat in Samoa is not only helping farmers to provide a staple food but is also enabling taro to become a very important export crop as well. It is the only product I know of that can used with safety to the crop."
David Browne, Venture Exports New Zealand. David has over 30 years of experience in the Pacific Islands. Venture Exports Ltd, PO Box 5724, Wellesley Street, Auckland, New Zealand, Venture-Exports@xtra.co.nz.
Paraquat is used to control weeds growing in taro fields before or after planting and between the crop rows. Paraquat controls most weeds, does not move from the area of contact on leaves, and is deactivated in soil.
You can read more about paraquat’s unique soil properties here.
Paraquat has advantages over hand-weeding in saving time, labour, and avoiding damage to shallow roots. Paraquat has advantages over other herbicides in that, unlike glyphosate, it can be used for inter-row weed control with much less risk of damaging the crop; and unlike soil residual herbicides such as triazines or phenylureas, it is applied at much lower rates and will not leach or affect following crops.
Paraquat is a broad spectrum herbicide. It’s mode of action is to inhibit photosynthesis, an essential process in plants, and means that paraquat destroys all green tissue. However, although it is termed ‘non-selective’, paraquat is safe to taro crops because, firstly, it is immobilised on contact with the soil meaning that it cannot move to roots and be taken up into plants. Secondly, even if small amounts of paraquat land on taro leaves there is little or no damage nor residues reaching the roots because paraquat does not move through plants systemically like the alternative non-selective herbicide glyphosate.
Paraquat is very fast acting and rainfast, unlike glyphosate.
A video showing paraquat’s unsurpassed speed of action can be viewed here.
Weeds sprayed in the morning will often show symptoms by the afternoon, making it easy for spray operators to see which areas have already been sprayed. This holds even if rain falls within 15-30 minutes, making it possible to spray for longer if rain is expected.
Paraquat has a very robust environmental profile. It does not leach because it is extremely tightly bound to soil particles immediately on contact, so it cannot move into groundwater or surface waters by run-off and similarly cannot affect soil animals or microorganisms.
Paraquat has a very robust environmental profile.
Details of paraquat’s safety to the environment, spray operators and consumers can be found by referring to the Paraquat Fact Sheet.
In recent years, intensive use of glyphosate has caused new weed problems as less well controlled species have ‘shifted’ to become more dominant and some species have evolved biotypes which are resistant to glyphosate. Using paraquat as an alternative non-selective herbicide with a different mode of action in integrated weed management systems is helping to avoid problems of weed shifts and resistance.
The importance of paraquat in fighting weed resistance to glyphosate and maintaining farmers’ options to use conservation tillage systems is discussed here.
You can read more about the benefits of using paraquat here.
What is Taro?
Taro (Colocasia esculenta (L.) Schott) is a starchy root crop also known as cocoyam, dasheen, koko, bari, dalo, wu tau, and poi, to name but a few. Other plants sometimes referred to collectively as taro include giant taro (Cyrtosperma spp.); tannia (Xanthosoma spp.); and poor man’s taro (Alocasia spp.).
The taro plant has heart-shaped green or purple leaves borne on long (0.5-2 m) petioles arising from underground bulbous, cylindrical stems called corms. These are the havested part, filled with starch, usually 30-40 cm long. Daughter corms, or cormels, form on lateral stolons (runners). There is a shallow fibrous root system, mostly in the top metre of soil. Taro only rarely flowers to set seed and is propagated vegetatively from corms. Taro corms are the main harvested part of the crop. Corms are boiled, baked, roasted or fried as a staple source of carbohydrate. Cooking is needed to destroy irritants naturally present in the plant. It has been called ‘the potato of the tropics’. In Samoa and Tonga, taro may provide 16-18% of daily calories; and in Africa around 7%. By comparison, in Europe, North America and Asia potatoes supply only 3-5% of daily calories.
Other parts of the taro are also used: leaves can be boiled as a green vegetable, providing a source of protein, vitamins and minerals. Peelings and waste are fed to livestock.
Taro is often produced as a subsistence crop, but considerable amounts are sold domestically and for export. Corms are bulky and perishable, but are the main form traded. The taro industry can be important in rural development, providing jobs in cleaning, sorting, packing and shipping. In some places, like Hawaii, taro is processed, often into a sour paste called ‘poi’.
Where is Taro Grown?
Taro is grown on almost 2 million ha. Countries with the largest harvested areas include Nigeria (0.73 million ha), Ghana (0.26 million ha), Cameroon (0.22 million ha) and China (0.09 million ha). While taro is a minor crop in these countries it assumes real significance in the agricultures of small tropical islands, especially in the Pacific, eg Samoa, Fiji and the Solomon Islands.
Taro grows best under hot and wet conditions, with temperatures always above 21ºC. It is sensitive to frost and therefore a lowland crop. Highest yields are found in regions with annual rainfall above 2,000 mm
Production is possible on soil types ranging from heavy clay loams to light volcanic soils. Ideally, fertile medium clay soils with high water holding capacity and rich in organic matter are preferred. Slightly acid soils of pH 5.5 - 6.5 give best results.
How is Taro Grown?
Taro crops are planted using either small unmarketable corms or setts (corm pieces), sometimes pre-sprouted. Leaves continuously appear and senesce, with new corms starting to form after about 3 months.
At around 6 months, shoot growth slows and corms start to bulk rapidly. Harvest is typically about 10 months after planting, but can vary between 5 and 15 months depending on location and production system.
Harvesting is usually done by hand and average yields of taro crops are between 6 and 12 tonnes/ha.
A key point about taro production is that it requires much water, partly because of the large leaves which transpire heavily. Also, taro prefers full sunlight which encourages water loss, although it can be grown in the shade of other crops like bananas, coconuts or breadfruit.
There are two main production systems: flooded and the less intensive dryland (upland).
Much higher yields come from flooded taro production systems. Flooding ensures sufficient water availability and helps to control weeds. This is a more intensive system, but it means that taro can be grown out of the rainy season to take advantage of better prices. In less sophisticated systems, taro is grown in marshy areas or on the banks of streams.
To create a flooded taro system, the land is first levelled and then the soil is puddled and bunds (levies) are built to retain the flood water. Taro may be planted just before or after flooding. While the crop is growing, fields are drained to apply fertilizer and then reflooded. This may happen several times as fertilizer application is best split to minimise leaching. It is important to keep the water flowing to remain oxygenated.
Flooded taro is grown as a monoculture for several years before being replaced by other crops such as rice or vegetables.
Dryland taro is the more common form of cropping system, reaching harvest faster, but with lower yields. In this system, taro must be planted at the start of the rainy season which needs to last through to harvest. Fields are prepared by ploughing and harrowing. The soil may be moved into ridges into which the taro corms are planted and mulches applied to help retain soil moisture and control weeds. Unlike flooded systems, dryland taro is sometimes inter-cropped with trees as noted above.
Pests and Diseases
Insect pests of taro include the taro beetle (Papuana uninodis) which has become a problem recently and has been blamed for the loss of 40% of harvested taro in Fiji. Cluster caterpillars (Spodoptera litura) and planthoppers (Tarophagus proserpina) are other important insect pests; and root knot nematodes (Meloidogyne incognita) can also be a problem. Taro leaf blight (Phytophthora colocasiae) can be a devastating disease.
Weed control is critical in dryland taro, particularly in the first 3 months. Weeds thrive in such climates and a huge range of annual and perennial species have to be controlled by hand-hoeing or by spraying herbicides. Opportunities for weed control are:
- Before planting
- After planting, but before the crop emerges
- Between the crop rows before the canopy closes smothering any newly emerging weeds
Herbicides used vary in their spectrum of weeds controlled, their degree of soil residual activity, and whether they have only contact, or systemic properties.
Controlling seedbed weeds
Paraquat controls a very broad spectrum of weeds at moderate rates of 540-900 g/ha. Glyphosate may also be used. Herbicides which will control weeds emerging after spraying (soil residuals) may also be used, including triazines and phenylureas applied at rates up to several kg/ha, but the risks of leaching and crop damage are often too great for their use to be recommended.
Because paraquat is deactivated immediately on contact with soil, and because small quantities of spray hitting crop tissue only cause localised temporary damage, it may be used confidently after planting up to when the crop starts to emerge. Close to emergence, the risk of damage from systemic non-selective herbicides like glyphosate contacting emerging shoots is too great.
Controlling weeds in the crop
If hand tools are used for weeding, care must be taken to avoid damaging the shallow roots and developing corms. Soil is often moved up around the base of plants prior to hand weeding to minimize damage.
Glyphosate cannot be used for spraying between rows because of inevitable contact with the crop. Glyphosate quickly moves from the point of contact throughout plants and small quantities can cause extensive damage.
Oxyfluorfen is a herbicide with soil residual and contact activity which can be used post-emergence if the spray is carefully directed between crop rows. However, there is some risk of damage as oxfluorfen is volatile and under some conditions where water is evaporating from soil, herbicide vapor may move out of the soil to damage crop shoots.
Paraquat, however, is widely used for inter-row weed control in taro. It cannot move through soil to damage roots; spray droplets hitting crop leaves will not damage yield; and it does not produce any vapor. Perhaps most important, especially for weed control in tropical rainy seasons, is that paraquat is rainfast within 15-30 minutes of application. As regular rain is essential to dryland taro production, rainfastness is a major advantage.