3.1 Planting Material
3.2 Production Systems I: Flooded Taro
3.3 Production Systems II: Dry-Land Taro
3.4 Weed Control
3.5 Fertilizer Application
The specific cultivation practices for each of the major taro-producing countries are discussed in subsequent chapters. This chapter seeks to set out some of the fundamentals that guide taro cultivation, and provides a background for a more effective appreciation of the specific cultivation practices in various areas.
There are essentially four types of planting material that are used in taro production:
i) Side suckers produced as a result of lateral proliferation of the main plant in the previous crop;The use of huli (Figure 1) is particularly advantageous because it does not entail the utilisation of much material that is otherwise edible. Moreover, huli establish very quickly and result in vigorous plants. However, huli are best adapted to situations where planting occurs shortly after harvesting, since protracted storage of huli is not advisable.
ii) Small corms (unmarketable) resulting from the main plant in the previous crop;
iii) Huli i.e. the apical 1-2 cm of the corm with the basal 15-20 cm of the petioles attached;
iv) corm pieces resulting when large corms are cut into smaller pieces.
Where corm pieces are used, it is sometimes advisable to pre-sprout the pieces in a nursery before they are planted in the field. This enables sprouts to appear on the pieces before they are moved to the field. Side suckers and small corms may also be kept in nurseries to develop good sprouts, especially if there is a long time between the previous harvest and the next planting.
The availability of planting material is an ever-present problem in taro production. This is particularly so in places like Tonga where occasional droughts reduce the quantity of available planting material for years after each drought.
Three strategies are currently available for the rapid multiplication of planting material. The first is to use a minisett technique analogous to the same technique used for yams. Essentially, small corm pieces 30-50g in weight are protected with seed dressing. They are sprouted in a nursery, and then planted in the field. The resulting small corms and suckers are used as subsequent planting material. The minisett technique can be carried out by the farmers themselves, since the level of technology required is well within their competence.
Figure 1. The Taro Huli, Used as Planting Material
The second rapid method of generating planting material is through meristem tissue culture. Starting from a single plant, thousands of plantlets can be generated in a few months. However, tissue culturing requires considerable scientific sophistication. While it is useful for multiplying and distributing elite clones, it has so far not become a routine method for the generation of commercial taro planting material.
A third method of rapid multiplication of taro planting material is the use of the true seed of taro for planting. This currently being tried by the Kauai Agricultural Research Station in Hawaii. Even though one successful taro crossing can produce hundreds of seeds, there are likely to be problems with segregation in subsequent generations, the smallness of the resulting seedlings, and the infrequent nature of taro flowering.
There are two main production systems used in taro cultivation:
i) Flooded or wetland taro productionFlooded taro cultivation (Figure 2) occurs in situations where water is abundant. The water may be supplied by irrigation, by the swampy nature of terrain, or from diverted rivers and streams. The soil must be heavy enough to permit the impounding of water without much loss through percolation. Apart from rice and lotus, taro is one of the few crops in the world that can be grown under flooded conditions. The large air spaces in the petiole permit the submerged parts to maintain gaseous exchange with the atmosphere. Also, it is important that the water in which the taro is growing is cool and continuously flowing, so that it can have a maximum of dissolved oxygen. Warm stagnant water results in a low oxygen content, and causes basal rotting of the taro.
ii) Dryland (unflooded) or upland taro production.
Figure 2. Flooded Cultivation of Taro
The best situation for flooded taro production is where irrigation water is available, and the water level can be controlled. This requires an initial levelling of the land and the construction of embankments so that water can be impounded. The field is puddled so as to retain water, and is flooded just before or just after planting. The water level is low at first, but it is progressively raised as the season progresses, so that the base of the plant is continually under water. The field is drained occasionally for fertilizer application, but is re-flooded after 2-3 days.
In many production situations, wetland taro is grown without adequate control of the terrain or the water supply. In such situations, taro is grown on stream banks or in low lying marshy areas with hydromorphic soils. The required inputs in these situations are much less than those for the controlled flooding described above. The yield output is also commensurately less.
Growing taro under controlled flooding has several advantages over normal dry-land taro production:
a) The corm yields are much higher (about double)However, flooded taro requires a longer time to mature, and involves a considerable investment in infrastructure and operational costs.
b) Weed infestation is minimised by flooding
c) Out-of -season production is possible, often resulting in very attractive prices for the taro.
Because of continuous water availability, time of planting is usually not critical in flooded taro production. Planting can occur at virtually any time of the year. Indeed many producers take advantage of this phenomenon by staggering their planting dates in various plots. Thus they can have corms for sale virtually all year round, even during off-season periods when prices are high.
Most flooded taro is grown as a sole crop, rather than intercropped. This is partly because of the intense specialised nature of the cultivation, and partly because very few other crops can sufficiently tolerate the flooded condition to share the field with taro. For the same reason, taro may be gown on the same field for several years (monoculture) before another crop such as rice or vegetable is introduced.
Dryland taro production implies that the taro is not grown in flooded or marshy conditions. Despite its advantages, flooded taro is restricted only to certain locations where the economics of production and water availability permit the system to thrive. By far the largest area and production of taro in the Asia/Pacific region occurs under dry-land conditions. This is also true of global taro production.
Dry-land taro is essentially rain-fed. Sprinklers or furrow irrigation may be used to supplement the rainfall, but the objective is mainly to keep the soil moist, not to get the field flooded.
The rainfed nature of dry-land taro cultivation means that the time of planting is critical. Planting is usually done at the onset of the rainy season, and the rainy season itself must last long enough (6-9 months) to enable the taro crop to mature.
Land preparation for dry-land taro starts with ploughing and harrowing. If the soil is deep and friable, the crop can be grown on the flat; otherwise, ridges are made. Ridges are usually 70-100 cm apart and plant spacing on the ridge is 50-90 cm. Planting in the furrows of the ridges is also practiced. Unlike flooded taro, dryland taro is quite frequently intercropped, although sole cropping is also common.
Planting in dryland taro production involves opening up the soil with a spade or digging stick, inserting the planting pieces, and closing up. Mulching is done to conserve moisture. Manures and composts may be applied after planting, or incorporated into the soil during the initial land preparation.
As indicated above, dryland taro matures earlier than flooded taro, but the yield is lower and the production inputs are also less.
For flooded taro, weed infestation is minimal, but some aquatic weeds do occur. Some of these are pulled out manually, although in high-technology production systems, herbicides may be added to the irrigation water. In Hawaii, Nitrofen at 3-6 kg/ha has been found to be effective.
For dryland taro, weed control is necessary only during the first three months or so, if crop spacing has been close enough. Thereafter, the crop closes canopy and further weed control is not necessary. In the last two months of the crops field life, average plant height diminishes and spaces open up again between plants. Weeds may re-appear but their potential for economic damage is very low.
Weed control with hand tools is the most prevalent practice in dryland taro. Care should be taken to confine the tools to the soil surface; taro roots are very shallow and can be very easily damaged by deep weeding or cultivation. Earthing up of soil around the bases of the plants is advisable during weeding, so that the developing corms are protected. Herbicide weed control is possible in dryland taro production. Recommended herbicides include Promtryne at 1.2kg/ha, Dalapon at 3kg/ha, Diuron at 3.4 kg/ha or Atrazine at 3.4 kg/ha.
The majority of taro growers in the Asia/Pacific region, especially those producing taro for subsistence, do not use any fertilizer. Some even believe that fertilizers diminish the quality and storability of their taro. All the same, taro has been found to respond well to fertilizers and to manures and composts. The specific fertilizer types and quantities recommended vary widely from place to place; they are therefore left till the next section where cultivation practices in various countries are discussed. In general, it is best to apply the fertilizer, compost or manure as a split dose. The first portion is applied at planting, possibly incorporated into the soil during land preparation. This first dose promotes early plant establishment and leaf elaboration. The second dose is supplied 3-4 months later when the corm enlargement is well under way. Splitting the fertilizer dose minimises the effects of leaching which is potentially high in the high-rainfall areas where taro is produced.
Taro is able to form mycorrhizal associations which promote phosphorus uptake. Also, in some flooded taro fields, Azolla is deliberately or inadvertently cultured in the field water, thereby improving the nitrogen supply to the taro. This is quite common in flooded taro fields in the Hanalei Valley, Hawaii.
Malnourished taro exhibits certain deficiency symptoms. Potassium deficiency causes chlorosis of leaf margins and death of the roots. Zinc deficiency results in inter-veinal chlorosis, while for phosphorus, a leaf petiole content below 0.23% signals the need to apply fertilizer. Various other nutritional deficiencies and toxicities of taro have been elaborated by OSullivan et al. (1995).
For dryland taro, maturity for harvest is signalled by a decline in the height of the plants and a general yellowing of the leaves. These same signals occur in flooded taro, but are less distinct. Because of the continuous and abundant water supply, the root system of flooded taro remains alive and active, and leaf senescence is only partial.
Time from planting to harvest ranges from 5-12 months for dryland taro and 12-15 months for flooded taro. Much depends on the cultivar and the prevailing conditions during the season.
Harvesting is most commonly done by means of hand tools. The soil around the corm is loosened, and the corm is pulled up by grabbing the base of the petioles. For flooded taro, harvesting is more tedious because of the need to sever the living roots that still anchor the corm to the soil. Even in mechanised production systems, harvesting is still mostly done by hand, thereby increasing the labour and cost of production.
Average yield of taro in Oceania is about 6.2 tonnes/ha, while that for Asia is 12.6 tonnes/ha. The global average is about 6.2 tonnes/ha.