Plant Protection
Tea Weed Management
Paraquat is an essential tool in tea

Tea cropping

The Boston Tea PartyThe Boston Tea PartyNext time you are relaxing with a nice cup of tea, remember that this beverage once sparked a revolution. In 1773, a group protesting against unfair taxes, dressed as Mohawk Indians and tipped the cargo of tea carried by ships of the British East India Company into Boston Harbor. The Boston Tea Party rallied support for the revolutionaries in the 13 Colonies and, some say, started the American War of Independence. Now, tea is a crop leading another revolution, one in sustainable agriculture.

The major environmental issues in growing tea include (Clay, 2004):

  • Loss of habitats and effects on biodiversity
  • Soil erosion on the often hilly terrain
  • Water pollution and reduction in soil health by agrochemicals
  • Deforestation as a result of the need for wood for drying tea leaves

Using the non-selective herbicide paraquat for weed control can address three of these four issues. Paraquat can be used to maintain a managed, non-competitive weed flora which provides habitats to encourage biodiversity and helps prevent soil erosion. Paraquat does not affect soil health and does not pollute soil or surface waters.

Paraquat is an essential tool in tea

Paraquat is a broad-spectrum herbicide. Its mode of action is to inhibit photosynthesis, an essential process in plants, and means that paraquat destroys all green tissue. Paraquat is safe to tea for several reasons:

  • It is immobilized on contact with the soil meaning that it cannot move to roots and be taken up into plants.
  • It is sprayed around the tea plants, which are protected by bark which paraquat cannot penetrate.
  • Even if small amounts of paraquat land on lower tea leaves there is little or no damage nor residues reaching the buds and young leaves because paraquat does not move through plants systemically like the alternative non-selective herbicide glyphosate.

Also unlike glyphosate, paraquat is very fast-acting and rainfast. Weeds sprayed with a paraquat solution in the morning will often show symptoms by the afternoon, making it easy for spray operators and plantation managers to see which areas have already been treated. This holds even if rain falls within 15-30 minutes, making it possible to spray for longer before rain is expected.

A video showing paraquat’s unsurpassed speed of action can be viewed here.

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.

In perennial plantation crops such as tea, emphasis is on the management of weeds rather than their permanent removal. This is because maintaining a particular balance of weeds in the plantation flora is important for sustainability by providing habitats for predators of insect pests and minimizing soil erosion though the anchoring effect of plant roots.

Paraquat 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.

You can read more about paraquat’s unique soil properties here.

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.

Paraquat is an important tool in farmers’ battle against weeds, while also having a good environmental profile and creating opportunities in rural communities. You can read more about the benefits of using paraquat here.

Where does my cup of tea come from?

What is tea?

Picking tea leaves by handPicking tea leaves by handTea plants (Camellia sinensis (L.) Kuntze) come from more than 1,500 different varieties. They are all evergreen bushes or small trees with a strong tap root and yellow-white flowers. The major varieties are:

  • Assam (Camellia sinensis var. assamica), tree-like with a single stem. Unpruned Assam tea trees can grow 6-20 m high.
  • China (Camellia sinensis var. sinensis), which is a bush.
  • Cambodian (Camellia sinensis var. parviflora) which is intermediate in characteristics and sometimes referred to as a hybrid between Assam and China tea.

In commercial production, tea plants are pruned to around 1.5 m high.

Teas are made from newly emerged leaves and buds, which are hand-picked every two to three weeks. After picking, leaves are spread on large trays and allowed to wilt for various lengths of time. They are then rolled or chopped to release enzymes followed by ‘fermentation’. Sugars and tannins are released as the tea ‘ferments’. Actually, this is an enzymic oxidation process as no alcohol is formed. The process is stopped by heating at certain stages to produce the various kinds of tea available, including:

  • White tea: a speciality tea, which is dried immediately after picking buds which have been shaded to prevent them from turning green as chlorophyll is produced.
  • Green tea: limited ‘fermentation’ is followed by steaming.
  • Oolong tea: partial ‘fermentation’ giving characteristics in between green and black tea.
  • Black tea: ‘fermentation’ is allowed to complete over two to four weeks before drying (‘firing’).

Green and, especially, white tea are high in catechins (types of tannin or polyphenols), which are powerful anti-oxidants, believed to have medicinal properties in fighting cancer and enhancing the immune system. Catechins are more concentrated in the younger, more tender leaf tissue picked for green and white tea, and are also better preserved in the processing of these teas which are steamed with little or no wilting or fermentation and then dried.

Although the caffeine concentration in tea leaves can be around 4%, and higher than in coffee beans, the caffeine content of a cup of tea is typically about half that of coffee.

Where is tea grown?

Global distribution of harvested areas of tea (FAOSTAT 2010)Global distribution of harvested areas of tea (FAOSTAT 2010)Tea grows best in regions that enjoy a warm, humid tropical climate with rainfall measuring at least 1000 mm per annum. Ideally, tea prefers deep, light, acidic and well-drained soils. In such conditions tea will grow in areas from sea level up to altitudes as high as 2,100 m.

Tea is overwhelmingly a crop of the eastern hemisphere with by far the largest areas in China and India, which grow nearly 1.5 million and over half a million hectares (ha), respectively. Sri Lanka grows more than 200,000 ha (Fig 1; Table 1).

Table 1. Top ten tea producers by area harvested (ha) (FAOSTAT, 2010).

1 China 1,419,530 6 Vietnam 113,200
2 India 579,000 7 Myanmar 78,746
3 Sri Lanka 221,969 8 Turkey 75,851
4 Kenya 171,916 9 Bangladesh 52,236
5 Indonesia 124,573 10 Japan 46,800

Figure 2. Changes in annual production of leading tea-growing countries (FAOSTAT).Figure 2. Changes in annual production of leading tea-growing countries (FAOSTAT).Over the past decade, production has increased particularly in China and Vietnam (Fig. 2). Half of all tea produced (80% is black tea) is exported with a total annual value of about $3 billion. In 2004, Kenya overtook Sri Lanka as the world’s leading exporter of tea. They were closely followed by China. India is also a major exporter, and Indonesia completes the top five. Russia, UK, Pakistan, Egypt and Japan are the leading importers. The British and the Irish drink most tea, averaging nearly 3 cups per person every day.

Sustainable tea production

Growing tea

Picking tea by machinePicking tea by machineTea is planted from seed or cuttings and many crops are still being harvested after almost 100 years. Crops grown from seed are first pruned when 2-3 years old and 70 cm high. From cuttings, this stage is reached after only one year. Further prunings are made aiming to produce a bushy plant with many shoots. When these terminal buds are picked, this encourages side-shoots to grow, which are picked in turn at subsequent harvests.

Pests and diseases

Some of the symptoms of insect damage to tea plants include folded, curled or deformed leaves; discolored, striped or spotted leaves; desiccated leaves; and sawdust on trunks, branches or at the base of plants. Insects damaging leaves include leaf hoppers (eg in Vietnam the Tea Green Leafhopper, Empoasca spp. is a major pest), mites, thrips and aphids. Termites (eg Postelectrotermes militaris, Coptotermes spp. and Macrotermes spp.) burrow into woody tissue.

Diseases affect leaves, buds, branches and roots. The most important leaf disease in Vietnam is Blister Blight caused by Exobasidium vexans. Buds are affected by Anthracnose which is caused by several fungi including Colletotrichum theae-sinensis. Pseudomonas spp. bacteria can cause cankers on trunks and branches. Root rots are caused by many fungi including Armillaria spp, Ganoderma spp and Hypoxylon spp.

Weed management and biodiversity

A large number of annual and perennial weeds infest tea plantations. Tropical climates with ample sunshine, heat and moisture mean that weeds thrive and may compete with crops for space, water and nutrients, and shade the crop plants, especially when they are young.

Prematilake et al. (2004) conducted field experiments with various methods of weed control in young tea. They found that paraquat-based systems were superior to those using the systemic non-selective herbicide glyphosate. Intensive use of glyphosate has led to changes in plantation weed flora (‘weed shifts’) as more tolerant species have become more dominant. ‘Soft’ weeds, typically prostrate annual grasses which are easily controlled, are replaced by re-invasion of cleared land by more aggressive ‘noxious’ species, typically creeping and climbing annual and perennial broad-leaved species. These aggressive weeds compete with the tea crop, reducing yields and make spray operations, fertilizer application and picking difficult.

Using paraquat, however, to manage the weed flora rather than eliminate it, can help to maintain a balanced flora that precludes the dominance of aggressive species. Paraquat only removes the top growth of well-established weeds and does not affect the germination of new seedlings allowing vegetation to re-establish after 1-2 months. A controlled presence of soft weeds maintains the balance of the weed flora and prevents weed shifts to noxious species simply because bare ground is less available for them to colonize. The presence of non-competitive vegetative cover also provides habitats to encourage biodiversity. The wildlife encouraged will include predators of insect pests, which would otherwise have to be controlled chemically.

Paraquat can be safely sprayed between the crop rows without fear of damaging the tea plants. It is immobile in soil and cannot move to the roots and up into the shoots. Bark cannot be penetrated by paraquat meaning that it can be sprayed right up to the base of the bushes.

Soil health and water

Paraquat’s unique combination of biological and physico-chemical properties, particularly its fast non-systemic action and extremely strong soil adsorption, give it a very robust environmental profile. More information on paraquat’ soil properties can be found here .

"Agrochemicals used on tea plantations kill many of the microorganisms that live in soil" according to Clay (2004, p103). However, as paraquat is not biologically active in the soil because of its binding properties, it has no adverse effects on soil fauna or microorganisms.

"The chemical inputs applied on tea plantations have had a deadly effect on soil biodiversity while simultaneously polluting river water, killing fish, and harming the animals and people who depend on the rivers for water" according to Clay (2004, p103). This does not apply to paraquat because it cannot leach into water being so tightly bound to soil, far more so than other agrochemicals. Used as recommended, paraquat is not hazardous to fish or invertebrates because, besides its immobility, even if paraquat spray should drift onto ponds, rivers or ditchwater, it is rapidly removed by adsorption to plants and sediments, and by microbial degradation. Extensive ecological studies have shown that paraquat is not a risk to aquatic environments (Roberts et al, 2002).

Soil erosion

Tea plantations are often on slopes susceptible to soil erosionTea plantations are often on slopes susceptible to soil erosionTea plantations are often found on sloping terrain in hilly areas receiving very high levels of rainfall. This can have dramatic consequences for soil erosion. For example, the United Nations Economic and Social Commission for Asia and the Pacific (UNESCAP) has reported that annual loses of soil by erosion in the upper Mahawelli catchment area of Sri Lanka can be as much as 75 t/ha.

This causes difficulties on and off the tea plantations. There are serious implications for productivity as soil erosion depletes reserves of nutrients and reduces water-holding capacity. Off site, eroded soil can silt-up reservoirs and water courses making problems for irrigation and hydroelectric power generation.

Using paraquat for weed management can help minimize soil erosion by maintaining a vegetative ground cover. It can be used in combination with grass strips between crop rows and weeds are only controlled along the crop rows. Because it has no systemic action or residual effects in soil, these properties allow re-colonisation of weeds by re-growth from roots or new flushes of germination. This means that weed root systems are always present and have an anchoring effect on the soil, resisting movement.

CASE STUDY: Paraquat sustains productivity for Sri Lankan tea farmers

P.B. Ekanayake, K.G. Prematilaka, and A.P.D.A. Jayasekara of the Tea Research Institute (TRI) of Sri Lanka told the 2005 Asian-Pacific Weed Science Society conference how they have found that paraquat can play a vital role in weed management for their tea farmers.

A six-year study in Sri Lankan tea estates concluded that systemic herbicides like glyphosate can be toxic to tea plants and adversely affect growth and yield. However, researchers from TRI found that “the effect was minimal with paraquat.”

Glyphosate has been increasingly used because tea growers have been under pressure to simplify their systems and cut costs. However, unless used very carefully, because glyphosate is systemic, spray drifting on to tea can damage young leaves and buds, and cause longer-term reductions in plant vigor, decreasing yields. Furthermore, unacceptable residues of glyphosate have also been detected in dried tea leaves. The weed control performance of glyphosate has also been compromised by the pressure it puts on the weed flora to shift to more tolerant and competitive species.

TRI recommends restrictions on the frequency of glyphosate use, an interval of 1-2 weeks between spraying and harvest (depending on rate), and the use of adjuvants to reduce the rate required. Because of this, the study authors recommend paraquat as a weed management solution for Sri Lankan tea, saying:

“Manual weeding and chemical weeding with paraquat in rotation could be advocated to sustain the productivity and maintain an eco-friendly environment in tea plantations.”

The results of this study confirm years of extensive research which support the safety of paraquat to crops and to the environment. The complete paper can be read here …
Impact of Weed Management Studies on the Productivity of Tea Plantations in Sri Lanka

CASE STUDY: Paraquat saves soil in China

Min An-min, Guo Hen-xiao, Li Hong-xia and Zhao Rong of the Sichuan Academy of Forestry have reported how using paraquat to control weeds has reduced soil erosion in sloping tea gardens.

Paraquat dramatically reduces soil erosionParaquat dramatically reduces soil erosionTea is grown at altitudes above 1500 m in the mountainous upper reaches of the Yangtze River. Tilling the soil to control weeds has resulted in serious soil erosion caused by surface run-off of water. Scientists from the Sichuan Academy of Forestry have found that using paraquat to control weeds is more than twice as effective as glyphosate at reducing erosion compared to conventional tillage.

This is because glyphosate leaves the soil exposed for longer than paraquat, which allows a non-competitive green cover to regenerate. The presence of vegetation disperses the impact of rain and weed roots provide an anchorage for the soil. In trials in tea gardens with slopes of 8o – 18o, the losses of nutrients and organic matter along with the soil were 80-100% less when using paraquat rather than glyphosate.

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