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Putting weeds to work

Weeds depend for their success on qualities that can be turned to the farmer's advantage. Fibrous and durable, able to seed heavily or spread by rhizomes, and capable of rapid growth during adverse conditions (many are deep rooted and can survive drought), some weed species can be put to practical use. Examples include the aptly named Prickly pear (Opuntia spp.), which has become an invasive weed in may places, but which can be utilised both as a living fence and as the food source of the cochineal beetle from which the high value red food dye can be made.

Vetiver grass used to control soil erosion in Sri Lanka
Vetiver grass used to control soil erosion in Sri Lanka

Another deep rooted and durable plant is Vetiver grass (Vetiveria spp.): tolerant to drought, flooding and even fire, Vetiver's deep, dense root system has long been used to bind and strengthen soil structures. Originating in subtropical Asia, the planting of Vetiver to protect irrigation canals from collapse is an indigenous practice for lowland rice farmers in The Philippines. In Africa, drilled lines of Vetiver have been used for over thirty years to stabilize new road cuttings and banks, the unpalatibility of the leaves to livestock improving the Vetiver's chances of becoming established. As a means for controlling soil erosion and increasing rainfall infiltration, planting of Vetiver hedges has many advantages over other methods, not least that all the work can be done by individual farmers without much labour requirement, and requires no special equipment. Tillers or seedlings are planted in shallow ditches dug along the contour, and once established form a dense hedge that needs little or no maintenance. The plants slow and disperse run off, increasing infiltration and preventing gully erosion, and also hinder soil creep. Grass mowings of Vetiver can be used to make an insect-repelling and moisture conserving mulch, and a pungent oil in the roots is known to repel rats and other pests.

Weeds that feed

Two East African roadside weeds, Tithonia diversifolia and Lantana camara, have been shown to have significant levels of nitrogen and phosphorus; when grown as a cover crop and incorporated into soil, they contribute to fertility at no cost except for the labour of planting and soil incorporation.

A weed which contributes to human nutrition is Purslane, which contains vitamins lacking in many conventional foods. Common Purslane (Portulaca oleracea), an invasive weed of urban and rural areas worldwide, is a good source of vitamins A, C and E, and is also extremely rich in alpha-linolenic acid. Also known as 'fish oil', this fatty acid plays an important role in disease prevention and in growth; researchers in the US are currently developing cultivation practices to increase the 'fish oil' content, and hence the 'nutraceutical' value of the plant. Purslane's ability to thrive in a wide variety of climatic conditions and soil types make it ideal as a source of leafy vegetable in dry areas, and in parts of central Europe and the Mediterranean Purslane is used in salads and soups. In parts of Ethiopia, Purslane is used as a famine food and as a feed for livestock. Purslane has densely matted multi-branched stems and thick, fleshy leaves which can form a valuable 'living mulch', protecting soil from erosion and conserving soil moisture levels.

Finally, one of the most notorious weeds of all, Water Hyacinth. In Bangladesh it is valued as feed for cattle, for composting, and for the cover that it gives, when managed correctly, to brood fish and fish fry in water bodies. In Uganda, where it became the curse of Lake Victoria, Water Hyacinth is now in demand by the Prisons Department for its Water Hyacinth Prisoners' Artcraft Project. Fishermen are paid to trawl the lake, collecting some 15 tons of the weed each week, which is dried in the sun for three to four days and is then steeped in sodium metabisulphite for one hour to discourage weevil attack. The fibres are then ready to be woven into cordage, which is used for weaving a range of handicrafts including bags, baskets, lampshades and mats, including sleeping mats. Men and women prisoners are involved in the handicraft work, and some of the money from sales is used to help released prisoners to resettle in their communities and to start up their own income-generating businesses: about US$50 is given to each prisoner on their release. If the eradication programme for Water Hyacinth proves too successful, the Project will switch to using Papyrus and Rattan, two other naturally occurring 'wild' plants.

It has long been observed that there is some good in everyone, however unpromising they may seem at first. The same would appear to apply to the plants that grow around us. But recognising benefit when others see only cost takes an open mind and a readiness to try new approaches.

Date published: March 2004


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