A Technical Guide for Wine Producers

RECENT ARTICLES   |   WYNBOER HOME Organic weed control in the vineyard Johan Fourie, ARC Infruitec-Nietvoorbij, Stellenbosch Key words: Vineyard, organic, weed control, cultivation A world population that keeps on increasing places more and more pressure on natural resources. Therefore it is essential for the cultivation of crops to occur in harmony with nature. Hence the growing interest in organic cultivation. Although soil cultivation is undertaken mainly to eliminate competition from weeds, it should be done in a way that will increase water infiltration, thereby limiting water run-off and erosion, to maintain or improve the soil structure and organic matter content of the soil, and improve aeration of the soil. Soil cultivation can be divided into two categories, namely mechanical cultivation and minimum cultivation. In the context of organic cultivation, both practices are recommended to bring about sensible weed control, since no organic herbicide for use in vineyards is currently registered. Any weed control programme must be environment friendly, economically viable, effective and sustainable. To obtain the maximum benefit from any weed control action, it must be undertaken at the correct stage(s) of the season. It is therefore important to know the growth pattern of the weeds, to ascertain whether they are annual or perennial, and to determine whether they reproduce with seed only or also have the ability to reproduce vegetatively (by means of root stocks, tubers, bulbs or stalks). Weeds with the ability to reproduce vegetatively are usually distributed by mechanical cultivation and have the potential to become problem weeds in an organically cultivated crop. Weeds that reproduce by means of tubers or bulbs, for example, should therefore be controlled mechanically before the development of these parts, i.e. in a very early growth stage. The control of such weeds must therefore receive preferential attention, especially while the distribution is limited to a small area. As soon as these weeds become established over large areas, it is very difficult and almost impossible to obtain effective control. It is also important to prevent weed seeds from being added to the existing seed bed. The weeds should therefore be controlled before seed production occurs. The existing seed population may be drastically reduced in the top 50 mm soil layer over the medium to long term by stimulating germination by means of mechanical cultivation, controlling the germinated weeds and limiting further germination by disturbing the soil as little as possible thereafter. By so doing the seed bed, especially in that particular soil layer, will be drastically reduced. This should be the long term strategy of any weed control programme. The negative influence of weeds Compete for water In the dry summer months, growing weeds cause the soil to dry out more quickly. It is critically important, especially under dryland conditions, to restrict water loss, since excessive water stress could have a negative impact on the vine's vegetative growth and production. Compete for nutrients Weeds may remove a lot of nutrients from the soil. This means that less nutrients are available for the vines. On marginal soils in particular, this may result in insufficient shoot growth and insufficient production of poor quality grapes, with the accompanying loss of income. Promote pests and diseases Harmful insects may hide and multiply in the weeds, from where they may infest the vines. A dense growth of weeds limits the movement of air and creates a micro-climate that is conducive to the development of diseases. This results in a smaller harvest and even total crop losses. Nematodes may multiply on the roots of certain weeds to numbers that are harmful to vines. Complicate the harvesting process Weeds such as mat sandbur (Cenchrus incertus Curtis), siny Emex (Emex australis Steinh.), bristly ox-tongue (Picris echioides L.) and bush stinging nettle (Urtica urens L.) that grow into vines (bush vines in particular), complicate the harvesting process. The seeds of other weeds such as sticky bristle bur (Setaria verticillata (L.) Beauv), Spanish blackjack (Bidens bipinnata L.) and common blackjack (Bidens pilosa L.) stick to workers' clothes and have to be removed with great difficulty. Various weed control options available within the framework of organic cultivation Mechanical weed control This is the oldest form of weed control and includes the use of ploughs, as well as weeding with spades or forks. Advantages Weeds are controlled immediately. In areas with low summer rainfall, where the soil surface dries out to such an extent that weed seeds do not germinate, a single mechanical cultivation in late spring may produce effective weed control. It can control annual and certain perennial weeds. It is relatively easy to apply. In warm, dry weather the roots of the uprooted weeds dry out quickly, thus limiting regrowth of perennial weeds that reproduce vegetatively. Disadvantages It creates an ideal seed bed for weeds to germinate. Perennial problem weeds such as field bindweed (Convolvulus arvensis L.), common couch grass (Cynodon dactylon (L.) Pers.) and nutsedges (Cyperus esculentis and C. rotundus) are propagated in this way. Weed control must be repeated regularly to be effective, especially in areas with regular rainfall or where irrigation takes place. The cost of mechanical cultivation can be quite high. The shallow roots (0-200 mm soil layer) of the vines are damaged. Regular cultivation of the top soil destroys the soil pores, compacts the soil and causes a decrease in organic material. In time the topsoil becomes pulverised and loses structure. This causes the formation of a crust on the soil surface. Rain or irrigation water cannot penetrate the crust, resulting in run-off, causing erosion. Slashing weeds (with slasher or manually) Advantages The slashed weeds may be left on the soil surface as a mulch layer to suppress further weed growth. By not disturbing the soil, the soil structure and organic matter content are maintained. Water run-off and accompanying erosion are limited. Disadvantages Weeds must be slashed regularly, especially before seeding, with a slasher or edge cutter. Since the action must be repeated frequently, it can become quite expensive. The weeds continue to grow and still compete with the vines for water and nutrition, although to a lesser extent than when there is no control. This reduces the harvest which result in a loss of income. Cover crop cultivation Dry matter is placed in the vine row to suppress the germination and growth of weeds. A cover crop (grain species or nitrogen fixing broadleaf species) is sown in the work row in winter and controlled in the growing season of the vine to create a mulch. The following materials can be used for mulching, namely straw, reeds, grass from the veld, tree bark, leaves of trees, plastic and even paper or carton. When using plant material, the plants should have no seeds, otherwise seeds must be removed beforehand. The weeds must be controlled (with a hoe) before the mulching material is packed in the vine row. To be effective, the thickness of the mulch should be approximately 50 mm. Advantages It limits evaporation from the soil surface, the soil moisture is therefore retained for longer. It improves water infiltration during rainfall and irrigation. It limits erosion. It prevents the soil from becoming too warm in summer and too cold in winter and in doing so, creates favourable conditions for root growth. When the mulch decomposes, nutrients are released. This increases the organic matter content and soil fertility of the topsoil. It controls most weeds effectively. A higher production of quality grapes compared to other practices increases the nett income per hectare. Disadvantages If the material has to be bought, it may be expensive. It may be difficult to obtain material. Spreading out of the cover layer is labour intensive and therefore expensive. The purchase of cover crop seed is an added expense. In regions where the winter rainfall is low, irrigation scheduling must be adjusted in winter to make provision for the needs of the cover crop as well. Weed control strategies A "Pre-planting" weed control strategy for organically grown vines Start controlling the weeds in the area to be planted at least one year before the establishment of the vines. It is good practice to establish a cover crop on the area to be planted, since it will help to keep the weed population down. Cover crops such as oats, rye, or triticale are good options. The cover crop must not be worked into the soil, but left as a mulch on the surface of the soil. If problem weeds such as field bindweed, common couch grass and nutsedges occur on the site, the actively growing weeds should be controlled chemically with a high dosage of glyphosate. This should be repeated as the situation necessitates, until four weeks before planting. It is important for effective and sustainable weed control that these weeds should be eradicated before establishing the vines, if at all possible. It is important to realise that in such cases the first two crops may possibly not be certified as organically cultivated grapes. The subsequent crops will, however, qualify. A short and long term weed control strategy Hoe the vine row thoroughly before planting and cover the one metre wide strip with a straw mulch of at least 50 mm, just after planting. Make sure that the mulch is thick enough to prevent any weed from germinating. Add additional fibre as soon as weeds start breaking through. With regard to the work row, the basic requirements for soil cultivation are best fulfilled by applying cover crop management to the work row. In order to produce the maximum fibre for organic matter build up and weed control, it is important to implement the following steps for sustainable cover crop management: The previous year's mulch should be disced at least four weeks before sowing. This will ensure that the initial increase in microbiological activity and the accompanying reduced nitrogen level in the topsoil (0 - 300 mm) do not coincide with sowing. By neglecting this, the cover crop may establish itself poorly, resulting in insufficient dry matter production (DMP). The addition of rock phosphate to the topsoil, for incorporation during seedbed preparation and in quantities suitable to the fertilisation programme, will help to provide in the nutritional demand of the cover crop. Soils that tend to form a surface crust should be cultivated with a cultivator just before sowing. After sowing, the seeds should be covered with a layer of soil. An effort should be made not to plant the seed deeper than 25 mm. In instances where full surface irrigation is available, the cover crops should be established before mid-April. If full surface irrigation is not available, producers have to wait for the first autumn rains. Full surface irrigation being available, a DMP of up to 8.9 tons.ha-1 might be achieved with oats species, if the cover crops receive a weekly irrigation or rainfall of 18 mm the first ten weeks after sowing. The nitrogen fixing broadleaf species in particular must receive a weekly irrigation or rainfall the first four weeks after sowing to achieve successful establishment. The grains should receive a maximum of 28 kg N (approximately two tons of compost) at the two to four leaf stage. In the case of the nitrogen fixing broadleaf species this is unnecessary, since these species fix nitrogen especially if inoculated with a suitable Rhizobium species. The grains should be sown at a density of 100 kg/ha, the Medicago species and pink Seradella at a density of 25 kg/ha and grazing vetch at a density of 50 kg/ha. Since success with the self-establishment of cover crops was precarious, it is recommended that the cover crops be sown annually. Crop rotation must be applied to limit accumulation of soil-borne diseases that threaten the cover crops. A two year cycle is recommended, in which rye or triticale for example are succeeded by oats species or nitrogen fixing broadleaf species. Grazing vetch and pink Seradella perform well on sandy to medium textured soils, while triticale performs well on medium to heavy textured soils. The broadleaf species perform poorly on soils with a pH (KCl) of 5.5 and less. At the very latest, the cover crop should be controlled mechanically at the beginning of October (the soil water status is a determining factor) with a disc harrow. The cultivation should be applied in such a way to retain as much fibre as possible on the surface. In cases where rye and triticale were sown early (both have a relatively short growth cycle), these species sometimes die back naturally as early as the end of September. In such instances, the dead cover crop can be flattened with a roller to form a mulch. In cases where wind is a big problem in newly established vineyards, the cover crop may be left upright to serve as a windbreak. The weed stand and spectrum of weeds should be determined at the beginning of November and the beginning of January at the least. In climatic conditions under which weeds proliferate, weed growth should be monitored on a monthly basis. The tempo of weed growth will determine how frequently mechanical weed control is required. It is important to leave the mulch on the surface for as long as practically possible, in order to achieve pre-emergence control of spring and summer growing weeds. Monitoring the weed spectrum enables the producer to adjust or finetune his weed control strategy timeously. In conclusion, it is important to realise that any crop growing in the work row during the growing season of the vine will compete vigorously with the vine for water and nutrients and cause grape production to decrease, drastically in the case of dry land viticulture. The use of a permanent cover crop in the work row is, therefore, not considered to be the ideal cultivation option in organic vineyards. For further queries contact Johan Fourie at Tel: (021) 809 3043, Fax: (021) 809 3002 or E-mail: johanf@infruit.agric.za

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