COVER CROPPING IN VINEYARDS BODEGAS URBINA
A cover crop of plants other than vines established in the vineyard, typically between the rows, generally for the benefit of the vineyard soil. Also known as a sward, or sod culture, it is an alternative to bare soil created by cultivation. Sometimes cover crops are not deliberately planted but weeds are allowed to grow instead. Cover crops are normally mown during the vine-growing season, and may be removed by cultivation or herbicide spray. Typical sown cover crops are grasses and legumes.
The grasses used may be native to the area or specially introduced species such as rye grass, fescue, or bent grasses, although sometimes cereals such as barley or oats are used. Legumes planted as cover crops include clovers, medics, peas, and beans. Cover crop management is more difficult in high-density, narrow-spaced vineyards, such as Bordeaux’s left bank and Champagne.
A common reason for sowing cover crops is to increase the organic matter in the soil and hence improve its structure and capacity to hold water. When this is the aim, the species sown should, like cereals, peas, and beans, grow quickly and produce plenty of bulk which can then be incorporated into the soil by shallow ploughing. Cover crops with a deep tap such as radish or hollow-stemmed such as oats maintain soil structure by facilitating water absorption and reduce the need for irrigation.
The cover crop sward also reduces the amount evaporation from the topsoil, even though some of this retained water will be used by the cover crop to survive. Vine roots are more likely to delve deeper into the soil where cover crops create competition for water and nutrients in the topsoil. Deep, rooting vines are less susceptible to sucking up rain water near harvest, thus preserving colour, acid, and flavour in grapes, and maintaining thicker grape skins, which in turn are more likely to resist pests such as grape worm and vinegar flies.
Cover crops are also commonly planted to stop soil erosion in areas with summer storms and are especially useful in sloping vineyards subject to alternate dry heat and sudden downpours, as in some parts of Alsace. The roots of the cover crop bind the soil and resist the flowing water. Cover crops are also used to combat wind erosion, which can cause severe damage to young plants.
But perhaps the most important use of cover crops is to encourage earlier ripening and improve wine quality. Slight water stress hastens the ripening process, so cover crops, which compete with the vines for water and nutrients, especially nitrogen, can help to generate this stress in areas of high summer rainfall. However, when winter legumes such as lupins, vetch, or beans are ploughed in during spring, they allow atmospheric nitrogen to be released into the soil.
Legumes are much more efficient at doing this than most broadleaved weed species, which do provide nutrients but not in the amounts needed for most commercial wine production. In certain environments, this could have negative consequences for vine vigour. Some mowers throw the cover crop clippings under the vines, thus forming a mulch.
Apparently weedy vineyards should not necessarily be dismissed as untidy. They may represent a deliberate ploy to improve wine quality. Deep-rooting crops such as mustard and chicory can be particularly useful to use up subsoil water which shallow-rooted grasses cannot reach.
Cover crops should be grown with caution during the vine’s growing season. In spring, they make the vineyard more prone to frost than if the soil is bare. The cover crop may pity host to insects which spread diseases such as flavescence dorée. In summer, the cover crop can use too much water or nitrogen and the vines can suffer as a result.
Because of competition with vines for nitrogen, the use of cover crops can also cause stuck fermentations. These effects can be offset by close mowing or by killing the cover crop by ploughing or herbicides. Cover crops reduce the problems caused by dust from traffic on bare soil s in the vineyard, dust can encourage.
