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why do they cover grape vines with plastic

Abstract: The horticultural market is characterized by an increasing demand of fresh extra-seasonal products and of early- or late-ripening fruits, both achieving a better economic income. In table grapes, the ripening period can be extended by selecting new varieties or by applying cultural techniques apt to force the plant growth under unfavorable climatic conditions or to protect the foliage and fruits from meteorological adversities. The harvest date may be advanced by inducing a praecocious budbreak. This is obtained by managing the vineyard as a protected cultivation, that is, by covering the roof and the lateral belts of vineyard with transparent plastic films, which allow a high fraction of solar radiation to pass through and then are able to retain a good portion of energy by limiting the convective and radiative thermal dispersions. Hence, the air temperature increases and induces a faster accumulation of growing degree days which, in turn, stimulate an earlier vine budbreak. The plastic covering is affixed on the vineyard at the end of winter; the earlier the covering, the earlier the budbreak.

This latter can be advanced by 10 to 40 days depending on the covering date, the grape cultivar and the climatic characteristics of growing environment. The maximum effect can be obtained by covering the vineyard 50 days before natural vine budbreak in open field; however, the advancing effect could be partially lost during the following phenological phases, when the internal thermal regime is too high respect to the requirements of the reproductive and photosynthetic processes. The plastic film optical properties greatly influence the canopy development and the qualitative traits of the grape clusters. This type of protected cultivation has proved to be particularly suitable for early-maturing cultivars and seedless grapes; these latter, in particular, may improve the berry quality, and any case, avoid the competition of seeded varieties with bigger and savoury berries. The harvest date may be delayed by protecting the ripening clusters from meteorological adversities.

This technique profits from the good preservability on vine which characterizes the grapevine cluster and improves this trait by protecting the berry sanitary status during ripening. In fact, starting from veraison, berry skin turns thinner and juice turns sweeter, thus berry becomes more sensitive to fungal attacks, especially after the wetting caused by the late summer rains. To preserve the crop, the vineyard roof in covered with plastic films. Under the best conditions, which are with healthy grapes and low rainy season, the berry integrity may be guarantied for a long period, and the harvest may be delayed by 3 months, without reducing yield and berry carpometric characteristics. For best results, it is very important to maintain berry hydration and turgidity as long as possible, since the grape freshness is the goal to achieve and the attribute which distinguishes this product from that coming from the cold storage. The protected cultivation to delay harvest is particularly convenient for late seeded cultivars, especially for grapes having an appropriate skin thickness, abundant pruine and a big berry with a relatively low surface:volume ratio, which are all traits helping to limit berry transpiration and thus favors its final hydration status.
Valley farmers are concerned over recent rainfall and the impact it can have on their crops.

However, a plan is in place to prevent damage. Plastic tarps effectively protected crimson seedless grapes from the rain. White plastic is tied down at the end of rows to cover this entire vineyard west of Porterville. The rain never touched the grapes and so crews were able to continue harvesting bunches off the vine. HMC Vineyard operations manager Tim McIntyre says the plastic covers help extend the growing season. "The plastic goes over the top and then the middle of this canopy, on this gable system trellis if we do get rain the water will run through the middle and come out between the bunches so they don't even get touched. " 30-percent of the crop still needs to be picked so the weather can cause problems with late varieties like the crimson seedless.

McIntyre explained, "If we get rain before the fruit matures it causes the quality to be unpackable for table grape quality and we get split skin and rot. " McIntyre says his grapes needed more time to develop their deep red color because of a very hot August and September. The plastic covers offer added protection. Without them, crews would have no work on this day. He said, "We wouldn't even be here. We'd be waiting to see what's happening with the crop. " Grapes damaged by rain could end up at the winery instead of the fresh fruit market. Mcintyre says the plastic covers give him peace of mind. He said, "I know growers who don't and it's a gamble and a risk you take and if you can get in through that window of opportunity without no weather you're successful. " McIntyre says he still has three weeks left in the harvest season. And again about a third of his crop is still on the vine so it's important the grapes stay dry.

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