Those of you who read some of my articles will know how often I speak of sunlight penetration into the grape vine. Most new grape growers I met do not really understand or take note of the importance of having enough sunlight inside the vine.
Now when I speak of sunlight, I do not needarily speak of direct sunlight but more about the UV rays of the sun, as direct sunlight can damage grapes. Not all grape varieties are intolerable to sunburn, but there are a few varieties that really can not take any direct sunlight, especially if you live in an area where the summer temperatures reach the high nineties (F) or high thirties (C).
To protect these grapes from sunburn, you need to have a well-planned vineyard, a well-developed canopy, a practical but effective trellis system, proper canopy management and also some knowledge of when and grapes are most vulnerable to sunburn.
The layout of the vineyard:
Properly laying out the vineyard will ensure that during the hottest time of the day, the leaves on the canopy will protect the grapes from hanging directly in the sun. Many factors will determine the "perfect" layout of the vineyard; things like the path of the sun in your area, the topography, slope, over shading trees, anticipating winds, altitude and even the variety.
The trellis system:
Having a trellis system that expose as many leaves possible, to direct sunlight is ideal, but at the same time, that trellis system should allow you to train the grape vine so the canopy will also protect the grapes from direct sunlight.
As said so many times before; managing the grape vine's canopy is one of the most underestimated cultivation practices when you grow grapes. You MUST do canopy management, no matter what variety you grow, if it is for wine or table grapes, or where you live.
You must know when to tie down the shoots, when to pull leaves, when to open up the canopy to allow direct sunlight to penetrate the row and how to manage a too vicious grape vine.
Grapes are most susceptible to sunburn from pea-size to just before veraison (colouring). During this stage you must ensure your grapes are protected. Handling or touching the grapes of susceptible varieties during this stage should be minimized. Naturally, the grape berries have a layer to protect them from sunlight and diseases, but will rub off when you touch the berries, making it more susceptible to sunburn. Please note, that is only true for really sensitive varieties, the less sensitive varieties can be handled during this time of the growing season.
Opening up the canopy for direct sunlight to penetrate the vine is well known practice throughout the world and will do wonders for the coloring and ripening of your grapes, BUT it needs to be done at exactly the right time.
When your grapes start to turn color, the sugar content within the berries will increase and the acid levels will start to drop. During this stage, the grapes become less tolerable to sunburn but is not sacred from sunburn, so you must still keep the grapes protected. Opening up the canopy at this stage is done by pulling more leaves or on the gable trellis system I use on my farm, it can easily be done by removing (pruning) overlapping shots from the top of the structure like in the picture below. This will create a 5 to 8 inch tunnel where the sun can shine through.
It's a known fact that most of the white grape varieties are more susceptible to sunburn than the black or purple varieties are. Whatever it is because you can more easily see the sunburn damage or if it is a genetic thing, does not matter. I found that white grapes that hang in direct sunlight can suffer what I call sugar stains (like in the picture below). The fact is; If you grow a white grape variety, you need to take special notice of this article.
Enjoy the day and happy grape growing!