This grape growing season, here in the Puget Sound, is off to a slow start. Most vine growth will occur with warmer temperatures in the summer, so the cool weather we’re seeing right now might not affect this year’s crop. Still, it’s got me thinking about making up for lost time – managing the vines to take advantage of every photon of sunlight that comes their way. One way to do that is leaf pulling.
Leaves in the shade: Like open windows with the AC on
The idea behind leaf pulling is that a leaf in the shade isn’t just idle, it’s holding back the vine by consuming sugar and nutrients that could be going to productive leaves or fruit. So if you pluck leaves that are in the shade, or those that shade other leaves, the vine will still get almost as much benefit from photosynthesis as before, but will not have the drain of non-productive leaves. Done correctly, this will mean better fruit. It might even mean earlier fruit.
Fruit in the shade: A more complicated trade off
Sunlight affects ripening fruit in different ways. Too much can burn, and not enough can delay ripening. Here, leaf pulling decisions are more tricky. What do you do about a leaf shading a grape cluster? If you pull it, the cluster will get more sun, but at the cost of reducing photosynthesis and sugar production. I don’t know that there’s a this-always-works-answer here. In cool seasons, the risk of burning may be less so you might be more inclined to pull leaves that shade fruit clusters. This might also be true later in the season, after the hotter July and August days are behind you. On the other hand, it’s those cooler seasons, or times of year, when you want to turn every photon of light into sugar. For what it’s worth, I’m planning to pull leaves for maximum photosynthesis through the hot summer, then open up the clusters to sunlight in late summer/early fall.
The flip side of maximizing photosynthesis is making the best use of the resulting sugar. I’ll talk about doing that with cluster thinning in my next canopy management article.
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