Why grapevine leaves matter so much to your wine

Grapes get all the attention. But every drop of wine starts in the leaves. Walk a vineyard with a good grower and you will see them look at the leaves before they look at the fruit.

The short answer: leaves make the sugar that ripens the grapes, control how much water the vine uses, decide how much sun and air reach the bunches, and are the first place disease shows. Too few leaves and the grapes won't ripen; too many and they sit in damp shade. Great wine needs a balanced canopy.

The vine's kitchen

In photosynthesis, leaves turn sunlight, water and carbon dioxide into sugar. The vine sends that sugar to the grapes, where it is stored as glucose and fructose, the sugars yeast later ferments into alcohol. Leaves also feed the trunk and roots with reserves for next spring.

How many leaves does a bunch need?

Researchers in California measured it. For vines with a single canopy, about 0.8 to 1.2 square metres of leaf per kilo of grapes gave the best sugar, berry size and colour. Vines with a divided canopy, where more leaves get direct light, needed only 0.5 to 0.8 (Kliewer and Dokoozlian, 2005).

• Too few leaves (after hail, disease or heavy leaf removal): grapes struggle to ripen.
• Too many (a very vigorous vine): shade, green flavours, damp bunches and more rot.
• Balanced: what growers aim for with pruning, shoot thinning and trimming.

Leaves and water

Leaves breathe through tiny pores called stomata, losing water as they take in carbon dioxide. When the soil dries out, the vine closes them to save water, and photosynthesis slows. A little water stress after fruit set keeps berries small, with more skin for each drop of juice, which can help red wines. Severe drought stops ripening altogether.

Leaves, sun and air around the bunches

Removing a few leaves around the bunches lets in light and air. Light builds colour and reduces green flavours; air dries the bunches and lowers the risk of rot. Overdo it in a heatwave, though, and the grapes burn. See how sunlight shapes wine.

What sick leaves tell a grower

• Downy mildew: yellow “oil spots” on top of the leaf and white down underneath.
• Powdery mildew (oidium): a grey-white powder on leaves and berries.
• Leafroll virus: in red varieties, leaves turn red between green veins from the base of the shoot upwards, and their edges roll down. Infected vines ripen later, with less sugar. Those beautiful red autumn vineyards are not always healthy ones.
• Flavescence dorée: a disease spread by a small leafhopper. Leaves yellow or redden and curl. It has no cure, it is a quarantine disease in the EU, and infected vines must be pulled out.
• Chlorosis: yellow leaves with green veins. It is a symptom, not a disease: on very chalky soils, vines often can't take up enough iron. Growers choose rootstocks that tolerate limestone.

More in grapevine diseases: fungal, bacterial, viral and phytoplasma.

Leaves in a warming climate

Hotter summers change the job. Growers who once stripped leaves to ripen their grapes now often keep more of them, as shade against sunburn, and some are trying trellis shapes that give the bunches more cover.

Leaves that tell you the grape

Each variety has its own leaf shape: the number of lobes, the teeth on the edge, the hairs underneath. Identifying vines by their leaves is called ampelography, and before DNA testing it was the main way to tell one grape from another.

Next time you stand in a vineyard, look up from the grapes. The leaves are telling you how the vintage is going. Then follow the whole season in a vine's year.

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