Water

By Jamie Goode | 20th May 2022

One of the challenges for plants is getting enough water, and then when they have got it, not losing it. When plants moved from seas to land some 470 million years ago, they began evolving mechanisms to protect themselves. The most primitive multicellular plants, mosses, are able to dehydrate and rehydrate, but most vascular plants can’t do this. So they need roots that can access underground water supplies, and then a waxy cuticle that keeps the water in once they’ve got it. But this creates another challenge: in order to carry out photosynthesis they need a supply of air into their tissues (which brings with it the vital CO2), and they do this with closeable pores called stomata. The plant does a calculation that then determines when these are opened and when they are closed. If it’s very warm, or warm and breezy, they can find themselves losing too much water – more than they can get from the soil to replace it. In this case they close their stomata and photosynthesis stops. Water relations like these are a critical part of plant physiology, and are highly relevant to vineyards which are often planted in dry places.

For three vintages - 2016, 2017 and 2018 – South Africa’s winelands were hit with a terrible drought. Dams emptied and the impact was even felt in Cape Town – it almost became the world’s first major city to run out of water. The old, mostly unirrigated vineyards that have been making some of South Africa’s most exciting wines, began to struggle.

‘I almost fell into depression because of the drought,’ says Eben Sadie, who is one of South Africa’s best-known winegrowers, making wine in the warm, dry Swartland region. ‘When you love your vineyards so much and you see them regress like that, it’s horrible.’

But Sadie also thinks that the drought has made many people much better farmers. ‘We would never have been to arrive at this point in our viticulture, were it not for the drought. We would have got there, but we’d have got there 20 years later.’ This is because although South Africa has been making wine for many hundreds of years, it only came out of a period of isolation in the mid-1990s, and its fine wine dimension is still growing, and its winegrowers are still in a learning and experimentation style.

‘The drought was just like a pressure cooker,’ says Sadie. ‘We had to change so much about our protocols and practices. In terms of working the soils, the big thing that has happened in the vineyards is the use of cover crops. We plant massive cover crops. It is my main focus: I’ve actually planted my last cover crops on Friday last week. There is a lot of composting, trying to build as much carbon in the soil. Carbon is the intelligence of the soil. Every percentage point increase in soil carbon, you almost double your water management capacity.’ This is one of the big changes in viticultural thinking. In the past, we used to think of the soils as merely being a support medium for the crop plant, the vine, and good viticulture was seen as growing just the vines in a tidy vineyard, with nothing else growing there. With no competition, the vine can flourish. But now we realise that having other things growing in the vineyard – encouraging biodiversity – is key to building soil carbon and soil structure, which helps with water infiltration, and helps keep the water there once it arrives, accessible to the vine. The soil microlife also helps feed the vine with vital nutrients.

Sadie also emphasizes that it’s important to plant the right varieties, and the drought has not been kind on varieties not well suited to the Cape’s climate. ‘In the world of grapes, there are two types of varieties, isohydric and anisohydric,’ he explains. ‘Isohydric varieties are ones that shut down at 10 am when it is getting warm. The vine knows it is going into stress when the tempo of the evapotranspiration through the stomata overshoots the capacity of the vine to take up water through the capillary action through its roots. Grape varieties like Grenache, Carignan, Assyrtiko, Grillo and Cataratto – all these hardcore Mediterranean varieties – are isohydric. Anisohydric varities will just keep on evapotranspirating and put their own systems into such an induced stress situation. It’s at this point that the vine starts burning off the acid. It’s the only way the plant can get a massive amount of energy to get through this stress.’

‘18 years ago I started a project of planting new varieties in the Swartland,’ he says. ‘It takes a long time to get the material through quarantine. You usally start with 18 eye buds, and this has to become plants, and these plants have to go through a nursery. I only started making the first wines from these projects in the last four or five years.’

Vines are actually very good at surviving with relatively little water. In a Mediterranean climate of the sort that most of the vineyards in South Africa experience, there’s usually very little rainfall during the growing season. The vines need to be able to tap into water supplies underground. But too much water can actually be a bad thing for wine quality. Just because a little is good, more isn’t better, and the ideal is that there’s enough water early in the season for the leaves to grow into an appropriately sized canopy (these leaves are needed to provide the sugar resources to ripen the grapes), but then at a certain point in the season there should be a slight deficit, so the canopy stops growing and the vine concentrates its resource on the fruit. This is because a growing shoot tip is what is known as a ‘sink’ for the products of carbohydrate, while the ripening grapes should actually be the sink.

A good vineyard seems to manage putting the vine under moderate drought stress towards the end of the season. But the key term here is ‘moderate’. Excessive drought stress, of the sort experienced in South Africa during the 2016, 2017 and 2018 seasons, is bad for vines. One reason is to do with carbohydrate reserves. After the vine ripens the grapes and they are harvested, its job is not done. The leaves still have work to do, sending the products of photosynthesis to the trunk and roots to build up carbohydrate reserves. These are needed to get things going the following season. After budbreak, the vine grows shoots and leaves, and for a while, more energy is used growing these shoots and leaves than they produce in terms of photosynthesis. This first stage of growth draws on stored reserves, and it carries on for quite a while. In drought conditions, the leaves shrivel and stop working soon after harvest, if not before, and there’s no replenishment of these much-needed resources. This results in uneven budbreak and growth the next season, and the vine gets progressively weaker. Some stress is good for quality, but too much is bad.

So now viticulturalists are breathing a sigh of relief that the drought is over. But they are refining the way they work, looking to build resilience into the vineyards. And water is at the heart of viticulture.