Minimising water woes

That's the advice of Dr Damian Martin from Plant & Food Research in Marlborough.

At the recent Organic and Biodynamic Winegrowing Conference, Dr Martin said water woes in the 2015 season saw many growers facing irrigation restrictions, which in turn placed water stress on the vines. The best way to combat that, if another such season occurs, is to ensure you don't have too much foliage going into the hottest months of the year. Less leaves, however, also means the vines need to carry less crop.

Past irrigation research undertaken in Marlborough showed that as a rule of thumb, every square metre of vineyard foliage transpires a litre of water, on a sunny day in the middle of summer.

"Large grape vines, big high capacity vines in the lower Wairau in Marlborough can have 10 to 12 square metres of foliage," Dr Martin said. "So the maximum water (the vine would require) is 10 to 12 litres per vine, per day. Conversely if you have quite a small vine which has say three square metres of foliage, then its water requirement is only about three litres a day. It's that simple."

However it is ultimately crop load that is the key determinate of water need he said. Roughly speaking, to ripen a kilogram of Sauvignon Blanc grapes in Marlborough, you would require a square metre of foliage.

"So a really big vine can at least sugar ripen 10 kgs of crop, and likewise a small vine with three square metres can only ripen three kgs of crop."
The ratio is different for premium reds, which proportionally require 50 percent more leaf to crop.

Dr Martin said low leaf areas of say three square metres are not often achieved. Instead the norm is large leafy vines with relatively high crop loads. These vines need a lot of water.

Add to the mix cover crops and undervine plants and even more water is required.

"The amount of water they use through the growing season can be about as much as the grape vine itself."

So what can you as a grower do to alleviate the need for constant watering via irrigation?

"The key thing," Dr Martin said, "is to reduce the leaf area and that will reduce the amount of water flowing through your vine, and conserving it in the subsoil."

If you reduce the shoot vigour early on in the growing season, you can reduce the leaf area of the canopy later on. One way of doing that is to utilize under vine competition in the period from bud burst up to pre- flowering .

"Those under vine plants don't actually use a lot of water at that time, but they are competing at a key period with the vine, when all that shoot growth potential is being set up. So you can influence the shoot growth very dramatically over quite a short period. You spend a little bit of that soil water in your under vine plant, but you are actually saving it for later in the year, because your vine's leaf area will be lower."
There is a caveat though, that under vine competition has to be removed before it gets too strong.

"Swards will use most of the spring and summer rain before the grape will be able to. In situations where you are under pressure for water or don't want to use a lot of water, then I think you need to consider whether to have a sward. You can consider cultivation, it is not always a bad thing. It refreshes root growth in the top soil, it aerates the soil and reduces compaction, all of which are positive. But don't over do it."

Trimming the foliage is the simplest way of controlling the leaf area

"Trimming hard and regularly is an effective tool. It is also a tool to reduce the cumulative growth potential over a number of years. Like a bonsai plant, if you trim a vine hard and often enough over a period of time, you will gradually reduce its overall capacity and you will change the balance between shoot development and root distribution."

Balance is the key here though. You still require a suitable leaf area to crop ratio – so if you take a lot of leaf off, you have to remove a lot of crop as well.

Encouraging the vine to dig deeper for water is something many growers are keen to achieve. Dr Martin says stimulating rainfall is one of the best ways of achieving that. By that he means, using the irrigation less frequently and for longer periods.

"By irrigating a lot less frequently you achieve partial root zone drying and the vine sets up some protective mechanisms to conserve water. When you irrigate regularly the vine is never set up to undergo any sort of deficit."

If you are starting from scratch and developing a new vineyard – think about the following:

• Short trunks
• Lower trim heights
• Drought rootstock
• Change planting densities

Or you could start transitioning to dry farming. (See next story). That isn't as easy as it appears, Dr Martin said. While it is a common method in Europe, he said in New Zealand it is more difficult.

"Our vineyard soils are not that great texturally here in New Zealand in terms of being suited to dry farming. They tend to be silty or sandy, which means the vines have easily available water all the way while the soil dries down and then suddenly they are completely dry."
He said comparing our growing conditions with Europe is like comparing apples with oranges. The evapotranspiration in this part of the world is far higher than that experienced in places like Bordeaux and Burgundy.

"In Marlborough we have 7mm of evapotranspiration on a nor-west day, whereas in places like Burgundy or Bordeaux, they will only have 5mm. It is tougher climatically for us to dry farm."

Dr Martin said the take home messages in an emergency drought situation are the following:

• Leaf area and then by extension crop load drives the vine's water requirement
• Small vines with low leaf areas and low crops need far less water
• Reduce leaf area and the crop load if your vines are coming under water stress
• Encourage deeper rooting
• Avoid frequently rewetting the topsoil 

Any queries, please contact Dr Martin at; This email address is being protected from spambots. You need JavaScript enabled to view it.