Water- How much is too much?

16 October, 2008

Global warming is affecting many parts of the world- more frequent extreme weather is predicted, along with altered rainfall patterns; an increase in winter rainfall and a decrease in summer rainfall.

There are already signs of significant changes in flowering and ripening patterns across the world. At Geisenheim, earlier bud break has been mapped since 1961 and flowering is currently 10 days earlier than the 30 year average. Research in Bordeaux, Alsace, New Zealand and USA are all showing trends towards earlier harvest (Cilby, 2005).The  effects of global warming will have a significant impact on viticulture practices; more than likely there will be changes in the flavour profile  of grapes from shorter ripening seasons, an imbalance between tannin and sugar ripeness, and changes in aroma profile as berries will be ripening through hot night temperatures.

Rivers are reaching critical levels in the Murray-Darling Basin, Australia; vineyards are starting to suffer from the consequences of prolonged drought - excessive salt accumulation in the root zone, impacting soil properties and compromising vineyard health and economic sustainability for the region and the industry.

However, ironically most growers in Marlborough have recently been asking the question “ When will it ever stop raining?”.

Rob Agnew from HortResearch says it has been the wettest winter since records began. Before the flooding could be absorbed, another flood was on its way, followed by continuous periods of steady rain. It is a winter most people want to forget. Even once the spring had officially begun, we had only one whole day without rain up until September 14. Everywhere you travelled throughout the province you saw vines sitting in water logged paddocks., So what effect will the floods have on young vines in Marlborough?

As welcome as the rain was to Marlborough, the Southern and Eastern side of the plains had a deluge of rain in a condensed period of time; a number of vineyards in the preparation stages of development are now faced with a prolonged period of “drying out” before machinery can begin work in these blocks.

The best advice is don’t be too hasty to start work in the vineyard.  Compaction is a result of cultivating soils that are too wet and/or the use of heavy machinery.  This degradation will have a big impact on the first year and future growth of the vine.

In essence, machinery and cultivation needs to be timely, even if planting is pushed out later than originally planned.

Waterlogged soils has a profound affect on the root system of vines:

• Vines in wet soils are more vulnerable to soilborne pathogens such  as Cylindrocarpon, Phytophtora, Pythium, Rizoctonia and Verticillium.
• Moist soils warm up quicker pre-empting an earlier budburst which may result in increased frost risk in some areas. However, prolonged wet soils stay cold longer and budburst can be delayed.
• Young vines are more susceptible to soil borne pathogens than older vines due to the difference in root mass.
• Soils holding water will become anaerobic, limiting oxygen available to the roots resulting in decomposition of the root system and eventual vine death.
• If the root system has been damaged by Grass grub, there is an increased chance of soil borne pathogens causing further damage to the root system.
• Coastal vineyards may experience rising salt levels due to the elevated ground water table. Depending on both the soil type and ground water table level, some salt may be flushed out, reducing the salinity content.

Marlborough has dealt with drought and frost conditions but the region has not dealt with extended periods of prolonged wet weather and the effects of this may not be immediately apparent on the vines.