Over the last weeks many Napa vintners have been wondering whether they should be irrigating to prevent possible forthcoming effects caused by the drought statewide. Following the rain over the week-end, the cumulated amount of water received during winter is now around 10 inches, which is typically the case by the second week of February. As of today, we have received more water than we had in 2009 at the same date. Weather is unpredictable and the recent rainfall proved that we should be cautious prior to jumping to conclusions before the season starts. In that context, here are 2 key ideas we wanted to share.
First, even with the current rainfall, it is too early to say how much water will be available in the root reservoir when the season starts and vine leaf area begins to unfold. Except in a few exceptionally warm soil situations, as of today, the vine is still dormant and no budbreak is visible. The question of early watering will really be critical once budbreak starts. During the period of leaf area development, it is critical to respect the natural balance between leaf area development rate and water available in the vine root reservoir. As such, triggering irrigation too early may have bad consequences later on.
If initially less water is available in the root reservoir, then shoot elongation and total vine leaf area will be reduced early. Even if this is not ideal, this will not necessarily be a bad thing everywhere considering that less water may be available for the rest of the season. In fact by developing a lower transpiring leaf area earlier, the vine will use less water later on. Consequently vine water supply is more likely to remain balanced over a greater period, even if the season started with a more limited root reservoir. If too much water is applied during leaf area development, this may be asking for trouble later when more water will be needed to sustain a larger sized canopy. Irrigation during leaf area development favors a greater transpiring leaf area earlier. In turn, it precipitates further the onset of water deficit …Soon the vine will ask for more water and it is not necessarily what you want in a year of exceptional water scarcity.
Second, the water applied weeks before budbreak – hoping it will benefit the vine later – may be a waste, particularly when cover crop is present. In an article published a few weeks ago, Centinari and his colleagues reported that the amount of water loss through cover crop evapotranspiration and soil evaporation can be greatly reduced if you practice bare soil tillage.
As such, when applicable, mowing should also be used as a short-term water management strategy to reduce cover crop competition on the vine.
In conclusion, stimulating vine leaf area development early without overdoing it and reducing cover crop competition early will be the key points to have in mind to adapt to the start of the 2014 vintage.
Do not forget: for better vine water use efficiency, it is better to water in larger but less frequent volumes. The goal is to refill more of the root reservoir when watering. As such, to avoid over watering and to optimize the timing of each irrigation, you should start monitoring water deficit very early during the season.