Too much irrigation... In a hot season?

In 2015, Washington State had one of the hottest years on record, but is there such a thing as too much irrigation in a hot season? We all know demand goes up, and models for irrigation are not perfect. My work has suggested there is such a thing as too much irrigation, even in a hot year. 

In much of Washington's growing area we have more control than most of the growing areas in the world. Our soils are naturally barren, and we have essentially no rainfall, or very little rainfall in the main part of the growing season. In a sense, we have a more empty canvas than many vineyards around the world; with that, we can choose to paint the picture we wish to paint. 

First, there are several types of control that we have in the vineyards we manage. Some of our control is within season, some over many seasons. If I make a mistake in a vineyard block and decide to place too much nitrogen, for example, depending on the formulation I use, liquid, dry or manure, it may take several seasons for me to get back to a level that matches my goals for that particular block. If I grow a canopy that is too large, I have another chance to try again from scratch the next year, not counting minor considerations to the plant structure that may have been created by overgrowth. Those are aspects that we as viticulturists can see physically, if not at the time surely soon after. The plant will show us there is an imbalance, or we just plain screwed up. There are, however, things we cannot see we have seasonal control of in the vineyard, and this is the topic I want to address. One such aspect we cannot see yet, we have control over in our spur pruned vineyards is the internal structure of each new cane that will grow from a bud each new season. The development of this cane over the season is ultimately under our control. So, how should we develop it, and why does it matter?

How we build these canes each year makes us more or less prone to heat stress issues in the vineyard. One of the hardest quality stress related issues that affects vineyards in hot seasons is Sour-Shrivel (You may say, "This happens in normal seasons, too", and I'll get to that.) This particular issue has large quality implications, even in hand-managed vineyards. It's hard for my guys to know which clusters are bad and which are good, as they all look the same. Removing them can be difficult and downright impossible for many machine harvested blocks.  Researchers are working on what may cause this particular issue but have yet to offer anything concrete and relatively nothing as a working theory either. All I can do is offer my opinion here, but I will say that we avoided this issue last season and will this season, as well, barring any major anomalies. 

I believe that this issue is related to stress management of the internal water column of each plant. The resultant shut down of a particular cluster is most likely caused by a cavitation in an area of the plant which is not repairable. In the case of Grenache, which uses water the opposite of many Bordeaux varietals, there is a heavy tendency towards cavitation inside the trunk. Researchers have proven that this is due to a larger natural xylem structure than other grape varieties. Cavitation in many cases is repairable, that is you will not kill the plant if it has a cavitation at some point depending on the stress level and how it was applied in the vineyard, either environmental or man-made. You will see a response in the canopy of yellowing basal leaves or worse within 10 days or so. But by adding water the problem goes away in many circumstances, and the plant goes back to its usual self. Using this as a model for cavitation on a large scale within a plant, there are locations in the plant either by distance from the root system or by a combination of distance and physiological stage where the plant will not repair but shut off and seal completely. This is exactly what I think is happening in the case of Sour-Shrivel.

How can we be less prone to cavitation in plants, which can ultimately lead to Sour-Shrivel in the vineyard? As with many things in viticulture, it's important for us to put the right symptoms with the right action, and as you may know, something you see today could have been caused by a decision made two months prior. This is the case with shrivel. Better early season water management leads to canes which are less prone to cavitation when seasonal stress is applied. 

One of the largest issues in our area last year was not only the drought but how the irrigation districts decided to go about restricting water. I am not here to defend or blame any of their management strategies in the valley. I am just here to explain why May irrigation (or the lack thereof) is possibly the most important period for quality management in Washington State. By scheduling water to be off in May last year, a man-made problem in the vineyard was created. Vineyard managers decided to "water up" prior to the shut off, unbeknownst that there was to be 1.8 inches of rain two weeks later. Too much water at this stage in the game allows the plants to create an internal structure that is larger than what is needed to be able to resist stress and cavitation later. A small change in the xylem size as it is being developed in the new canes results in a plant that is far less stress hardy later on. Controlling growth through this period is very important. Moderation as with many things in life is the key. Plenty of water for growth but too much can lead to problems later. 

What this looks like in the vineyard is a drop dead date for all normal irrigation beginning May 1st of each year for no further irrigation unless necessary. If it is necessary, then only enough water should be used to get the plants to the next week and so on, keeping an eye on the weather for rain and an eye on the plants for stress and growth. There should be no major stress in the plants at this point anyway, as they are utilizing some of the deeper soil moisture. This is beneficial to the viticulturist, because we can never know how much water is in the profile. Only when we are empty or close to empty can we estimate, and we are using this period of time to get rid of any excess that may exist. The only irrigation that should happen in this time period is to make sure there is plenty of water in the soil at least by a few days before bloom. This will keep you from having set and shatter issues related to dry soils. It will not stop you from having set or shatter issues created by inclement weather or proximity to major heart during this period. That is uncontrollable. 

Management during this time period alone can help considerably, however, it's not the whole story. Berry size and crop load seem strongly correlated to Sour-Shrivel. Maintaining tight control on the water after the bloom period and reducing berry size are an important part of the equation, as well. Although, I think it is less of the problem than that of the cane and rachis development, which occurs in May. The real key to this final point is that having an unbalanced crop when stress hits can be a major cause of cavitation, as well. On that note, it is important to reduce crop loads timely for anything that is heavily out of balance. 

The last question which you may have been asking is what kind of stress are we trying to reduce. Largely the type of stress that I am talking about is stress that the viticulturist cannot control much of. If we build a resilient plant structure each season, we can make it through many of the heavy stress periods we do not have control of as viticulturists, giving us a little more choice in our quality than we had before. For example, last season we had 16 days of 100+ degree F days beginning in mid June and extending well into July before we had some relief. This was definitely a stressful time but not the most stressful time of the season for the plants. The most stressful time for the plants was the only 1-2 days of heat right at version for the plant as it is switching from utilization of sugars to utilization of malic acid for respiration. The heat during this period seems to be where Sour-Shrivel is created, and the resiliency of our plants to stress at this period can be managed at a much earlier point in the season. 

I'll attach one caveat to this post. There are many things to consider when making decisions in a field such as these. You still need to make decisions that reflect the characteristics of each vineyard. Not just the soils, varieties and location but also the management, water rights, and logistics of the site as well.

Leave a comment, and tell me what you think.