You would think that we already know enough about water management of grapevines that articles on the subject would not be necessary each and every year. But that is not the case. I continually see growers making the same mistakes year after year. Thankfully, it is not necessarily the same growers making the same mistakes, but not all of us in the industry seem to be enlightened just yet. And to be fair, I have found some mistakes in the way that I've thought about water management that I'm not afraid to admit.
Please note that I used the phrase "water management" and not irrigation. Water management goes well beyond irrigation and, in large part, involves the practice of not irrigating.
Allow me to go over what I feel are some of the main fallacies and foibles shared by both growers and us so-called water management experts.
1. It's late spring, and Just a little irrigation won't hurt...
Yes, it will. The most critical time in a vineyard's growing season, as it pertains to water (and fertilizer) management, is during the active growth phase of development. In North Coast vineyards the soil begins each season with a profile full of available moisture. Most growers do not even think about irrigating up through May, but come June, the temperatures get warm and the irrigation trigger finger gets itchy. It happens to everyone.
I've been asked numerous times, "Can I just put a little on, just in case it gets hot?" My answer is usually no: because there is still ample moisture in the soil; because the vines are not stressed and will cope with the heat without irrigation; and (most importantly) because the shoots are still bright green and not showing any signs of slowing down.
In my opinion, 75 percent of water management of North Coast vineyards concerns holding off on any application of supplemental water, at least until the soil moisture is depleted to a level where the vines will slow down and stop vegetative growth. Applying "just a little" before shoot growth ceases just prolongs the desired outcome, possibly (or probably) allowing the canopy to become overgrown and delaying the redirection of carbohydrates away from the shoot tips and into the fruit, dormant buds and permanent structure of the vine.
The best news is that it doesn't take a pressure chamber, soil moisture probe or weather station to determine how long to refrain from the irrigation itch. Just walk the vineyard frequently, looking upward (hopefully upward!) at the shoot tips and observe whether they are actively growing, slowing down or stopped. A scoring system may be used, if desired, such as the shoot tip rating system I wrote about last year1. The decision is trickier in non-uniform vineyards as shoot growth in some areas may be stopped while others are still growing. A decision must be made based on the condition of the majority of the vines. Better yet, if irrigation lines can be installed to irrigate the different zones separately, the vineyard irrigation initiation decisions can be customized to zones within the block.
Note that what I just wrote does not necessarily apply in the same manner to vineyards along much of the Central Coast, such as Salinas Valley, Eastern Paso Robles or Santa Maria-Santa Barbara. Those areas commonly receive only 8-10 inches of precipitation during the rainy season and often require some supplemental irrigation to get reasonable shoot length (about 3.5 to 4 feet or about 18 nodes) and to avoid water stress during fruit set.
But the same principle applies: get control of shoot growth before it gets excessive and then maintain the vines during the remainder of the season with restrained irrigation, avoiding visible signs of stress responses.
2. Stressed vines make the best wines...
I readily admit that I have been guilty of this common fallacy. In reality, "stress" is an overused term in viticulture. What really is vine stress? Environmental stresses occur every season in the vineyard, including heat, wind, cold, nutrient and water. The way the vines respond to those stresses is what we observe and/or measure. And, while we might wish to impose some stresses on the vines, such as limiting water availability, we do not want our vines to be negatively impacted by those stresses. Shedding leaves or losing fruit to dehydration is not a path to high wine quality, nor is it a means to attain sustainable viticulture.
At the other extreme, we want to avoid providing more resources to our vines than they require. Much in the same way we limit ourselves in eating hot fudge sundaes, we should avoid providing our grapevines with luxurious amounts of water and nutrients so they produce and maintain a small, yet healthy and effective, canopy for developing the fruit that they carry. But no more than that.
Besides environmental stresses, another way we stress our vines is by overcropping them. Overcropping increases the vineyard's requirement for nutrients and water. Cutting back water availability to what should be reasonable for a moderately cropped vineyard will induce a stress response in a vineyard that is carrying too much fruit. In other words, a heavily cropped vineyard that is dropping leaves by harvest is not making better wine than its more lightly cropped and similarly irrigated neighbor that retains a green canopy.
Just after harvest, it is easy to see if a vineyard has been overcropped and overstressed: most of the leaves fall off just days after the fruit is removed. Vines that are not stressed in this manner will hang onto their leaves for weeks after harvest. The benefit to the vines (and their yields) over the long-term cannot be overstated.
3. The pressure chamber is the best instrument for monitoring vineyard water status...
I love the pressure chamber (a.k.a. pressure "bomb"). It goes wherever I go, and I rely on it because it provides a quantitative measurement of vine water status. It is field rugged so that when I drop it off the back of my tailgate (which I have done) it keeps on working. I don't plan on putting my pressure chamber in storage anytime soon. As someone who has made literally thousands of measurements, I can impress people by estimating vine water status usually to within a half of a bar (pressure unit) just by looking at the vine before I make the measurement. Nevertheless, the pressure chamber measurement can often be misinterpreted because it is only a part of the water status picture.
The pressure chamber measures leaf water potential, which is loosely defined as the sap tension (or suction) within the xylem vessels of the stem. The tension arises from evaporation from the leaf's stomatal cavities (transpiration) as well as the matrix forces holding onto the water in the soil-a tug-of-war situation. The drier the soil, the tighter the water is held and the more energy is required to extract the water to feed the transpiration stream.
But leaf water potential only describes the energy state of the water in the vine. We don't know how the vine is responding to the condition it is in. Ideally, we would like to know about the state of all of the physiological processes in the leaves and in the fruit, but that is not at all practical. However, an instrument that I really like is the porometer, which measures the degree of stomatal opening or closing. Stomata (tiny leaf pores) are very important because they regulate the amount of water vapor loss from the leaves but also affect the rate of CO2 gas that is available to be assimilated. So, knowledge of the stomatal conductance, as it is referred to, gives us very good insight into the response of the vine to current water and environmental stresses.
I have said that if the pressure chamber is my left arm, then the porometer is my right (and I'm right-handed). So, if I absolutely had to give one up, it would be the pressure chamber. But porometers are expensive and tend to be less rugged than other field equipment. That may change soon, however, as I am aware of an inexpensive unit that will become available soon.
I am also regaining interest in soil moisture monitoring. I have long eschewed measuring soil moisture due to the spatial heterogeneity (complexity and variability) of most vineyard soils and drip-irrigation wetting patterns. Add to that my desire to measure the vine, which is the focus of attention, and not the soil, which is part of the environment. However, soil moisture devices can do one thing that I cannot do with either a pressure chamber or porometer: make measurements when I'm not there.
I have experienced problems with making pressure chamber measurements on days during irrigation events versus those made between irrigation applications. Vine water status during irrigation application will be misleadingly high when the vines are being irrigated. Yet, it is difficult for viticulturists to plan vineyard visits around irrigation schedules of individual blocks. On the other hand, growers with their own technical staff will have the ability to make measurements of vine water status multiple times per week in each vineyard, thereby eliminating that concern.
The most valuable soil moisture measurements are those made continuously and recorded frequently, such as hourly. Continuous data logging of soil moisture used to be expensive-about $2,000 per monitoring site, plus data logging equipment. However, new and cheaper sensors and data logger technology are dropping the cost of continuous soil moisture measurement down to the vicinity of hundreds of dollars per site-a good value.
Careful installation of soil moisture sensors is essential, including site placement and proximity to the drip emitters. Universal calibrations to soil texture are available but will only serve as approximations. Rather than relying on the soil moisture measurements as absolute quantities, it is more useful to develop customized thresholds as well as to look at patterns of soil moisture wetting and depletion over time.
Next year, I'll let you know what other mistakes we've been making. In the meantime, relax, your vines aren't as thirsty as you think. wbm
1 Greenspan, M.D. Integrated Irrigation Management in California. Practical Winery and Vineyard. Part 1: Monitoring and Scheduling. March/April 2005
