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G r AP e G ro WING is placed inside the pressure bomb. The chamber is pressurized until sap is exuded from the end of the petiole. The amount of pressure needed to move sap from the leaf corresponds to how tightly moisture is being held by the leaf. This is measured in bars or atmospheres. Most grapevines begin to show stress at -12 bars; others may not show stress until -15 bars. Gen- erally, growers with white cultivars and those from northern climates (such as Pi- not Noir and Merlot) use -12 bars as the threshold to begin irrigation. Red cultivars from warmer climates (such as Zinfandel, Syrah, Mourvedre and Sangiovese) can use an irrigation threshold of -15 bars. Vi- sual inspection should confirm that shoot tip growth is slowing, with shoots show- ing small internodes and tendrils. Once irrigation begins, many growers Most drip irrigation systems are designed to deliver about 1 gallon of water per vine per hour, usually with one or two emitters. until a young vineyard has fully expanded and filled the trellis system, which usually takes three to five growing seasons. Dur- ing the growth phase of the vineyard, wa- ter should be available to allow for maxi- mum vegetative growth. Irrigation should be limited in late summer, so that the vine wood can mature and harden off to pre- vent cold damage in the winter. To begin the RDI program, the vineyard soil profile should be fully charged with water at bud break. Most years, rainfall is adequate to do this in many coastal vineyards. In drier climates, preseason ir- rigation may be necessary. Normally, wa- ter is then withheld while the vine canopy is actively elongating and setting fruit to reduce vegetative growth and crop load. Often, when RDI is used, grape clusters are lighter and looser, which can improve wine quality and reduce the potential for bunch rot. Canopies tend to be more open, and red fruit often colors well un- der these conditions. In an RDI approach, a deficit threshold (a predetermined level of midday water deficit) is measured by using a pressure chamber (pressure bomb) to measure vine leaf water status. Usually a fully expanded leaf growing in the sunshine is detached from the vine, placed in a plastic bag, the end of the petiole is cut, and then the leaf apply water based on the percentage of the ground that is shaded by the vineyard canopy multiplied by the evapotranspira- tion rates for a given period of time. For instance, if ET for the week is 15 gallons of water per vine, but the vineyard canopy only covers 60% of the vineyard floor, 15 gallons x .6 = 9 gallons applied per vine. This topic is well covered in "Wine- grape Irrigation Scheduling Using Defi- cit Irrigation Techniques" by Terry Prichard and should be consulted for more details. You can download this publication at ucce.ucdavis.edu/files/ filelibrary/2019/13563.pdf Finally, continue to irrigate right up until harvest. It is important to keep the canopy in good condition so that leaves photosyn- thesize and accumulate sugar in the fruit. If sugar maturity is occurring too rapidly, extra irrigations can slow down ripening, allowing more time for tannins to mature and fruit flavors to develop. Conclusion Irrigation practices continue to evolve in winegrowing. RDI represents a way to conserve water and energy while control- ling vine vigor and improving fruit quali- ty. If you aren't already using this prac- tice, you should learn more about it and see if it is the right thing for you to do in your vineyard. Glenn McGourty is the UC Cooperative Extension winegrowing and plant science advisor for Lake and Mendocino counties. He also tends a 1 acre vineyard of the aro- matic Italian winegrape variety Arneis on his property along the Russian River near Uki- ah, Calif. To comment on this column, email edit@winesandvines.com. Wines & Vines JUne 2011 83 ANDREW TAYLOR