VINE’S WATER NEEDS

Both are excellent methods to better understand the vines’ water needs, Ferry-Abee says. “Leaf water potential is measured as quickly as possible after removal from the vine to prevent excess evapotranspiration.

“Stem water potential tends to be slightly more accurate, but is also more time-consuming, Leaves are covered with a foil-covered bag and left on the vine for an hour or more before measurement. This extra step allows transpiration to stop and water levels in the leaf to equalize with the rest of the vine.”

She lists the basic steps for each method:

Mid-day leaf water potential: Measure from 12:00 noon to 2:00 p.m.; use leaves fully exposed to the sun. Wrap leaves tightly in a clear bag and cut off the leaf with a sharp razor blade. Quickly insert the leaf into the pressure chamber, and slowly pressurize the chamber. Carefully watch the petiole with a magnifying loupe. As soon as you see sap beginning to exude from the petiole, stop increasing the pressure. The pressure in negative centibars is the leaf water potential.

Mid-day stem water potential: Measure from 12:00 noon to 2:00 p.m. Select leaves in the shade.

Wrap leaves tightly in a bag covered in aluminum foil to block out the sun and prevent the bag from becoming a mini-greenhouse.Leave the leaf and bag on the stem for 1½ to 2 hours.

Cut off the leaf with a sharp razor blade and measure as above.

Regardless of the method used, Ferry-Abee recommends taking at least three measurements per vineyard block or soil type, then averaging the results. Select leaves that are fully expanded and healthy, with no holes or pest damage. Also, try to keep the timing between cutting the petioles and beginning pressurization consistent, and raise the pressure in the chamber at the same rate for all samples.

Accurate results require consistency, she notes. The measurements should be taken by someone — preferably the same person — who is well-trained in the procedures.

“Pick one of these methods and stick with it,” Ferry-Abee says. “Don’t switch between leaf and stem water potential throughout the season.”