The answer?

"It depends...literally. It depends on the circumstances and the time of year," Hellman told the crowd at the recent Newsom Grape Day in Plains, Texas. Hellman holds a joint research and extension appointment with Extension, the Texas Agricultural Experiment Station and Texas Tech University.

The annual event sponsored by Extension and Newsom Vineyard featured presentations on wine grape research, irrigation, weed control, barrel reconditioning, and labels. There was also a regional Texas Wine Grape Growers Association meeting.

Other grape days slated across Texas this year include: May 19, McKinney; June 8, Monte Alto; June 10, Spicewood; June 13, Palestine; and Aug. 10, El Paso.

"Water is the most influential factor in growing grapes, especially in West Texas or on the High Plains," Hellman said. "In order to meet the vine's water needs, we have to know when and how much to irrigate."

Technology can help on both counts.

Hellman recommended growers use soil moisture monitors to gauge when vines are thirsty and a Web site, http://texaset.tamu.edu/ , to calculate how much water they should apply.

"The Watermark Sensor is a reliable, easy-to-use and maintain, and affordable soil moisture monitor. It measures how tightly moisture is held to soil particles," Hellman said. "A good approach is to install several monitoring stations in a vineyard."

Two moisture sensors, one placed shallow and one placed deep, comprise a monitoring station. Wires, or leads, from each sensor terminate above the soil surface.

To take a moisture reading, a grower or vineyard worker attaches a meter to the leads and notes the read out. Values recorded by the meter are compared to benchmark values provided by the manufacturer to determine if the soil is wet, dry or somewhere in between.

"The benchmark values are a guide," Hellman said. "By monitoring soil moisture values over time you can key the readings to your climate and soil conditions. In effect, soil moisture monitoring becomes our on-and-off switch for irrigation.

"It's not an expensive tool. The hand-held meter costs about $200 and the sensors cost less than $25 each."

Grape growers can use the Texas Evapotranspiration Network Web site to gauge how much irrigation water to apply. The network and Web site are projects of the Irrigation Technology Center run by the Texas Water Resources Institute and Texas A&M University System scientists.

Evapotranspiration, or ET, is a combination of evaporation and transpiration (plant respiration). ET values express how much water a plant uses.

"ET values are recorded at several weather stations across Texas. You can pick the weather station nearest you at the Web site and note the ET value, which changes frequently," Hellman said. "The Web site also lists crop coefficient values. Multiply the ET value by your crop coefficient and you get a Crop ET, an approximation of how much water it will take to replace what the plants are using.

"But we also have to factor in the efficiency of your irrigation system. The efficiency of drip irrigation typically ranges from 85 percent to 95 percent. Sprinkler efficiency ranges from 60 percent to 75 percent, and furrow efficiency is only 40 percent to 50 percent. Divide the Crop ET by your system's efficiency and you get an approximate amount of water to apply, minus any rain that has fallen."

The Texas ET Web site includes a calculator to make the process easier. Site users can set preferences such as weather station location, crop coefficients, system efficiency and daily or weekly ET reference values to simplify the process, he said.

Some growers practice deficit irrigation by applying less than 100 percent of the plants' water needs. Hellman advised caution.

"Deficit irrigation is viable, but only with mature vines," he said. "We can apply 80 percent of the ET recommendation with no yield loss or vine damage. Some literature touts 60 percent to 70 percent irrigation, but that is not something we recommend except for very experienced producers.

"Deficit irrigation is not something you want to do with young vines, while you are establishing the vineyard. Don't skimp on water with young vines."

More information on growing grapes in Texas is available through the online Texas Winegrape Network, at http://winegrapes.tamu.edu/ . The site features fact sheets and guidelines for grape production, resource information links, weather data, viticulture and enology research reports, and wine industry news.

For more information on the upcoming grape days in Texas, contact Hellman at (806) 746-6101. For more information about the June 10 Wine Grape Field Day in Spicewood, contact Jim Kamas, Extension horticulturist, at (830) 997-3452 ormailto:j-kamas@tamu.edu