What Is the Permanent Wilting Point for Grapevines?

Grapevine Water Needs

• Grapevines have well-developed root systems, which they need to anchor their heavy fruit-bearing vines. Although vines are fairly drought-tolerant, they need adequate irrigation for successful fruit set and development. Insufficient water not only decreases yields but also reduces fruit quality and sweetness. Water-stressed fruits may experience irreversible effects, and delayed irrigation may come too late to restore them. Grapes exposed to sun may burn and shrivel, forming premature raisins.

Field Capacity

• When soil is saturated to the point that it can hold no more water, it’s said to be at “field capacity.” Depending on soil type and texture, moisture content ranges from 10 to 50 percent, according to the University of Minnesota. Sandy soils are porous, with a field capacity of 10 to 30 percent, and do not retain water as long as heavier clay soils, which have a field capacity of 30 to 50 percent. Muscadine grapes (Vitis rotundifolia) are native to the Southeastern U.S., where they thrive in native clay soil. Clay is moisture-retentive and better able to sustain muscadines during drought.

Permanent Wilting Point

• As field capacity diminishes during periods of insufficient rainfall or mechanical irrigation, soils begin to dry and roots cannot take in enough water to hydrate plants. This is called the “permanent wilting point,” beyond which plants cannot recover, even if they receive water. Depending on soil type, soil moisture content at the permanent wilting point reaches 5 to 25 percent, according to the Virginia Cooperative Extension, and plant mortality is inevitable. Because root competition from other plants pulls moisture away from grapevines, keeping vineyards weeded is vital to conserve water.

Solution

• Commercial viticulture, or grape cultivation, must maintain proper levels of soil moisture to ensure profitable yields. Plants lose water through soil evaporation and through leaf transpiration, collectively called "evapotranspiration." According to the Texas AgriLife Extension, the Evapotranspiration (ET) Method uses local weather data for actual and projected rainfall, plus calculations for ET losses, to determine if there is a need for supplemental vineyard irrigation. Soil moisture sensors are also used to determine levels of available moisture to prevent grapevines from reaching their permanent wilting point.