Bleeding (inset photo) caused by root pressure is associated with bud swell and bud break. Bleeding sap is collected from a potted Merlot grapevine in a Washington State University study looking at the consequences of dry soil during bud swell and bud break.

Bleeding (inset photo) caused by root pressure is associated with bud swell and bud break. Bleeding sap is collected from a potted Merlot grapevine in a Washington State University study looking at the consequences of dry soil during bud swell and bud break.

Adequate soil moisture in the spring is even more important for grapevines than previously thought. Washington State University scientists have confirmed that low soil moisture delays bud break and slows shoot growth, but early spring irrigation can overcome problems from dry soils.

After an especially dry winter in 2005, where the Pacific Northwest received less than half of normal precipitation, many Washington vineyards experienced stunted shoot growth and compromised yield. That’s not surprising, according to Dr. Markus Keller, WSU horticulturist. It’s known that bud break and growth require water for cell division, and if there’s not enough soil moisture in the spring, bud break is delayed, shoot growth is stunted, and there is poor flower development.

“But in the past, we’ve underestimated the significance of adequate soil moisture in the spring,” he said. Keller, who specializes in grapevine physiology, explained that WSU research helped show the connection between bleeding, or sap flow, and bud break.

“In grapevines, root pressure is the driving force for water flow to swelling buds and young shoots in spring,” he explained. Root pressure arises from remobilization of stored nutrient reserves (starch, proteins, organic acids, potassium, and calcium) into the xylem to raise sap osmotic pressure. It’s this osmotic pressure that draws in water from the soil, which results in a hydrostatic pressure in the xylem that pushes water up the vine and leads to sap flow.

Studies have found that sap pressure of 0.1 megapascals can support a water column 33 feet high, he said, which is why pruning wounds bleed.

It’s the root pressure that restores the xylem function, dissolving and pushing out air bubbles from winter freezes, and rehydrating and reactivating grape buds, Keller explained.

“The problem is that winter precipitation is not always sufficient to replenish soil moisture for adequate root pressure, which may then hinder bud break and restrict shoot growth,” Keller said in a phone interview with Good Fruit Grower.


Keller, along with WSU colleague Dr. Bhaskar Bondada, WSU graduate student Colin Lee, and Italian intern Giulio Carmassi, conducted a study to determine if bleeding is a prerequisite for bud break and canopy development. The research team wanted to test if spring shoot vigor and fruit set are related to soil moisture before or during bud break and to develop practical recommendations for early season irrigation management.

Potted Merlot vines were grown in a greenhouse and exposed to a range of soil moisture levels, from field capacity to permanent wilting point. Soil moisture levels were maintained from before bud swell through bloom. Loamy sand and sandy loam soil textures were used because of their different water-holding capacities. Bleeding sap flow and sugar content were measured and pot weight, soil temperature, vine phenology, growth, and yield data recorded.

“Our research found that the link of delayed bud break and vine development to dry winters is real,” Keller said.

In their study, sap flow and composition seemed little affected by soil moisture and soil type when moisture levels were greater than the permanent wilting point. However, bleeding and bud break were not possible when soil moisture was near permanent wilting point. Soil moisture levels 2 percent above permanent wilting point resulted in delayed bud break. Subsequent shoot growth and fruit set increased up to soil moisture levels that were 4 percent below field capacity.

Low soil moisture also resulted in abortion of clusters, decreased percentage of fruit set, and increased the incidence and severity of inflorescence necrosis.

In both soil types, variation in the soil moisture accounted for about 75 percent of the variation in shoot and internode length, leaf number and size, and total leaf area.

The scientists also found that rapid sap flow was correlated with rapid shoot growth.

“We confirmed that after dry winters, vines may be unable to generate adequate root pressure, which leads to malfunction of their hydraulic system,” Keller reported.

In essence, no, or low, bleeding leads to delayed and uneven bud break, stunted shoot growth, slow canopy establishment, aborted flowers, and poor fruit set, he said. Poor shoot growth leads to low yields and, ultimately, to reduced profits.

Early season ­irrigation

While a lack of bleeding indicates dry soil and potential problems, the remedy is fairly easy—­irrigate early.

Keller advises growers to know their soil types and water-holding capacity. Loamy soils have higher water-holding capacity but require higher soil moisture levels than sandy soils for grapevine bleeding. This means that even the best soil moisture measurements are useless without knowledge of the soil’s moisture profile. Therefore, growers should also determine the field capacity and permanent wilting point for each soil type.

“Measure soil moisture before bud break and irrigate if the soil is 4 percent below field capacity,” he recommends. “Also, monitor vine growth and irrigate if you don’t see bleeding or bud swell despite rising temperatures.”

Early irrigation—filling the top three feet of the soil profile to 3 percent below field capacity—may prevent dieback of young vines and severe crop loss in mature vines.

Funding for the research was received through the U.S. Department of Agriculture’s Northwest Center for Small Fruits Research and the Wine Advisory Board/Washington State University.