Good Fruit Grower

February 15

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Irrigate early if winter is dry 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. Inadequate soil moisture in spring can delay grape bud break and vine growth. by Melissa Hansen dequate 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. Research 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. WSU grape irrigation bulletin Photos by Giulio Carmassi W 30 FEBRUARY 15, 2013 GOOD FRUIT GROWER ashington State University Extension has released a new irrigation manual for vineyards, clarifying irrigation options and strategies for juice and wine grape producers in Washington State. ���Irrigation Basics for Eastern Washington Vineyards��� is the title of the bulletin that seeks to help growers better understand what���s behind grape irrigation. Two companion pocket manuals are also available: ���Estimating Soil Moisture by Feel and Appearance,��� published by the U.S. Department of Agriculture���s Natural Resources Conservation Service, and from the National Center for Appropriate Technology, ���Water Management: the Pacific Northwest Irrigator���s Pocket Guide.��� Both manuals and the new bulletin are available through Washington State University���s Extension Irrigation Web site at

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