For crop production, water is everything. Although water is critical to plant growth and survival, water sources and control over them can vary substantially across the production regions of the world. For grapes, many regions in New World production areas would not be so successful without irrigation and the control over grape growth and productivity it has provided. Irrigation has significantly changed the way grapes are grown and managed in large-scale production systems by allowing grape production to expand into arid regions and by improving control of plant nutrient status, water status, and frost and disease management. Grape producers in Washington State commonly use irrigation for either juice or wine grapes. However, the various irrigation options available can often appear contradictory, or are poorly explained in terms of practical application.
This Washington State University Extension publication presents and further clarifies irrigation options and strategies for both juice and wine grape production in Washington State. Companion pocket manuals that supplement this publication include the USDA-NRCS Program Aid 1619: Estimating Soil Moisture by Feel and Appearance, and the National Center for Appropriate Technology: Water Management: The Pacific Northwest Irrigator’s Pocket Guide. Both manuals are available upon request through Washington State University’s Irrigation Extension website at http://irrigation.wsu.edu.
Irrigation Strategies for Wine and Juice Grapes
Wine grapes (Vitis vinifera) adapt well to water stress because their roots can effectively search out moisture. In general, when supplied with enough or excess water, all grape varieties preferentially grow vegetatively (e.g., shoots).
Irrigation strategies for wine grapes in arid grape-growing regions are designed to optimize fruit quality, influence potential wine style, and control canopy vigor. For example, many components of wine aroma come from the production of isoprenoid compounds (e.g., monoterpenoids). These aromatics are typically produced late in the berry-ripening period. Some are produced in the skin of the grape berry, others in the flesh of the berry. Consequently, changes in berry volume (size) may alter the ratios of these particular compounds, which can be productive or counterproductive, depending on wine-style goals. Alterations in canopy development, and thus, canopy microclimate and fruit exposure, can also alter the production of these compounds, as
Juice grapes (Vitis x labruscana ‘Concord’ and ‘Niagara’) are better adapted to the typically high annual precipitation of eastern North America (30 to 50+ inches). Irrigation strategies for juice grapes reflect this evolutionary difference and are designed to optimize sugar accumulation and encourage high yields.
General Irrigation Strategies in Grape Production
Understanding the way vines use water is paramount to understanding irrigation strategies. During different stages of vine development, water is needed for different physiological processes. These water requirements can be manipulated to control vegetative and reproductive growth of the vine. Since these plants have adapted to water stress, mild water stress can actually help the plant become more efficient in its use of water. It does this because stress increases abscisic acid (ABA), a plant hormone that is commonly produced as a response to environmental stressors, and in this case, is produced in the roots as a result of dry soil. The ABA is transported through the plant xylem (water-conducting vessels) to the shoots of the plant, signaling the leaves to close their stomata (tiny pores on the undersides of leaves that facilitate the intake of carbon dioxide (CO2) and export of water and oxygen). ABA can inhibit shoot growth and may also inhibit other hormonal activity. Unfortunately, one consequence of this reaction to water stress is that while the closing of stomata prevents water loss, it also reduces the intake of CO2. This reduction in CO2 intake reduces photosynthesis and subsequent carbon assimilation (production of carbohydrates for immediate use and storage) in the plant.
Budbreak to bloom. At the beginning of the growing season, water uptake by the plant roots is required to hydrate young buds and aid in an even budbreak. Water is also essential for the uptake of soil-available nutrients. Thus, early-season fertilization will be ineffective, unless it is applied with appropriate levels of plant-available water.
Between budbreak and bloom, the vine is undergoing many developmental changes that are sensitive to plant water levels. Shoots are growing rapidly, and the development of a fully functional canopy that maximizes sunlight absorption is critical (Figure 1A). This rapid growth also requires substantial nutrient uptake from the soil, which is achieved through