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p r a c t i c a l w i n e r y & v i n e ya r d a p r i l 2 0 1 4 57 g r a p e g r o w i n g SUPPLIERS OF QUALITY VIRUS CERTIFIED GRAFTED GRAPEVINES Deadline for Custom Grafted Order For Spring Planting is January of Previous Year For Further Information & Pricing Please Call or Write 1912 Concession 4 R.R. #2 Niagara-on-the-Lake Ontario, Canada LOS 1J0 Phone: 905-468-0822 Fax: 905-468-0344 Mori_Nov05.qxt 9/19/05 3:29 PM Page 1 Figure 3: When vines were grown near permanent wilting point (8% water content in loam sand) but then watered back to field capacity, the canopy growth resumed but flowering clusters were aborted (inset). dients included field capacity (FC) to permanent wilting point (PWP). At FC, the soil has the maximum amount of water it can hold; any additional water is usually lost to deep percolation or runs off. If a soil is near the PWP, plants have limited ability to extract water from the soil and can potentially suffer irreversible damage to the plant (die). In loamy sand, FC is 15% water content, and PWP is 7% water content by volume. In sandy loam, FC is 23% water content, and PWP is 11% water content by volume. In this experiment, the soil moisture gra- dients tested were at 1% intervals between FC and PWP. Those levels of soil moisture were maintained through the trial. The plants were then tested for their ability to bleed and for the quantity of sap they could produce (Figure 2). Bleeding sap was monitored after bud break to deter- mine if vigor (the rate of shoot growth, total leaves produced and fruit set) was correlated with the rate of sap flow. Soil moisture influences early season growth Sap flow started almost immediately (within one to three days) in vines that were irrigated to FC. This was followed by rapid and uniform bud break and shoot growth. In vines maintained near PWP after trans- fer from cold storage, delayed bud break occurred and sap flow was not observed. Preliminary tests showed that if vines initially maintained at PWP were watered back to FC (before the buds died), they would resume normal bud break and shoot development. Shoot growth and sap bleeding rate were strongly correlated with soil mois- ture in both soil types. Shoot vigor (elon- gation and leaf expansion) was the greatest when soil moisture levels were between FC and 2% less than FC (13% -15% for loamy sand, and 21% -23% for sandy loam). When soil moisture declined, vine vigor declined and fruit set also declined. Overall, vines in soil that was near PWP (7% water content in loamy sand and 11% water content in sandy loam) had very poor growth or neared death. However, vines at 1% above the PWP (8% water content in loamy sand and 12% in sandy loam) survived although inflorescence abortion was seen (Figure 3, inset). This study will continue in 2014 to see when drought-stressed vines should be watered to prevent long-term detrimental effects. In other words, at what point can vines at PWP be watered to FC and still pro- duce a viable crop and functional canopy? Irrigate early if the soil is dry Based on these results, growers are advised to measure soil moisture before bud break and to irrigate if the moisture level is at 3% below FC or lower. This threshold is especially important because sandy soils have a narrow "window" of available soil water. Alternatively, irrigation could be initi- ated if sap bleeding does not start despite warming soil temperatures (above 45° F) or if bud break seems erratic despite favorable air temperatures (above 50° F). While spring conditions are typically wet in the Pacific Northwest, do not forget the importance of recouping from a dry winter with added irriga- tion. This will optimize early canopy development and minimize crop loss. To overcome the potential problem of winter drought-stress, test your soil moisture and, if necessary, irrigate when water is available. PWV This text was edited from first publication in the fall 2013 Washington State University Viticulture & Enology Extension News. Issues can be downloaded at wine.wsu.edu/ research-extension/publications/newsletter. Photo BY hemAnt gohil