Maintaining Winter Soil Moisture Levels
January 10, 2013 Towards the end of last season as we moved through late summer and into the fall, our grapevines were doing their job by maturing their crop, and we, as growers, started to relax. But this is not a time when growers should become complacent. Most of the time I see growers getting so absorbed with the logistics of harvest that they are not paying attention to the health of their vines. In the time leading up to harvest, soils should be kept reasonably moist to allow for normal vine functions. After harvest, restoring soil moisture should be a top priority. If you don’t maintain relatively moist soils, your vines miss out on an opportunity to improve overall health and ensure the best possible crop yield next season.
Vines Busy After Harvest
When we look at grapevines after harvest it doesn’t seem like much is happening but there are several important things going on.
• Fall Root Flush: Within a few days after harvest vines start their postharvest root flush. This is the larger of the two roots flushes during the season (the other is just after bloom). Without adequate soil moisture this root flush is inhibited, so irrigating right after harvest is important to long-term vine health.
• Acquiring Nutrients: Vines acquire significant amounts of nitrogen and potassium as well as some micronutrients in the fall after the crop has been removed. This process is delayed until there is adequate moisture in the soils.
• Building Carbohydrate Reserves: During crop ripening, vines draw on their carbohydrate reserves. By the time we reach harvest carbohydrates are at their lowest level of the season (particularly if the crop was larger than normal). The postharvest period is a time when the vine is rebuilding its carbohydrates for next season’s crop and preparing for winter dormancy. This process is slowed with dryer soils, and low carbohydrate levels can lead to delay spring growth (DSG).
Mid-Winter Irrigations
In more arid grape growing areas, mid-winter irrigations are beneficial for the vines unless winter rainfall amounts are well above normal.
• Winter Injury: Maintaining moist soil conditions during the winter can help grapevines retain moisture that is lost through freezing and dry weather. Desiccation (drying out of the vine itself) can injure canes, spurs, and buds on mature vines and injury to the trunks and cordons on young vines. The bottom line is that maintaining soil moisture levels results in vines that have adequate moisture content which can then withstand colder temperature without injury.
• Boron Deficiencies: When grapevines experience very dry soil conditions during the early part of the winter, they can suffer from a drought-induced boron deficiency. This is known as Barnes Syndrome and it appears at bud break with erratic, slow, and distorted shoot growth. The shoots will be dwarfed, with short internodes that may grow in a zigzag manner.
In The Springtime
In the spring at bud break and through early cane growth we receive the first report card that tells us how good a job we did last fall and during the winter.
• Delayed Spring Growth (DSG): Insufficient carbohydrate reserves at bud break can result in DSG. The symptoms of DSG can include irregular and poor bud break and low shoot vigor. If bud break normally occurs over a one-week period for a given vineyard, with severe DSG bud break can take three weeks to complete. Also with severe symptoms there is typically a reduced crop.
• Winter Injury: The vine symptoms from winter injury (i.e. freeze damage) can sometimes look like DSG particularly for younger vines (less than five years old). One way to tell the difference is to cut into the wood (the trunk, canes, or spur) and look for sections of the wood that have a light brown, spongy appearance.
It pays to take care of your vines through the winter because once you see the symptoms of DSG or winter injury, there is nothing you can do about them.
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