Cover Crops to the Rescue

With winter rains already starting and more wet weather expected throughout the season, cover crops can help offset overly wet vineyard soils
Dec 11, 2009

The storm door opened for California this last week and it has been wet. Although the main rainmaker in this series of storms is forecast for tomorrow night, the ground is already starting to produce runoff. The combined total for the mid-October storm and this week's rain is approaching 6 inches. Considering that in 2007 we received little more than 6 inches for the entire winter and that our main rain-producing months of January through March are still ahead of us, a wet winter preceding the 2010 vintage appears to be developing. (Just as forecast by the current El Niño conditions in the Pacific Ocean.)

After this weekend's storm, sunshine returns along with a warmer week. Perfect conditions for producing pasture in the hills for the cattle and for getting a good strong cover crop, which is already growing between the vines, firmly established.

Too much available water in the soils can cause problems by creating excess vigor in the vineyard. It prevents us from being able to control the vines using deficit irrigation in the spring and summer when the vines are growing. The result of vine vigor is overgrown canopies and shaded fruit with vegetal flavors and green tannins, along with large clusters and large berries that result in less concentrated wines that lack good flavor and color. Instead of feeling an urgency to ripen fruit in the late summer, vines with excess available water will continue to grow longer canes and will remain in a vegetative growth phase.

Now that our Pinot Noir vines have established roots that fully explore the 2.5 feet of soil over calcareous bedrock beneath them, the optimal amount of rainfall for good wine quality in our typical soil conditions seems to be around 12 to 14 inches on a normal year here in the coastal climate of the Sta. Rita Hills. This varies depending on soil depth, water-retention capacity, crop load and how heat is distributed through the growing season.

To show the complexity of the issue, I am going to stray a bit and provide a little background that may help you catch a glimpse into winegrowing and this notion of vine vigor. One of the most important tools for balancing vine vigor is the amount of fruit the vines are carrying. The fruit is a sink for the vine's energy produced by converting sunlight, carbon dioxide and water into sugars.

Consider that Mother Nature times the amount of sunlight, and the cool and warm periods of weather during the growing season always varies. Because the weather is unpredictable, without a crystal ball it is always a best guess as to how much fruit the vines should be carrying to balance the apparent vigor and available soil moisture over the growing season. The sweet spot in terms of what is the perfect amount of fruit hanging on the vine is a moving target.

A key for the interested wine drinker in understanding the winemaking that occurs in vineyards begins with the fact that the development of flavors in the grape clusters is on a different trajectory than the accumulation of sugars. The goal is to have all of these disparate components that are flavors and flavor precursors ripen simultaneously with the developing sugars that the vines are storing. This is done through many techniques that were learned from observations of the "dying vine makes the best wine" historical myth, briefly discussed in the previous blog.

With the inherent variability that fickle Mother Nature bestows, the ability to have some kind of control over vine vigor and growth during the growing and the ripening of the numerous "wine" precursors that the fruit represents is obviously beneficial. This brings us to the potential for excess rainfall and too much water in the soil limiting the ability to harness the tool of controlled water deficits and from applying techniques learned from "the dying vine."

This is why I am very happy to see the vineyard cover crop beginning to grow and develop so early in the year! The cover crop is the tool that allows us to remove water from the soil. With a little luck and good timing with the weather, we will use the weeds, grasses and forbs (small, herbaceous flowering plants) growing between the vines to reestablish control of vine vigor at some point during the summer growing season by removing excess soil water.

This means more frequent mowing and providing nutrients to the cover crop itself. Rather than waiting to mow once, as I typically do when the cover crop has set its seeds and died in the late spring, I will need to mow earlier and more often. This will help create a thicker and more branching habit to the cover crop plants, prevent seed formation and senescence (the biological changes brought on by aging). By keeping the cover crop healthy and growing into the late spring and early summer, I hope to deplete the soil water using plant transpiration and to reestablish soil-water deficits.

Using the cover crop is an important tool in addition to removing excess water. It also prevents erosion and loss of soil to water movement and wind. It then decomposes to provide the nutrients for the soil fungi, bacteria and protozoa that form the soil ecosystem. As roots decay, pores form in the ground where the plant roots were and allow water and air in to improve the soil.

Keep the cover crop growing! (At least that's the plan at this early juncture after seeing Mother Nature deal two cards …)

Harvest United States California Santa Barbara County 2009

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