After a winter such as this past one, with record setting snowfall and heavy rains, one would think drought conditions in the summer months would be improbable. And one would be correct; if one were not a grapevine. While the water table and other reserves of drinking water currently remain at stable levels, the lack of rain combined with record high temperatures have created ‘drought like’ conditions in most vineyards in the area.
These conditions, each taken on their own, would not be cause for concern. We have always told you that grapevines prefer hot, dry weather. This is very true in regard to disease pressure and ripening potential — and it is these very conditions that make California an ideal environment for winegrowing. Growers in the Golden State have it pretty easy in many regards compared to their eastern counterparts. However, one thing they must be diligent about is water management. After all, nothing will live for long without water.
Here in the east, liberal amounts of rainfall (combined with soils that have a high water-holding capacity) create conditions where surface soils above 18” or so remain reasonably moist. This causes a vine to root rather shallow, since it does not have to look to deeper soils to find water. But in those rare years where water is not readily available, and extreme temperatures cause surface soils to dry out quickly, vines with a shallow root system will begin to show signs of water stress earlier than those with deeper roots.
This is also the case in vineyards that are regularly irrigated. In the West, this is very often the case as there is no choice but to irrigate in regions with little to no summer rainfall. In Virginia, irrigations systems are installed mostly as an insurance policy. They are used regularly in the first few years while vineyards are being established, assuring that a vine grows to reach the wires and develop strong roots. Prudent growers will begin to restrict water applications by the third year, forcing the vines to look deeper for moist soils. Many growers (ourselves included,) will encourage “cover crops” on the vineyard floor to compete with the vines for surface moisture; again, causing the roots to look deep for water and further increasing the vines natural drought tolerance.
Many readers may remember winemakers across the state singing the praises of the 2007 vintage for its dry, ‘California like’ weather conditions; so you may ask, “Why are you concerned now?” There are two distinct differences between 2007 and the current year:
First is the timing of the dry weather. In ’07 we had moderate to normal amounts of rainfall through most of the growing season. It wasn’t until late summer that things began to dry up and conditions became ideal for ripening fruit. Even then, many vineyards (particularly those in the central and southern parts of the state) began to exhibit signs of water stress. Water stress, especially after veraison (the point at which grapes begin to change color and build sugar) can inhibit fruit maturation… so those growers with irrigations systems were advised to turn them on to get their grapes to harvest.
The second difference between 2007 and 2010 are the extreme temperatures. High ambient temperatures can cause surface soils to dry out more rapidly, reducing available water for the roots to access. This condition is further compounded by increased transpiration of water through the vines leaves as it tries to cool itself, much as people do when they sweat. This creates a situation where the vine requires more water than it normally would and cannot find it in the now arid soil; a dangerous combination to say the least.
Here at ‘The Cellars,’ we are using a two-pronged approach to deal with the hot, dry weather. First, we had to address the cover crop in the area just beneath the vine row.
Over the past several years, our non-chemical weed control program has encouraged the establishment of native grasses and broad leaf plants in this area. This cover has provided several benefits over the years: competing with the vines for water and thereby reducing the vigor of the vine while increasing its drought tolerance (our vineyard is showing less stress than some of our fellow growers that have no cover crop); eliminating soil-damaging herbicide use in the vineyard; and creating a habitat for beneficial insects that reduce our dependence on harmful insecticide sprays. Now though, in these very dry conditions, this competitive cover will begin to work against us by sucking up what little rainwater is delivered to the vineyard, as well as what we can deliver through irrigation. So, the cover crop must be eliminated!
This brings us to the second prong of our strategy; turning on the water. Certainly, with enough water delivered through the irrigation system, eventually it would reach the roots of the vine — despite the competition from cover crops. In extreme heat though, the vines are transpiring water very rapidly (upwards of 3.25 gallons per vine, per day.) With water being such a precious resource (especially without some rain to replenish our well) and considering the energy consumption needed to continually run our pump, we need to make the process of replacing this water as efficient as possible.
Our vineyard is broken up into 10 individual irrigation zones and the initial watering in each zone lasted about 24 hours. Subsequent applications will be done on an ‘as needed’ basis in 12 hour increments; during the evening and night time hours when temperatures are cooler and evaporation potential at a minimum. If dry conditions and the need for irrigation continue, moisture monitoring sensors can be installed to further increase irrigation efficiency.
Beyond that, we can address the permanent, grassy cover crop in the areas between the rows and/or spray the vines with a heavy coating of fine clay to reflect the suns rays reducing the rate of transpiration. These are more extreme measures that could have lasting repercussions such as erosion (in the case of the former) and reduced or delayed ripening in the case of the latter. These would be “last-ditch” efforts to combat a prolonged drought in the case that we had to shut down the irrigation system.
Fortunately, we are a long way from having to take such extreme measures, as the dry weather has set the clock ahead for us and we are rapidly approaching harvest. Careful monitoring of the vineyard will be necessary as always, we simply have one more thing to be mindful of.
Until next time….
-Tom
Tom – Thanks for such an informative post! In talking with a few other wine makers, they too seemed concerned with the higher than usual temperatures coupled with the lack of rain. At what point do you make the decision to eliminate the cover crop in the rows, and more importantly, how do you go about removing the cover crop (via spray, or tilling?).
Frank,
Thanks so much for your interests and question. As for the first part of your question, removal of the cover crop should be done at the first sign of water stress, unless there is a high probability of rain in the immediate forecast. Vines under stress will exhibit several early symptoms including cessation of shoot tip growth, yellowing and eventual loss of older basil leaves around the fruit zone and upward cupping of younger leaves.
Tillage would be the preferred method of removing the cover in a sustainable vineyard; unfortunately, we do not possess any equipment to perform in-row cultivation so we had no choice but to use herbicide. I am confident though, given the amount of work we do to build healthy, living soils that the negative impacts of a single herbicide application will be minimal.
An ancillary benefit to removing the existing cover is that I will now have the opportunity it replace it with more specific species of plants. Rather than allowing native broad leaf “weeds” and tall growing pasture grass to repopulate this area, I will sow in a low growing grass and clover blend that will provide several benefits: First, I can customize the grass species to the characteristics of each variety. For example, Seyval Blanc, which has relatively low vigor, will receive a species that is less competitive while the more vigorous Cabernet Franc will get one that is more competitive. Also, a low growing grass will not need to be mowed down throughout the growing season there by reducing the amount of tractor passes through the vineyard, which in turn will reduce the potential for soil compaction, reduce consumption of fossil fuels and reduce labor costs. Second, the addition of clover, which fixes atmospheric nitrogen into the soil, will help to reduce the need for supplemental nitrogen applications. All of which, I feel, will improve the health of the vineyard and the environment.
Tom