In the previous article about growing high-quality grapes in a covered environment (see “Beating Mother Nature at her Own Game” in the March 2013 issue of Wines & Vines), I presented evidence that winegrape quality was more than sufficient to examine the business/financial and physical aspects of growing grapes in a three-season tunnel.

Can a high tunnel pay for itself?
The simple answer to whether a high tunnel can pay for itself is that, in many environments, it may take up to three years—or it might take one day. Most growers use tunnels to mitigate or lessen some form of environmental upset that occurs at a vineyard location on too regular a basis, such as early or late frosts or too much rain.

Grapegrowers should assess their vineyards from a financial standpoint: How often is there a spring frost event at the wrong time? How often is there a vineyard loss of 50% of the fruit for the year? If a grower is harvesting high-value crops with an average price of $2,000 or more per ton, the annual return per acre is $8,000 or more. A 50% loss means the grower turned over cash but did not make any money, with the intangible loss of a year of his or her life working to produce high-quality fruit. With grapes, a grower can’t make up the difference the next year by increasing the vineyard’s yield.

There is also the possibility that the grower will lose not only the current year’s profit but also have additional costs for rebuilding the vineyard for several years after the event. If a spring frost wipes out a season of production, averting one catastrophic loss could potentially pay for the tunnel.

However, if the business includes both a vineyard and a winery, then the loss is magnified by the value these crops provide to the winery’s bottom line. To the farmer the loss is $4,000 per acre, but for the winery side of the business, that loss represents about 100 cases of wine, or the equivalent of least a $24,000 drop in revenue. The more important that particular wine is to the winery’s bottom line, the more significantly that number can increase.

Another major benefit of the tunnel is an increased yield of equal or better quality fruit. In the previous article, I explained there was nearly a doubling of the yield for grapes grown in the tunnel as opposed to those grown outside. With little increase in farming cost, the per-acre yield should easily double to $16,000 per acre.

The tunnel environment has several other advantages. For example, we all know the soil is a fundamental factor in the growing of high-quality grapes. One of the major issues in all commercial farming is soil compaction due to heavy equipment accessing the fields. The tunnel structure provides an excellent opportunity to modify farming practices to mitigate compaction to nearly negligible levels.

In addition, the tunnel can be viewed as a containment vessel. If your vineyard is in a location where surrounding property owners are worried about spray drift, the tunnel can provide an excellent physical barrier. Just roll down the sides, shut the door and spray.

Other secondary reasons for purchasing tunnels will become evident as we look at the various aspects of designing and installing a tunnel system.

Tunnel design factors
Several considerations must be evaluated when choosing a tunnel type. Existing terrain is certainly important, but the most important consideration is working height. It is possible to work inside a tunnel with mechanized equipment, and with the exception of the largest pieces of equipment, just about all types of grapegrowing devices can be used in a tunnel. The most important limitation is how to work the rows next to the pole-line. The solution is to leave more row edge to get vertical height (see “Basic Hoop Layout” on page 90.)

The next element to evaluate is the degree of water control desired. In the next section, where I discuss how to grow grapes in a tunnel, you will find information about managing the plastic skin. The skin can be rolled up or down depending on the needs of the vines. When the sides are rolled up, water will enter the tunnel at the drip line if it rains. In many cases this type of water can be managed. However, if this is an issue, there is a tunnel design (shown in the photo on page 89) that allows for more control of this water.

The slope of a vineyard is not a hindrance to installing a high tunnel and, in fact, sloping ground can provide unanticipated benefits for tunnel consideration. Orienting the tunnel along the slope line can provide a warming effect that can help save crops during a freeze event. There are places where tunnels have been installed on slopes up to 20% where, on days when outside temperatures were at or near freezing, the tunnels were blasting warm air out the top of the tunnel. The infrared light was trapped in the tunnel, which warmed the air; warm air rises and so it moved up slope. The cooler air entered the tunnel and was warmed, repeating the cycle. In critical times, artificial heat can be used to create a directed airflow to prevent frost or freeze damage. In some cases, growers have placed baffles in the tunnel to segment and slow the air down so the warmth remained in each baffled area.

One factor that must be taken into account with tunnels is the height of the tunnel. The higher the tunnel, the lower the wind velocity the covered tunnel will tolerate. Some growers have chosen to use narrow-width bays to get the height needed. Others have chosen to use both wide bays and high tunnels and then installed windbreaks to lessen the chance of wind damage.

Designing your tunnel layout
It may seem counter-intuitive to think about growing grapes in a tunnel in the first place, let alone being able to make money at it, too. The physics of environmental control are straightforward. However, getting the right balance of structure cost to performance is the key factor.

The experimental vineyard described in Part I of this series was laid out with a plant density of 1,500 plants per acre. It should be noted that when vines are planted in a tunnel, a small amount of useable land is “lost” with each bay of a tunnel system. Thus, it is important to know as much as possible about the objectives desired so that you make the most efficient use of the space under the tunnel.

Tunnel bays come in a variety of widths. Because of the cost of infrastructure, it does not make economic sense to use tunnels with a width of less than 24 feet, and there are several reasons to use wider bays. Basic Hoop Layout on page 90 illustrates row configurations for widths from 20 feet to 28 feet. Approximately 0.8-1.0 meters of clear space from the pole line toward the row on either side of the pole is needed to allow for pruning, harvesting and other activities to service the needs of both the vines and the structure.

Once the bay size is determined, the frame is installed by drilling hoop posts in the appropriate spots and installing the correct cross bracing. How much bracing is needed is determined by the maximum velocity of wind the tunnel is expected to withstand before removal of the covering is demanded for structural safety. Many bays are rated to withstand up to 60 mph of sustained winds.

Although tunnels can be retrofitted onto an existing vineyard, it is easier to install a tunnel prior to planting. However, only the hoops need be in place before planting begins. A grower should expect to apply the same techniques used in any high-quality planting regimen. It is even possible to laser plant the vineyard. The height of the tunnel allows small equipment inside for inserting trellis materials. There is a choice of whether or not to cover the end posts with the tunnel. This decision is based on how, when and how tightly the tunnel will need to be closed during frost or freeze events.

After vines and trellis are planted, drip irrigation can be installed using buried or elevated soaking hoses or by button drippers. The tunnel floor can be left bare. In our experience, the plant growth under the vines is relatively easy to maintain, but by far the best way is to lay down the black agricultural fabric for weed control.

The plastic skin is the last item to be installed, and contractors will do the initial installation. While it is not terribly difficult to install, it is a good idea to pay attention when they do the installation so that the grower and/or vineyard workers will be familiar with the procedure.

The plastic used is a special blend specifically designed to block ultraviolet light. It also scatters the light very effectively within the tunnel, which results in less influence of light on row orientation. In addition, more light penetrates deeper into the canopy than indirect sunlight, and there is virtually no chance of sunburn on the crop, yet the clusters attain full color for both red and white grapes. Lignification of the stems happens in the normal course of plant development. The plastic will last for about three to four years.

How to grow grapes in a tunnel
In large measure, growers using tunnels have the same goals and parameters as conventional vineyards. Tunnels allow for several “outside the box” concepts that have the possibility of providing significant modification to “normal” grapegrowing.

Pruning: High-quality fruit is not generally subject to mechanical pruning, and so any limitation presented by a tunnel with this aspect of grapegrowing is not going to be an issue. That being said, there do not seem to be any limitations to current pruning methods that would pose any limitation in a tunnel from conventional methods. Timing of pruning will need modification due to the desires for early bud push and crop management. The vineyard will need to be season-ready considerably earlier than the comparable outside vines.

Canopy management: When growing grapes, a tunnel gets the vines off to a racing start. Each season when the vines begin to push, we have vines transition from bud swelling to shoot growth very quickly. The same vines outside the tunnel lag by days. For the grapes used in the trial at Tamanend Winery in Pennsylvania, we established a Double Guyot cordon-training system, where one cordon from each side of last year’s growth is removed during pruning and a new cordon is chosen to replace it out of the current year’s growth. Then the remaining cordon on each side is used for the current crop. This system has been determined to minimize buildup of diseases by having the cordons regularly replaced.

Preparation of the tunnel for the growing season requires getting the plastic skin on the hoops. The time to do this should be based upon when you want the grapes to ripen and the risk tolerance for a heavy spring snow event. Up to several inches of snow is tolerable, provided there is enough personnel to knock off the snow before it accumulates on top of the tunnel to any degree. During this time of year it is best to keep the doors closed in order to trap the heat within the tunnel. It is also recommended that the sides of the tunnel remain fully down.

The orientation of the vines into a traditional VSP system is straightforward. The canopy fills in quickly, making it necessary to begin hedging much earlier than vines grown outside.

After bud push, the plastic sides should remain down until after the latest date for a spring freeze. After that date, the plastic sides should be rolled up to get as much air and diffuse light into the tunnel as possible. Avoid rolling up the top’s skin during cold weather and when water avoidance is necessary.

Spraying: Tunnels do not require as many sprays or even the same sprays. For example, a grower will not have to spray for Japanese beetles. The beetles don’t like the diffused light within the tunnel and won’t enter. Downy mildew is much less of a problem in tunnels, but powdery mildew can be a problem. There is virtually no weed control necessary.

Many growers use an airblast sprayer set up at one end of the tunnel. All you need to do is turn it on and calculate the time it would take to get to about half way down the tunnel and let the sprayer spray. Move the sprayer from row to row and then move it to the opposite end of the tunnel and repeat. This has been done successfully elsewhere with no perceptible difference between interior plants and perimeter plants.

Another system was developed by a group of inventive people where sprays are introduced via a monorail attached to the central high point of the tunnel. A bar was constructed that hung from the rail and spread over the rows. A spray rig was fashioned over each row to cover the vines from top to bottom, and flexible hose was then run up each spray rig to the top of the monorail and back to a hose-containment box. This hose was then fed by a spray tank hooked up to the PTO of the tractor. A pulley system was also integrated into the monorail so that the spray rig could be moved from one end of the tunnel to the other. No one had to be in the tunnel when it was sprayed, and there was no drift outside the enclosure.

Fertilization: Whereas it is possible to add nutrients directly into the soil profile in the same manner as is usually done in the vineyard, the tunnel provides unique opportunities to fine-tune the soil nutrition profile. Drip irrigation allows for the injection of nutrients into the irrigation water. Thus nutritional deficits can be managed relatively easily.

Bird and pest control: Tunnels offer a unique opportunity to manage bird and pest damage. The perimeter of the tunnel has plastic walls that deer, rodents and other small mammals can’t see through; as a result, usually these pests are not much of a problem. The tunnel’s biggest advantage in pest control is for protection from birds. The rigid structure of the tunnel allows for the attachment of netting to the hoops so birds will not be able to gain entry into the enclosure. The perimeter attachment also allows free access to the fruit.

Harvesting: In many cases, high-quality fruit is hand picked. There is no difference in the methods of harvest in a tunnel environment from that outside the tunnel. It is possible to use mechanical harvesting equipment in a tunnel; the only significant limitation is row height on the boundary rows of each bay. While extensions of the legs are available on certain tunnel hoops, the increase in height will require downgrading the maximum wind the tunnel will be able to sustain. In the future, there will likely be equipment developed that can handle these limitations.

Conclusion  
As demonstrated here, tunnels can provide an alternative mechanism for growing higher valued crops. The primary benefits the tunnel provides are frost protection against crop loss and an increased yield with equal or higher quality fruit than from vines grown outside the tunnel. Just with the increased yield, the tunnel pays for itself in three years. Additional benefits include adjusting ripening times for process convenience, water control, spray reduction and greater sustainability of the vineyard from farming practices.

This article is the second in a series to explore the means and potential for changing viticultural practices to enhance quality, consistency, value and sustainability. Dr. Richard Carey is president of Vitis Wine Center and winemaker for Tamanend Winery in Lancaster, Pa. He has written numerous articles on new technologies for the grape and wine industry as well as a series of articles on laboratory analyses in Wine East.