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Winter Cold Injury to Grapevine Canes & TrunksAvoidance Strategies |
Strategies to avoid trunk injury - Site effect
Site selection is covered in a separate document (go there), but a few basics should be kept in mind.
Air and Water Drainage
Both internal soil drainage and external air drainage are very important. Cold air is more dense than warm air and tends to sink and flow down hill just as water does. Vineyard blocks should be sited so that cold air is deflected before entering the block, and any barriers to movement of cold air out of the lower side of the vineyard should be eliminated. Low areas and flat areas collect dense cold air. These should not be planted to cold sensitive varieties.
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Cold injury to a Washington state vineyard. Note that the down slope flow of dense cold air is reflected in the pattern of injury |
As mentioned previously, soils which are periodically wet result in shallow root systems and reduced vine survival. Vineyards should be sited where there is adequate depth of top soil, and water entering the vineyard should be controlled. The Federal Natural Resources Conservation Service provides assistance in planning and sometimes financing a vineyard conservation program. In addition to controlling erosion and reducing environmental impact, these programs tend to improve surface water management.
In the Finger Lakes region of New York vineyards are planted on slopes alongside very deep lakes which rarely freeze. The relative impact of the Finger Lakes and the Great Lakes (Ontario and Erie) on winter temperature are hotly debated by growers. The figure above is from Dieter et. al. and illustrates his view of the "local lake effect".
Dense cold air moving down hill is compressed and warmed. When there is unfrozen water at the base of the slope, the water will be warmer than the arriving cold air (open water is no colder than 32 degrees). The water further warms arriving air. This warm air rises above the lake surface, is slightly cooled and descends along the lakeside slopes. A convection pattern is produced which results in warm air impacting lower, mid-slope regions. There is no question that vineyards immediately adjacent to the Finger Lakes have less cold injury than those higher up the slope. However, it is also a fact that the Lakes provide a very large, barrier free surface for cold, dense air to collect, and the natural outlets for water also are outlets for cold air. In the process cold air is moved away from the vineyard district.
Having recently moved to a house alongside the west side of Seneca Lake, I have had another insight regarding Finger Lake impact on adjacent vineyards.
Above is a view from my porch at sunrise of a clear, cold, still day. The lake is "steaming" because air in contact with the water is warmed and moistened. Warm air can hold more water than cold air. The steam is formed by water condensing as the air rises and is cooled. The same effect on a larger scale causes a cap of clouds above the entire length of the lake. The cap forms when rising air is cooled to the dew point. In very cold still weather this cap of clouds can rise a as much as a mile above each Finger Lake. Still air is required for the cap to form, but air is never so still that it doesn't drift. Some mornings I leave my west side home in bright sun and travel to the east side of the lake and have to turn on the car lights because I am driving in fog formed from eastward drifting of the cap. Most often the direction of drift is east, but other directions are frequent.
This cap decreases vineyard cold in two ways. First it is visible evidence of air warmed by the lake moving over the lakeside slopes and directly transferring heat energy. The other effect has to do with radiational loss of heat from the ground surface. During the day radiant sunlight energy is absorbed by the soil surface. On a clear night this energy is re-radiated to space resulting in dramatic cooling of the air near the ground. If the cap of "steam" has moved over a vineyard, the radiated energy is reflected back to the ground, and cooling rate is much reduced.
I adapted the above figure from one produced by Dieter It shows the expected winter minimum isotherms (based on expected 10 year minimum) in New York state. The Great Lakes (Ontario and Erie) contain much more water than the Finger Lakes. Cold, arctic air passing over the Great Lakes is warmed and modifies the climate of the Lake Erie shore (home of the juice grape) and the Finger Lakes. Long Island Sound and the Atlantic Ocean surround Long Island and provide even more moderation.
The impact of elevation can also be seen. Water collects in low areas. The Great Lakes, the Finger Lakes, Long Island sound all represent relatively low elevation sites.
Aspect
The aspect is the compass direction of the site (the slope direction). In the northern hemisphere people are told that south facing slopes are preferred for vineyards. At very high (more northerly) locations a steep south facing slope may substantially increase the amount of sunlight energy received by the vineyard. However at more moderate latitudes, such as are found in California and New York, the beneficial effects are less apparent. In fact excessive exposure to sunlight in winter can be a problem. The radiation load on the southern portion of an upright trunk in winter can be higher than in summer (because of a low sun angle). In late winter sun heated trunks may lose cold hardiness and be injured. This type of injury is called southwest injury, because that is the trunk sector where major heating happens and where injury is usually found.
Trunks are a very vulnerable tissue and with cold sensitive varieties the impact of trunk injury is often more devastating than injury to other parts of the vine. This is due to several facts.
For this reason many people are willing to invest more effort in protecting the tender trunk from cold exposure. The several approaches are based upon:
Preventing tissues from becoming warmed on a sunny day.
Sun warming may make the local tissues become de-hardened. Cold sensitive tissues will be more likely to become injured during the night. This is often called southwest injury because that portion of a vine or tree will experience the most sun warming and hence the most subsequent cold injury
Trunks can be coated with reflective paint to prevent the absorption of sun energy. This is effective for trees, but is more difficult for grapevines. The bark of grapevines is continuously shed in strips, so it is difficult to establish and maintain a painted surface. We have tried reflective materials like aluminum foil, but I am not aware of anyone who has been able to show a commercial benefit.
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Various forms of winter trunk protection used in tests performed by Gary Howard of the New York State Agricultural Experiment Station with a goal of reducing sun heating of trunks. (Some of these are also reflective) The variety is Chardonnay and methods include white paint, reflective foil and foam wraps. |
The problem here is that the insulation doesn't prevent the tissues from reaching air temperature, it only slows the rate of heat loss from the trunks. In a typical winter cold episode, the vines have a long time to lose heat. They may start at a temperature within 5-10 degrees of the killing temperature. A subsequent drop in air temperature need only reduce the tissue temperature a few degrees to cause damage. Wind has an effect here of aiding heat loss. In New York, trunk injury is often the result of cold air masses which are accompanied by wind and are of sufficient duration to bring even insulated temperatures to air temperature.
A side aspect of insulation is what happens following tissue injury. The conditions within the insulation (probably high humidity) favor callus growth. Small injuries under insulation often result in unusually extensive crown gall.
Covering vines with plastic or some other material which will provide a greenhouse effect or will reflect heat.
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Plastic covered grapevines in Japan. Similar covers are used extensively in southern Europe and other regions to cause early bud break and to hasten grape maturity. |
I don't have any direct experience with the possible winter protection from such covers, but the structures have to be able to resist harsh winter conditions. It might be of benefit, but the cost to benefit ratio might be quite high.
High grafting to a cold hardy rootstock or interstem.
We have tried to produce nursery plants with scions grafted to 3 or more feet of rootstock. We have had no success. High field budding, cleft or T-bud grafting has been suggested, but the cool springs and short season in the northeastern states makes it very difficult to achieve success.
Another problem with the concept of a cold hardy trunk is what to do should the high scions become injured or killed. In New York we not only have to deal with cold winters, but we also have a high potential for eutypa die-back disease. If the scion variety is killed, re-grafting would be expensive and very difficult.
Burial in earth or mulch
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Vines which have been hilled-up so that their graft unions are covered by earth. This ensures that scion buds above the graft union will survive even should the above ground portions of the vine be killed. |
Burial is a most practical solution. In very cold regions whole vines are buried, but whole vine burial is not done to any extent in New York. The most common practice is to "hill-up" the vines with a mound of earth in the fall. The earth should cover the graft union to ensure that some scion buds will survive should the above ground tissues all be killed.
Avoiding scion rooting
Usually the hill is removed in the spring, but some people remove the hill on an alternate year basis. Because scion roots which form during the growing season must be removed to ensure that phylloxera sensitive roots do not replace the resistant rootstock, alternate year uncovering is probably not cost efficient.
Canes or other portions of vines may also be covered. (see bud cold injury).
