Almost every climatic region grows grapes. Grapes are grown in extremely hot regions and very cold regions, but for a long time it has been known that some cultivars perform better in warm areas and others better in cooler areas. Cool versus cold climate locations produce different characters in the same type of grape grown, but cold climates create the additional challenge of grapevine survival through the winter period. Therefore, a realistic acknowledgment of the viticultural potential of a region is required: while it may be possible to grow grapes in a site, that does not mean that it is possible to make a living out of it.
Low temperature is one environmental factor that presents significant challenges, but also potential advantages. Cold climate viticulture is defined by the winter low temperatures in the region, a parameter that is a major concern of site selection. Data exist on the historical low temperatures in most regions likely to be planted to grapes. Figure 4.1 shows the 100-year minimum
temperature probability for Geneva, NY, USA. In this example, in half of the past 100 years the low temperature was ⫺22°C or less, which is a pivotal temperature for native American grapevine survival. This information can be used to assess the degree of risk a prospective grower might be willing to take to grow a specific cultivar.
The effects of cold temperatures in the dormant season are not usually apparent until the following spring, when buds do not grow, or grow weakly and then stall. Shoots may fail to develop normally in the spring if one or more of the shoot primordia have been killed by the low temperatures (see Plate 16).
Plate 17 (left) shows a photograph of a cold-damaged vine well after budbreak. Its shoots developed normally for a short while and then faltered.
Closer examination showed that the cambial layer in the cane had been killed during the winter (Plate 17, right). Since the xylem had not been affected, early shoot development was normal but, since there was no functional phloem in the cane, carbohydrates were not able to be moved to the developing shoots.
Grape species and cultivars differ in their adaptation to low winter tempera- tures (Pool et al., 1990) and, as one might consider cool climate viticulture being defined by summer temperatures and frost during bloom, cold climate viticulture is directly associated with the minimum low temperature reached in the winter dormant period. Grape species have dormant survival ranges from a
9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0
Probability
–35 –33 –31 –29 –27 –25 –23 –21 –19 Temperature (°C)
Fig. 4.1. One hundred-year probabilities for different levels of winter minimum temperatures for Geneva, New York.
few degrees above freezing to less than 40°F below zero (⫺45°C) (Pierquet and Stushnoff, 1980). Grapevines enter winter by gradually developing hardiness (the ability to withstand temperatures that the growing plant could not survive). Grapes acclimate to winter temperatures at different rates: some harden off more rapidly than others and some lose cold hardiness in the spring more quickly than others (Wolf and Cook, 1992). Not all grape buds on a vine are killed at the same low temperature, so some proportion of the buds may survive to produce a crop or, failing that, allow the vine to survive and grow another year.
Grape buds that have had time to develop hardiness generally survive lower temperatures, so therefore the more basal buds (for all cultivars) and buds on canes that stop growth earlier (on early-maturing cultivars) tend to survive lower temperatures. Any environmental event that shortens the leaf activity season, such as an early frost, will delay the development of maximum hardiness (Howell, 2001). A gradual change in temperature, as usually occurs during the transition from summer to autumn to winter, is most desirable in maximizing cold hardiness, as periods of warmer temperatures can de-acclimate the buds and make them more susceptible to cold damage.
It is possible to measure the killing temperature of grape buds during development of hardiness and in the depth of winter through a freezing isotherm procedure (Wolf and Pool, 1987), which has been used to predict the potential damage caused by a freeze event. With this knowledge, viticulturists can then adjust their pruning levels to try to compensate for the damaged buds.
Canes may also be collected and their buds examined microscopically for damage (Goffinet, 2004). Percentage bud survival can then be determined and pruning to a proportionally greater number of nodes delayed until the danger of cold temperature is reduced. As grapevines have compound buds at each node position (with primary, secondary and tertiary buds enclosed within one set of bud scales) and the degree of hardiness of the buds varies, the effects of some freeze events can be overcome at pruning.
The possibility of damage from low winter temperatures can also be managed. The first defence (aside from selecting a site in an area that does not become so cold) is in choosing grape cultivars known to tolerate the expected low temperatures of the region. The second defence is to protect the vines from damage by the low temperatures through avoidance. One way of doing this uses the same technique as used for spring frost mitigation (Chapter 6), as long as an inversion layer, where warm air is on top of the cold air near the ground, exists. Fans can mix the warmer upper air with the colder lower air and prevent the temperature from reaching as low as it might have otherwise done.
However, if no inversion layer is present, there is no possibility of warming up the air near the vines.
Another method of managing winter cold damage is to leave multiple trunks, frequently three to five, each a different age so that if some are killed (a selection of trunk ages spreads the risk of damage) others can provide fruit
during the next growing season. As the trunks of grapevines are just as vulnerable to winter cold injury as the canes and buds, if they are damaged they are susceptible to the development of crown gall (see Chapter 9), which can result in vine death during the subsequent growing season.
A more labour-intensive method is covering or burying the vines or parts of the vines. Canes trained to a low wire, or those trained to the fruiting wire, can be taken off, laid on the ground and covered with soil using a grape hoe or plough (see Plate 18). The soil provides insulation from the cold air tempera- tures, but is also kept warm by the large mass of soil beneath. Straw mulch applied over canes has also been used successfully for this purpose (Zabadal and Dittmer, 2003). This ensures at least a minimal number of bearing canes for the next season, although there can be a reduction in bud viability and cluster number as a result of this practice (Goffinet and Martinson, 2007).
Another approach does not protect the bearing canes but buries the vine crowns, assuring that new growth can arise from the crown. However, if there are killing temperatures, there will be a delay in getting a crop off the vines as new shoots will need to be trained up on to the trellis. Because burying protects the crown from cold damage, infection by crown gall-causing bacteria is also limited, compared with methods leaving the canes exposed.
In the methods involving burying of vines, if they are grafted it is possible that scion rooting may take place. This is where the scion develops roots from above the graft union and, as these tend to be quite vigorous, they can take over from the rootstock roots, resulting in an own-rooted vine. For this reason it is important to remove all soil from near the graft union following the winter period.
To summarize, grape production in cold climates is accomplished by (i) avoiding damage, usually by growing extremely hardy cultivars or by burying vines; and (ii) managing the damage by delayed pruning or leaving extra buds based on bud survival counting.