The following is a guest post by Gary Strachan, an international vineyard and winery consultant specializing in the start up of vineyards and wineries. Please see his complete bio at the end of this post!
The popularity of the BC (British Columbia) wine industry has encouraged new entrants to establish vineyards on sites that are less expensive, sites that have not formerly supported viticulture. ¬†This factor, along with the more extreme weather events that seem to accompany climate change, has made us more aware of the risk of late spring frost that occurs after grape vines have emerged from dormancy and buds have begun to swell.There are several passive ways to minimize this risk. One way is to locate on a slope with good air drainage, not in a valley bottom where cold air can collect like water in a puddle. ¬†Another is to examine the surrounding terrain. ¬†Is there a slope or trough above the site that will channel cold air from higher elevations toward the property? ¬†Look below the site. ¬†Are there features that will block air flow from leaving the property . . . features such as a wooded area or a nearby hill?
On the site itself, are there depressions in the terrain? ¬†Even though the depressions may seem shallow and insignificant, if they allow cold air to puddle, they can lower the temperature of the canopy above the depression and contribute to frost damage or delayed ripening. ¬†Get rid of the depressions by carefully removing the top soil.¬† Fill the depressions with typical sub soil and restore the top soil.
Even on a well drained, uniform slope that has been undisturbed, there can be problems. ¬†Soil composition is typically more sandy at the top of a slope and more loamy at the bottom. ¬†The loamy soil will hold more water and nutrient and vines will typically break bud earlier than the less vigorous plants at the top of the slope. ¬†It may take years to overcome this problem by building up soil humus at the upper part of the slope by careful observation and good vineyard management.
The temperature of fruiting buds is less likely to drop as low if the renewal zone is raised higher off the ground.¬† Forexample, the fruiting zone of a hanging cane trellis is more resistant to frost injury than a VSP (a type of vertical trellis system). ¬†In addition, canes harden off earlier and have deeper dormancy on a hanging cane trellis than VSP.
None of these strategies will succeed if the vineyard is located in a region with a strongly continental climate. ¬†The presence of a large body of water removes the extremes of temperature in a region because water has such a high specific heat and also because the heat of evaporation and heat of freezing are so large. ¬†Some regions may have a similar mean temperature but the extremes can be greater. ¬†This is known as a continental climate, in contrast to the alternative which is referred to as a marine climate. ¬†Visualize the contrast between the climates of Vancouver and Winnipeg.
Inherited properties between different grape varieties and different clones can make a difference. ¬†Riesling is known for its late bud break and winter hardiness, but it is also known for its late harvest date, which makes it inappropriate for short season sites.¬† Many vinifera and hybrid vines have short vegetative seasons and have information available on their winter hardiness, but information about the time of budbreak is often hard to find.¬† The most winter hardy Vitis species often have shallow dormancy and this trait can inadvertently be passed on to their selected progeny. ¬†By shallow dormancy I refer to the property that the vine emerges rapidly from dormancy when the soil temperature begins to increase.
If passive methods don‚Äôt safeguard your buds, then you may have to use an active method. ¬†The best known of thesestrategies is delayed pruning. ¬†The number of days that budbreak can be delayed is variable, but a week‚Äôs delay in not an unreasonable expectation for most circumstances. ¬†Under extreme conditions when winter cold injury is expected, coupled with spring frost damage, pruning can be delayed until after budbreak. ¬†It may create an ugly trellis but enough canes or spurs can be selected to produce a reasonable crop in spite of heavy damage. ¬†The new buds are quite fragile, so pruning must be done with care to avoid damaging the remaining live buds.
Other active methods such as oil sprays, alginate sprays, ice crystallization bacteria, and plant hormone treatments are not yet mainstream but information is available about their use and potential effectiveness. ¬†I have seen no information about whether the effect of these treatments are additive, i.e. would an alginate treatment extend the effectiveness of late pruning?
The loss of buds due to a late frost can be costly. ¬†If it happens once, try to not let it happen again. ¬†Perhaps you need a wind machine?
Gary Strachan is an international vineyard and winery consultant who lives in Summerland, British Columbia. ¬†He is a former Agriculture Canada research scientist who studied cool climate grape varieties and and winemaking. ¬†He taught viticulture and enology courses at Okanagan College and Vancouver Island University for over 25 years. ¬†He is the former Canadian delegate to the O.I.V. and is current Chairman of BC’s Sustainable Practices Committee. ¬†Much of his professional practice is centered on the design and¬†start up¬†of vineyards and wineries.
Gary Strachan is listed on LinkedIn.