Wednesday, March 15, 2017
Temperature Variations and How They Affect the Grapes
(bud break - 2016)
One of the more popular questions I get from customers is: "Are the grapes affected by the fluctuation in temperature we see in the early Spring?" I wish the answer could be provided with a simple yes or no, but the fact is that only a long-winded answer provides the detail needed to fully understand what happens.
To begin we have to go back to October or November. It is after harvest now, and the grapevines have been collecting the needed carbohydrates to get them through the winter and jump start their growth the following Spring. As the temperatures start to drop, there is something called "cold weather hardening" where the shoots begin to change from their green color over to brown, and as time goes on a covering of bark will develop. The buds, as small as they appear to be, begin going through a dehydrating process that will provide a kind of protection from the icing conditions that are apt to follow during the winter months.
I am not sure when we arrived at our "rule of thumb", but what we have found is that when temperatures begin to warm up, we forecast our bud break (or bud burst) when we have 10 straight days of 50 degree average temperatures.During these warmer days, the air and the soil begin their climb, which influences the flow of water and nutrients from the soil up into the trunk, and eventually to the buds. To put it in more scientific terms, the warming of the soil is associated with the increased osmotic pressures in the xylem, with the upward movement of the water containing various cellular signaling molecules, hormones, and nutrients. The buds become re-hydrated and they become deacclimated (i.e they get re-introduced to warmer temperatures and less resistant to cold spells). You know all this is in high gear when you go into the field and find that the vines are bleeding from all the pruning cuts made during the winter.
If the vines have started to wean themselves out from their winter mode, there is apt to be plenty of water now in the trunks and buds. A reduction in temperature in and of itself is not really a problem here.A mild short term cold front is likely to just calm them down a bit, but a freeze has the potential of causing severe frost damage by killing the newly formed buds or splitting the trunks. A prolonged cold period can also cause problems, because once a vine has decided to come out of dormancy, it is very difficult to return to it's resting phase.
So what can be done? Prayer is on the list of viable options. There are other choices however, and one has to consider the cost vs. the potential losses. For a consistent year to year problem, wind mills have been set up to keep air flowing through the vines. Having a helicopter hover over the vineyard is a variation of this. Burning tires(certainly not preferred) or having other small fires may raise the temperature in the vineyard. I have heard of spraying oil on the buds, which may delay bud break by 2 to 20 days. And yet another option might be to spray water on the vines, thinking that a thin layer of ice might protect the buds from even colder damage.
What we did last year was to "long prune", which recognizes that we will loose some buds but hopefully we have enough good ones left to achieve the harvest levels we need. Delayed pruning was not really an option for us because we have a lot of vines but not a lot of hands to perform all the pruning required in a limited time before bud break.
Now let me add two other variables to this. Grape types come into play. Our Seyval Blanc has already seen some bud swell, which puts it on the teetering edge of popping open. Our other varieties show bud formation by their hard knobby presence, but they haven't shown signs of increasing in size due to tissue wetting. Some say that younger vines have bud break sooner than older vines; I have not seen any consistent example of this. Lastly, how much water is in the soil has an influence too. Wet Falls, melting snows, or Spring rains all add water to the root absorption zones, and it this water that finds it's way up to the trunks, buds, and shoots via capillary action or transpiration.
I have looked at last year's temperature charts which confirm my 10-day hypothesis (a sample of one year I might note). In this month of March 2017, even though we have had fond memories of warm spells, the fact is that the longest period of 50+ average daily temperatures has been only 3 days (March 7th through 9th). What I do see down the pike in the last week of March is a forecast of a week long string of very warm 80+ days but sub-freezing temperatures at night. These all satisfy our 50+ rule. All I can say is: "Pray".