Do large crops impact vines?

Large crops in 2014, followed by a dry year. Low crops in 2015 and one of the worst droughts on record. Then large crops again in 2016. What is the likely impact of those past years on the vines themselves and how can you prevent placing them under more stress in the season to come?

While the past two years of dry may have caused concern, Dr Marc Greven a scientist at Plant & Food Research in Marlborough says recent projects have shown that vines are resilient to water stress.

“We have shown in principal that the vines can handle quite a bit of time without water. If you stop irrigation for two or three weeks around veraison there doesn’t seem to be carryover effect on yield the next year. So the drought is not essentially going to be a major driver.”

However the same can’t be said about large crop loads and the effect of that on the quality of wood available to pruners, according to Dr Damian Martin, Plant & Food’s Science Group Leader in Marlborough.

“Generally what happens after a big crop is you have less good wood to choose from for pruning. That is one of the things that drives the lower yields (the following year). You might
be wanting to lay down four
good canes, but you can only find two good ones, one skinny, ratty one and the fourth cane you cannot find.”

There is also the issue that after large crops, the carbohydrate reserves of the vine can be low going into the following spring, just when they are needed for shoot development.

“The vines use all their reserves to grow up to a point and produce all the leaves until a couple of weeks before flowering. So if the vines have run out of carbohydrates by that point and don’t have sufficient leaf area to drive the system, and then you get cold weather, that is going to have a big impact on the potential crop load. But you need two or three disruptive events to line up to have a dramatic impact in terms of reduction,” Martin says.

If carbohydrate reserves are low, they will show up in the vine the next spring.

“If the bud burst is irregular and early season growth is struggling to get up to the wire, then that is a sure sign the carbohydrate supply is low. The best thing you can do is shoot thin as early as possible. (If you don’t) you are spreading a limited amount of carbohydrates over too many shoots. By thinning, the remaining shoots grow individually better. You will have a bit of an impact on yield, but you will not have another carry-over effect the following year.”

Martin says Sauvignon Blanc is one of the grape varieties that is most susceptible to the carry-over effect, because it is known to throw big crops in some years. And that is just what happened in 2016. The 39 percent increase in Marlborough yields (with the majority of that coming from Sauvignon Blanc) has a lot to do with the low yields in 2015. But also the timely bursts of rain in January and March impacted on the size of the berries, something that Greven says was not considered an issue some years ago.

“We always used to consider that berry size was more or less stable. But in the last few years we have come back on that in a big way.”
Martin concurs saying that berry size, out of all the yield prediction formulae, was considered to have a plus or minus 10 percent effect.

“But berry weight has influenced (the yield size) by up to 20 percent a couple of times. Both in 2014 and 2016.”

The saving grace for the upcoming season though is Mother Nature, delivering a long, drawn out autumn.

“It takes time for any carbohydrates that are left in your vine to become reserves. The shoots harden up from the bottom upwards, so they lock in their reserves. But there are a whole lot of carbohydrates in the leaves and the green tissue that need time to migrate back down into the plant. If the vine gets frosted and the leaves fall off then you don’t benefit from that.

“But this year we certainly had time for everything to move back down and probably right through the vine.”
Experience, he says, would say that we don’t need to be panicking a lot.

“But the proof will be how the canopies grow early in spring.”

Avoiding knee jerk reactions to high and low yields is also advisable, Greven says. Especially when it comes to letting yields influence how many canes you lay down that year.

He advises growers to learn the capacity of the vineyard, what level of yield it can comfortably deliver, and avoid making too severe adjustments based on the previous year’s yield. After a low yield year do not lay down more canes as the vine is already likely to produce more the following year. Similarly after a high yield year the vine will produce less fruit (relatively), the year after.

“You might have to adjust it a little bit, but don’t overdo it.”

While there is no current research that looks at early spring growth, Martin is hopeful that will change with new technology. He says currently the logistics of going out and doing shoot growth measurements in the spring makes it difficult to provide all important data to the industry.

“It is just not practically possible. But with a bit of luck in coming years we might have the technology and cameras that can provide us with that data. There are some things we should be able to do in that space to provide better decision support for the grower, especially around that early season growth – because it is really important.”