Frosts in California (and elsewhere!)

I have mentioned frosts before (August 14 and October 17, 2009), but some recent news brings this subject up again.

Early season frost is a big problem in New Zealand, but also in a lot of other growing regions around the world, such as in Ontario, which I've talked about before. The latest news from California, in an article from Western Farm Press, details the aftermath of a couple of frost events on April 8-10 in areas south of San Francisco. Damage estimates suggested that about one-quarter of the 10,500 hectares of grapes in the Paso Robles area will have a crop that won't be commercially worth harvesting.

Any grower worth his weight in quality secateurs should know what the frost risk is when looking to develop a vineyard, and if the risk is perceived as being great enough, some sort of management system will be planned for the development

Results of a severe spring frost - all green tissues of the vine are completely decimated

If the frosts are mild, then passive means, such as having bare soil between the rows in the spring, which will maximise heat accumulation during the day and release at night, can give from 0.5 to 1.0°C protection (of course, careful decision making in site selection is the best means of passive management!).

Active means of management include such things as taking advantage of an inversion layer (where a layer of warm air sits above the cold air at vine level, and which tend to form on still nights with clear skies) and sending up a helicopter to push the warm air down to mix with the cold.

However, if the frosts are very frequent, then installing a permanent system becomes cost effective, such as the wind machines I've already written about, or using water sprinkled over the vines.

A frost fan installed in Central Otago. Plenty of snow-capped mountains around the area!

There are some big differences between the wind and water methods. The movement of air method relies on mixing the warmer air in an inversion layer with the cold air on the ground. This works great unless 1) the inversion layer doesn't have enough warm air to raise the temperature of the cold air to above freezing or 2) there is no inversion layer.

The former is heartbreaking, as you might be successfully fighting off the freezing temperatures, then just before dawn and the temperature rise that comes with it, the inversion layer is all used up and the grape buds freeze.

The latter has the same outcome, but then you don't have to spend all night running around trying to combat the frost!!

In some cases, the conditions are not right for an inversion layer to form, or the cold might be coming in as a mass of freezing air, for example, from nearby hills or mountains that have recently been covered by snow (this appears to be what happened in California recently). In this case, since there is no warm air around, fans and helicopters won't help. However, the application of water to the plants will work for all types of frost events.

The idea behind the application of water is that as water goes from liquid to solid form (freezes) it releases heat (heat of fusion).

So as long as water is freezing on the plants, heat is being released, effectively keeping the temperature at 0°C. Plant tissues can survive this temperature and even a bit lower without damage, so as long as enough water is supplied to always have some in the process of crystallising, the ice, and grape buds or shoots inside, will be kept from temperatures that might damage them.

Two types of post-mounted micro-sprinklers - these use quite a bit less water than the traditional impact-type overhead sprinklers

The down side is that if enough water is not applied, the temperature can continue to decrease, so if the application system breaks, or the water runs out, before the air temperature rises sufficiently, you can lose the new growth.

In the foreground row visible in this picture there is a number of vines where less growth is visible. This is because the sprinkler at the top of the post (centre left in the photo) failed during a frost event, meaning that the vines were not continuously covered with water - thus once the freezing stopped, the temperature of the ice and tissues within dropped to damaging levels. Note that there is a frost fan in the background - the fan isn't able to cover the area that these vines are in, so the sprinklers were put in to provide protection there.

My preference, if I were to plant in an area that was prone to frost events, would be to use water sprinklers. However, in some cases there wouldn't be enough water available for this. Large dams can be build to provide a source of water, but sprinkling even a hectare of ground with water for a decent frost can require up to 40 cubic metres of water per hour depending on the system used!

The concerning thing is that with global changes to climate, we should be prepared for more extreme weather events - in this context, we need to be ready for more severe frosts later in the season. Seeing as how some grape growing regions in New Zealand can have a potentially damaging frost in almost every month of the year (historically, anyway), this isn't good news!!!

Hands up those who thought being a viticulturist was a cushy job!!

Prepping for harvest

After dealing to the powdery mildew in our little vineyard, there were a few things that we needed to get ready in the preparation for eventual harvesting!

The first order of the day was to give the vineyard cover crop a good mow! For me, this is mainly a comfort thing, as it's not that nice to walk through the vineyard if the grass is very tall. This is especially true if the grass is damp from morning dew!

As well as this, the shoots needed a general tidy-up, as they had grown a bit since the last time we were through, and were getting a little tall. This is despite it being not that long ago that we did the shoot topping and positioning. We had some rain recently, and the vines were responding with a fresh spurt of growth.

Then, as the grapes were just starting to colour up (Pinot noir and Pinot meunier), it was time to get the nets on.

Timing of this is a bit tricky, as if you put the nets on too early, the vines grow shoots and tendrils through the netting, which makes it a lot harder to take the net off after harvest, and the vine-y bits cause the netting to tangle.

If you put the nets on too late, the birds discover how yummy the grapes are, and they'll pick off the grapes as they are colouring up. If you're not looking closely, you might wander through the vineyard and think that veraison is being delayed, but in reality, all the coloured berries are being picked off by the birds, leaving little clue that ripening is at hand! As a result, you can lose a lot of crop! Also, if the birds discover the grapes, they will tell their family and friends, and in a matter of hours, decimate the crop of a vineyard.

So in putting on nets, it's better to err on the side of being too early!!

So that's what we've done - just a few berries are starting to turn, so we put the nets on last weekend.

 

Side netting being attached to the upper foliage wires. These lucky workers are the front line to defense against birds damaging the fruit!

The job is not done, however! We still need to finish clipping the nets - that is, making sure there are no holes in the netting that birds can get through. And they're very clever at getting through spaces you wouldn't think they could!!

Frost in the late throes of ripening

Just a quick note about late season frosts, while they're happening in Niagara. In New Zealand, early season frosts are a common problem, but not so much late season ones. I've already written about the use of wind machines in the dormant season (Aug14), but this season Niagara is in the situation of having many grapes (Chardonnay, Riesling, Cabernet franc, Syrah...) still out on the vines, and yet temperatures are dipping to below zero.

One reasonably strong frost event can kill all the green leaves on the vines, which means that the ripening process effectively stops (a small amount of sugar may accumulate in the berries due to the vines dipping into their carbohydrate reserves, which isn't good for long term vine health, sugar concentration may increase due to berry shrivelling, and then there may be small changes to acidity and flavour). Leaving the fruit on the vine won't help when you're trying to make normal table wines (icewines are a special case!), and if the vine tries to push a bit more sugar into the fruit using its reserves, it will be less resistant to cold temperatures during the winter, and may suffer a bit in the early season following.

So the fruit has to come off, ready or not!

To prevent the leaves from making an early departure, the wind machines can be used (providing there is an appropriate inversion layer) to keep the temperature dipping below zero, and keeping the leaves on the vines so they can photosythesise a bit more, hopefully ripening the fruit.

Even though the daytime temperatures are cool at best right not, there can be a significant amount of activity in the vines and fruit if it's sunny. The sun's radiation can heat fruit and the leaves to above the air temperature, so enzymes can be rockin' along quite nicely if it's not overcast or too windy.

Ironically, clear skies and lack of wind are exactly the kind of conditions under which frosts form, so what you want during the day isn't what you want and night!

We're keeping our finger's crossed for still air and sunshine during the day and clouds and a gentle breeze at night!!

Wind Machines

Since I'm here in Ontario for a while, one does get to touring around vineyards (and wineries!) a bit.

In NZ, wind machines are popping up all over the viticultural landscape, as the most common type of frost occurring in the spring are radiation frosts. Something that moves the air can be very effective in avoiding freeze damage to the vines, because in such a frost cold air develops near the ground, but warmer air accumulates on top of that cold air. This is called an inversion layer, with the warm air stacked on top of the cold.

A wind machine sticks up far enough above the ground (say, 10m) to draw this warmer air down and mix it with the colder air. In most cases, there is a significant reservoir of heat in the inversion layer, meaning that the mixed air is warm enough to prevent freezing of the vine tissues for many hours.

Helicopters are also used to mix the cold and warm layers, but if a vineyard is in a location that is hit consistently with frosts, it can be worth the capital cost to buy, install, and maintain the fans.

So in NZ there are a lot of wind machines being put in to help manage spring freeze events, but here in the Niagara region, they are also putting them in. However, spring frosts are not so much of a concern here (due to the general climate and also the location of the vineyards). So why are people investing in these expensive machines?

Here the main problem is freezing temperatures during the dormant season. Grapes can be very hardy plants, surviving very low temperatures (for certain species, this can be down to less than -40 degrees C!), but vinifera vines tend to be more tender, with a critical temperature of around -18C. So if the temperature in a region tends to get lower than this on a regular basis, you had better have some methods to manage the vines to prevent damage!

One method relies on the presence of the inversion layer, which can appear in the middle of winter as well as in the early spring (or autumn). By mixing the air, freezing temperatures low enough to damage dormant vine tissues can be avoided, leading to productive vines the next season. Without some form of protection, crop losses can be 100 percent, with the added burden of having to re-train the vines (if you're lucky) or re-plant them (if you're not lucky!).

I will write on other strategies to avoid winter freeze damage in another post...