Here in Minnesota and across much of the Upper Midwest we have had two dramatic frost damage events in the past year and a half. One on Mother’s Day Weekend in Spring 2010 and one this past September 15, 2011. Some growers escaped. Others did not but this has demonstrated clearly how serious these events can be and has made us look toward ways to reduce or escape the damaging effects of late spring and early fall frosts.
Late spring frost can freeze off the emerging shoots from fruiting buds left after pruning and thus can reduce and even eliminate the crop for the coming growing season. Some new growers may fear their young vines have been killed by an event like this, but in fact, each bud site has a secondary bud which emerges when the primary is killed and takes over growth. While it will keep the vine alive and able to carry on, generally secondaries contain fewer fruit clusters and thus give the grower a much smaller crop. There is even a third “Tertiary Bud” at each site which will emerge if there were to be a second frost event. Tertiary Buds are largely unfruitful and will maintain the structure of the vine but offer little to no crop.
Fall frost, on the other hand, can occur while the crop is still not ripe damaging and often destroying the leaf canopy. This can bring ripening to a halt requiring that the fruit be harvested early, before it is fully ripe. It can even reduce the vine’s ability to harden off or lignify wood in preparation for winter. Thus either event, spring or fall, can be costly and enormously damaging to the vineyard, both for the current season and seasons to come.
Large growers of many fruits face these problems annually. Spraying the vines with water using overhead irrigation is one well known way to avert damage to blossoms in the event of spring frost. When an irrigation system is turned on and frost occurs it will blanket strawberries, grapes, citris, peaches with ice. While it looks like a disaster in morning, a crop covered with ice, when it melts, miraculously the blossoms are saved, as is the crop. However, few small-scale Minnesota or Midwestern growers can justify the investment it requires to install overhead irrigation that this technique requires.
Another useful technique that is often used is the installation of windmachines on high towers that can be turned on to mix the warmer air aloft with the cold air that has settled upon the crop at ground level. Another technique designed to mix or displace cold air at ground level is to hire helicopters to pass over and hover over vineyards. The down blast from the blades is effective in displacing cold air with warmer air aloft and saving endangered blossoms. Again these techniques require large investments and few small growers can justify the costs involved.
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