Growing vinifera in cool climates requires extra care. One additional step is hilling up of the soil around the base of the vine trunk. The goal is to cover the base of the vine up above the graft union where the rootstock is joined to the scion. If there is a hard freeze, there is a better chance of the vine surviving. The trunk will likely die, but enough wood might be insulated and preserved beneath the soil to grow a new trunk from the scion. This first photo shows two recently hilled vine rows.
There are several ways to hill up vines. Here, Michael Amigoni (aka the GoMV) at Amigoni Vineyards uses a small tractor with a side-mount grape hoe to push the mounds of soil up against the base of the vines. There are custom-designed rear-disc systems as well. The wider the mound, the greater it's insulating factor. Hilling up is essential, as a hard freeze can be devastating. In the Finger Lakes region, 25 percent of the vinifera were killed a freeze in 2004.
Planting your graft unions an inch or two off of the ground is essential for ease of hilling and to get the maximum amount of soil over the graft union, increasing the insulation factor. Here is a close-up of the hoe blade. Care must be taken to avoid damage to the vines, and one key is to ensure that the trunks are as straight as possible from proper planting and training. On a side note, multiple trunks help fend off cold damage. If one trunk is split or killed during a hard freeze, there's a chance that the second trunk might survive, ensuring that you don't lose an entire crop to that freeze event. Snow can also help insulate vines, but we don't get permanent snow cover here in central Missouri. Mounding can also be accomplished with mulch or straw, but soil has a greater insulation factor. It can be done by hand, as in this backyard vineyard.
Hilling vines is just one technique in reducing winter damage in cool climates. Trunk renewal is another technique; this involves training a new cane every few years to replace an older trunk so that you don't have all trunks of the same age. That way a freeze that kills the older trunks might spare the younger wood that has undergone fewer freezing winters and less mechanical (tractor) damage. Here's a great summary of some cold weather practices in Canada, where they're growing Chardonnay, Sauvignon Blanc, Merlot and both Cabernets where it is not uncommon for winters to dip below -18 degrees. As rare as it is to find vinifera in our region, it is also as easy to forget that these European grape varietals has been growing in colder regions for many years and with much success. It's a challenge, but few things that are easy are worth doing.
* Update - 1/12/08 Here's an article on hilling in the OSU Wine-Grape newsletter