In the vineyard

Winter protection: hilling, burial and the wind machine

Ontario is one of the few serious wine regions where vines are routinely buried to survive the winter. Everything else in this province's viticulture follows from the same fact: the cold here can kill, and the grower's job is to decide how much to spend insuring against it.

The graft union is the vulnerable point

Almost all Vitis vinifera in Ontario is grafted onto rootstock, because phylloxera is present in the soil. That graft union, a few centimetres above ground, is the least cold-hardy part of the vine and the part that cannot be replaced: if the trunk dies above the union but the roots survive, the grower has a vine that can be retrained over two or three seasons. If the union itself dies, the vine is gone and what regrows is rootstock.

Hilling up is the standard response. Soil is ploughed over the base of each vine in late autumn to bury the union, insulating it from the air temperature, and ploughed away again in spring. It is an annual cost in fuel, time and soil disturbance, and it is close to universal on vinifera blocks in this province. The climate page covers the temperatures involved and why lake effect matters so much over short distances.

Full burial, in the coldest sites

Hilling protects the union. In Prince Edward County, where the winters are harder than in Niagara or on the Lake Erie North Shore, that is not enough, and whole canes are laid down and buried each autumn, then lifted and retied in spring.

The labour involved is the reason the County's vineyards are small and its wines are not cheap. It also shapes how the vines are trained: a vine that has to be laid down every year needs flexible canes and a trunk architecture that will bend, which rules out some of the training systems used elsewhere. The trellis page covers that constraint.

Wind machines

The other common defence works on air rather than soil. On a clear, still, very cold night the air stratifies: a layer of colder, denser air settles over the vineyard floor while slightly warmer air sits above it. A wind machine — a large fan on a tower — mixes the two, and can lift the temperature at bud height by a couple of degrees.

Two degrees sounds trivial and is not. Bud survival is not linear: across the range where damage begins, a small difference in minimum temperature can be the difference between losing a tenth of the primary buds and losing most of them. The same machines are used in spring against radiation frost after bud break, which makes them a defence at both ends of the season. They only work in stratified conditions, though — in wind, or when a cold air mass sits over the whole region, there is no warmer layer to bring down.

Knowing when to worry

Winter hardiness is usually expressed as LT50: the temperature at which half the primary buds in a sample are killed. The crucial thing about it is that it is not a fixed property of a variety. It moves through the season as the vine acclimates in autumn, deepens through midwinter, and deacclimates during a thaw — so the same block can differ by several degrees between January and a mild March, and a vine that survived minus 22°C in January can be damaged at a warmer temperature in March after a week above freezing.

Brock University's CCOVI runs the VineAlert programme, sampling buds through each dormant season across the Niagara sub-appellations, the Lake Erie North Shore and Prince Edward County, and publishing current values by variety and site. That is what tells a grower whether an approaching cold night is a problem for their variety this week — and whether it is worth the diesel to run the machines.

Insurance built into the vine

The cheapest protection is structural. Many Ontario growers train two or more trunks per vine rather than one, so that if a trunk is lost the vine is not. Others retain extra buds at pruning and delay the final cut until bud survival can be assessed, which is the single most common Ontario adaptation to winter and the reason pruning here runs later than the textbooks suggest.

Site selection is the decision that precedes all of these and outweighs them. Air drainage, proximity to a lake, elevation and slope determine how cold a block actually gets, and no amount of hilling or fanning will rescue a tender variety planted in a frost pocket. That is why the province's tender reds sit where they do — the regions page maps the pattern.