Climate

Ontario vineyard climate: lake effect, winter injury and the growing season

Ontario grows grapes at a latitude where it should not be possible, and the explanation is water. This page covers what the Great Lakes do for a vineyard, what they cannot prevent, and what growers do about the rest.

The latitude problem

Niagara sits at about 43 degrees north, the Lake Erie North Shore at about 42, Prince Edward County at nearly 44. On latitude alone those are the numbers of Tuscany, Rome and the Rhône, and if latitude were the whole story Ontario would be a warm-climate region. It is not, because the continental interior of North America has no ocean to moderate it. Summers are hot and winters are severe, with cold air from the Arctic arriving unobstructed across the prairies. The growing season is warm enough to ripen almost anything; the problem is what happens between November and March.

What the lakes do

The Great Lakes contain enough water to behave like small seas. Through the summer they absorb heat, and in autumn, when the air cools faster than the water, they give it back. A vineyard within a few kilometres of Lake Ontario or Lake Erie gets a longer, milder autumn than a vineyard inland, the first killing frost arrives weeks later, and late-ripening varieties get the time they need. In spring the effect reverses: the lakes are cold, the air over them stays cool, and bud break on the vines is delayed past the period of greatest frost risk. In winter, so long as the lake is open water, it keeps the air above freezing point more often than the surrounding land and takes the edge off the coldest nights.

That last clause is the catch. Lake Erie is shallow, averaging under twenty metres, and in a hard winter it can freeze across. When it does, the moderating effect vanishes at the moment it is most needed. Lake Ontario is deep and effectively never freezes, which is one reason Niagara's winters, though severe, are slightly more predictable than Essex County's.

What the Escarpment does

Niagara has a second climate engine that the other regions lack. The Niagara Escarpment, a limestone ridge a hundred metres or so above the lake plain, runs parallel to the shore a few kilometres inland. On clear, still nights, when frost forms, cold air drains downslope off the Escarpment towards the lake, and the warmer air over the lake rises and circulates back. The vineyards on the benches and the plain sit inside that circulation, which keeps cold air from pooling on them and reduces both spring frost and the severity of winter lows. The sub-appellations on the benches owe a great deal of their character to it.

The growing season

The standard measure is growing degree days, the accumulated warmth above ten degrees Celsius through the season. Ontario's regions range from roughly 1,300 to 1,700 degree days in a typical year, which places them alongside Burgundy and the Loire at the cooler end and Bordeaux at the warmer. The Lake Erie North Shore is the warmest, the Niagara-on-the-Lake plain next, the benches cooler, Vinemount Ridge and Prince Edward County coolest. Frost-free periods run from around 160 days in the coolest sites to over 200 on Pelee Island. Rainfall is moderate and spread through the year, so irrigation is uncommon, and the main growing-season risks are rain at flowering in June, which reduces fruit set, and rain at harvest in September and October, which brings rot.

Those are averages across sites and years, and they conceal the point: Ontario's vintages vary a great deal. A warm, dry autumn makes a year the reds are remembered for; a cool, wet one makes a year for Riesling and sparkling wine. Growers plant with that variation in mind, which is one reason the province leans towards varieties that make something worthwhile in either kind of year.

Winter injury

The defining hazard of Ontario viticulture is cold damage to buds, canes and trunks. Grapevines harden off in autumn and become progressively more cold-tolerant into midwinter, then lose that tolerance as spring approaches. A fully hardened vinifera bud typically survives to somewhere between minus twenty and minus twenty-five degrees Celsius, with hardy varieties such as Riesling and Cabernet Franc at the more tolerant end and Merlot, Sauvignon Blanc and Syrah at the tender end. Hybrids survive several degrees colder; labrusca colder still. The trunk and the graft union are damaged at temperatures a few degrees lower than the buds, and trunk damage is the serious kind, because a vine that loses its buds loses a crop while a vine that loses its trunk may be lost altogether.

Every Ontario region reaches those temperatures in some winters. The historical record has repeated episodes in which a single arctic outbreak killed a large fraction of the buds across whole regions, and occasional winters in which entire blocks of tender varieties had to be retrained from the root or replanted. It is also why the period of greatest anxiety is not January but a warm spell in February or March followed by a sharp freeze, when the vines have begun to lose their hardiness and are caught unprepared.

What growers do about it

Site selection comes first: slopes that drain cold air, proximity to open water, and soils that do not hold water into freezing. Much of the geography of Ontario's vineyards is the map of where winter is least severe.

Wind machines are the most visible tool, tall fans standing over the vineyards that on still, cold nights pull the layer of warmer air that sits a few metres up back down onto the vines. They raise the temperature at vine height by a few degrees, which is often the difference between damage and none, and they are effective against both spring frost and winter lows so long as there is warmer air above to bring down. They are expensive and loud, and they are everywhere in Niagara.

Hilling up means ploughing soil over the base of the vine in autumn to bury the graft union, so that even if the top of the vine is killed the union and the scion buds below the soil survive and the vine can be retrained. It is standard practice for vinifera across the province.

Burying takes hilling to its conclusion: in Prince Edward County, where the cold is most severe, growers lay the whole vine down and cover it with soil every autumn, uncovering it in spring. It is laborious, it limits the training systems that can be used, and it is the reason the County can grow vinifera at all.

Monitoring has become the fourth tool. Brock University's Cool Climate Oenology and Viticulture Institute samples buds through the winter across the regions and publishes how cold they can currently withstand, so growers know on any given night whether to run the fans. The programme is linked from the resources page.

Regional summary

A changing baseline

Ontario's growing seasons have lengthened and warmed measurably over the past several decades, and the province now ripens varieties in ordinary years that were once confined to the best ones. The winters have not become reliably milder in the way that matters, however: the severe outbreaks still come, and a warmer February followed by a March freeze is if anything a greater hazard than it was. The climate story of Ontario viticulture is not one of the winter problem going away but of a longer season on one side of the ledger and the same risk on the other.

The original ontariograpes.com kept a page of winter observations from Essex County vineyards. That address now redirects here.