TORONTO -- Environment Canada is warning of frost overnight in much of southern Ontario.

The weather agency says a cool ridge of high pressure will drift across southern Ontario on Thursday night, bringing frost to many areas.

Agency meteorologist Geoff Coulson says the frost advisory covers areas from the Ottawa Valley down to areas north of Toronto and also through northeastern Ontario.

But Coulson says Toronto and its immediate surrounding areas will escape the frost, which he says is coming earlier in the year than is typical.

He says the forecast overnight low for the Ottawa area is 0 C.

If that is reached, he says it could rival the record coldest temperature for Sept. 19 in Ottawa, set back in 1943 -- a chilly 0.6 C.

"It is a little bit early to see temperatures dipping down this low in parts of southern Ontario," Coulson said.

Those in the affected areas are advised to take preventive measures to protect frost-sensitive plants and trees, such as covering up outdoor plants. Frost may also damage crops in some areas.