We may get frost tonight in Collingwood. Environment Canada hasn't issued a frost warning yet, but unless the forecasted conditions change, they will at some point today.

Hopefully you were watching the weather and haven't put out any tender plants. If you have planted any heat loving flowers or vegetables, you will need to cover them for the night.

Frost is likely when:
1) the day is warm and so the humidity will be moderate, when the temperature drops at night, the moisture in the air, collects on surfaces as dew. If it's cold enough, dew becomes ice crystals.
2) If the night is clear. Clear skies means any heat absorbed by the soil will escape into the atmosphere making the air around the plants colder than if it were cloudy
3) If there is no wind, wind can prevent dew from settling on surfaces.

Depending on the plant, the cells that make up a plant's foliage can withstand a certain level of cold. Tender plants can't take any and their cells break down at the slightest frost and die. Others can withstand light frost and recover fine when the day warms up. Plants in the mustard family: kale, collards, cabbage, brussels sprouts, rocket, mizuma and mustard greens, can actually freeze solid and be just fine when they thaw.

Here's some more information about frost from the Farmer's Almanac.