Here in eastern Ontario, we could get our first frost in a couple of weeks (although the forecast looks pretty good so far). When freezing temperatures occur, the water inside plant cells freezes and expands, bursting cell walls. Frost is certain death for many types of plants. Cool season vegetables, on the other hand, produce more sugar as the temperature drops. The sugary water doesn’t freeze until the temperature drops much lower, and the additional sugar improves the flavour. That’s why root vegetables such as carrots and rutabagas are often harvested after a frost, or left in the ground late into fall.
At this time of year, monitor the weather forecast carefully. If a frost warning is issued, you may choose to harvest your frost-sensitive plants, or cover them for the night, if the cold spell will be brief. To cover plants, loosely drape them with blankets or plastic sheets (prop heavy covers up with stakes or something similar, so they do not crush the plants), or place plastic containers or the like over each plant. Remove the covers in the morning so that your plants don’t suffocate.
These plants are extremely sensitive to frost:
- Pumpkins and winter squash (the fruit will survive the frost, but the vines won’t, so you can play the odds and wait until the next day to see if the frost actually hits)
- Zucchini and other summer squash
The following plants can tolerate a light frost:
- Bok choy
- Chinese cabbage
- Swiss chard
These plants will tolerate a heavy frost, and in fact, most of them prefer the cold weather:
- Brussels sprouts
- Garlic (although it is usually ready before the frost hits)