Vegetables vary widely in how much cold they can tolerate as they reach maturity.
Crops that came from warmer parts of the world cannot tolerate even a light frost, and may even begin to suffer at temperatures below 50°F. Basil is an extreme example of this, as the leaves can turn black when the temperatures dip into the 40’s. Other plants are a little more tolerant, and warm temperature following a short cold exposure can reverse some of the injury. For example, pumpkins can withstand 35° F nights if daytime temperatures reach 70° F or more.
The following vegetables are not cold tolerant, and should be protected from frost or harvested before even a light frost:
- Sweet Corn
- Pepper, Eggplant
- Cucurbits: Cucumbers; Summer squash; Melons (Pumpkins and winter squash may be able to tolerate very light frosts, but it is better to protect them if possible.)
- Sweet potatoes (ideally should be harvested before soil temperatures drop below 50° F)
These vegetables also will not tolerate frost, but they will ripen further after picking, so can be harvested before they are fully mature:
- Tomatoes: must be at least “mature green” (changed from the deep green to almost turning red)
- Muskmelon: must “slip” easily from the vine, but can ripen further at room temperatures
- Pumpkins: if they are starting to turn color, they will continue to turn color. However, their keeping quality will generally not be as good as those that were able to turn color while on a healthy green vine.
Crops from more temperate regions can withstand light freezes (28-32°F):
- Onion, Garlic, Chives
- Swiss chard
Some crops will improve flavor with exposure to cooler temperatures; Root crops develop more sugars when the soil temperature is below 40° F; and Brussels sprouts are often best after a light freeze.
Among the hardiest vegetables are cole crops, which can tolerate moderate (24-28°F) freezes:
- Broccoli; Cauliflower (26°F)
- Chinese cabbage; Cabbage; Kale; Kohlrabi; Turnip
For best quality, harvest after several moderate (24-28°F) freezes:
- Parsnip; Salsify
- Brussels sprouts
Harvest after several severe (under 24°F) freezes:
- Horseradish (it can be stored in ground overwinter with mulch)