East Idaho’s growing season is notoriously short most years, and home gardeners usually have an abundance of produce that is not yet ready to harvest when that first prediction of frost comes in the fall. So the first frost of the fall season , or even the forecast of such, is a cause of great anxiety for the home gardener. But the first killing frost doesn’t necessarily mean the end of your garden harvest. Taking a few minutes to protect tender crops can often extend the harvest well into October.
Light Frost (29-32)
Usually the first frost of fall is a light one and you can cover your tender plants with things as light as a bed sheet or plastic sheeting to protect them, even plastic garbage bags may work. The trick is to anchor the covering so wind gusts don’t remove your protection. The idea when covering your plants is to trap heat from the soil in the air immediately around the plants. It’s important to uncover them when temperatures warm up, especially if you used plastic for protection. Plastic will trap the sun’s heat the next day and plants may scorch. Watering your crops before the frost is also helpful. Well hydrated plants are less susceptible to frost damage.
If you are unable to cover your plants try turning on the sprinkler just before the temperature drops below freezing and keep it running until the ice has melted the next day. The heat given off as the ice forms can actually keep the plant from freezing.
Hard Frost (28 degrees or lower)
When temperatures drop below 28 degrees simply watering with a sprinkler or covering with bed sheets will not be sufficient. You will need to cover with blankets or something with some real insulation value for sufficient protection. If possible, remove the blankets during the day to allow the sun (if there is any) to warm up the soil and provide light for the plants to stay healthy. If it freezes several nights in a row you will need to cover them each night.
The most frost sensitive veggies include cucurbits (cucumbers, squash, pumpkin, melons), beans, eggplant, peppers, and tomatoes. Sweet corn is also quite tender, but difficult to cover.
Tender flowers include impatiens, marigolds, geraniums, begonias, coleus, dahlia, and zinnia.
Frost Tolerant Plants
Frost tolerant crops include cole crops (cabbage, broccoli, cauliflower, brussel sprouts, kohlrabi), root crops (radish, beet, carrot, parsnips, turnips), leaf crops (lettuce, spinach, chard), chives, Jerusalem artichokes, and peas. Onions are frost tolerant but may be damaged if the bulb has pushed above ground, which they often do. Potato vines will freeze easily but the tubers are generally okay until the ground freezes down a couple of inches. Brussels sprouts are actually better after a light frost. Root crops can overwinter in the garden.
Hardy flowers include petunia, snapdragon, pansy, alyssum, dusty miller. Click here for a list of flowers and their cold hardiness.
Harvesting and Storage
When frost threatens, your first step is to harvest all ripe fruit before the frost.
Winter squash and pumpkins intended for storage should be harvested before a hard frost because temperatures in the low 20’s or frost for an extended period will shorten their storage life. If they are subjected to a severe freeze, harvest them immediately and cook, freeze or can them as soon as possible.
Tomatoes that get lightly frosted can be canned or frozen right away, but they will deteriorate rapidly if you try to hold them. Green tomatoes picked before frost can be stored at 50 to 55 degrees F and ripened for several weeks. Those picked after frost will not ripen or keep long, however.
Pepper plants blacken at the first frost. If you can’t cover them, pick the peppers before frost.
Though bean plants are killed by frost or a hard freeze, the pods do not show immediate damage. Pick and use or process them as soon as possible, however, for best results.
Summer squash plants don’t tolerate freezing at all. Pick summer squash before frost.
Sweet corn can usually be harvested and eaten, canned, or frozen after a light frost, but a hard frost may ruin it.
The cole crops (broccoli, cabbage, Chinese cabbage, cauliflower and Brussels sprouts), will tolerate fairly heavy frost; though a really hard freeze will reduce the keeping quality of even these hardy veggies. But unless they are predicting temperatures in the low 20’s, it’s not necessary to rush out and cover or harvest them.