In fall, temperatures dipping below freezing signal the end of the growing season, but if you take a few simple steps, it doesn’t have to mean the end of your gardening.
What’s a Frost, A Freeze, a Killing Freeze, and More?
But first, let’s get our terms straight. Most people use “frost” and “freeze” interchangeably, but a frost is lighter—it’s just dew freezing on plantsk when temperatures hit 29° to 32° F. Many plants recover nicely.
A light freeze occurs when temperatures hit 32° to 29° F. A light freeze kills tender annual plants, such as tomatoes, peppers, impatiens, marigolds, and more. (Perennials have tougher roots and tops, and survive the winter.) Many other plants will survive, especially if they are in a sheltered place or a place with radiant heat, like right up next to a house.
When we are going to have a light freeze, it makes sense to cover up your favorite, tender annuals and try to get a few more days, or maybe even weeks, out of them.
A hard freeze, sometimes referred to as a killing frost, is when temperatures get below 28 degrees for several hours. This type of frost kills all annuals, even those that will tolerate a light frost. Also, there’s little you can do to protect tender annuals from a hard freeze.
When to Expect Frosts and Freezes
When freezes and frosts occur in Iowa can vary wildly, depending on the year and your location. There have been blizzards on Halloween in Iowa and other years we’ve gotten well into November before frost hits.
In the coldest part of Iowa, northwest Iowa, Sioux City, the first freeze on average can happen between Sept. 16 and October 16. In central Iowa, in Polk County, an early fall freeze would be Sept. 26. An average first freeze would be Oct. 10 And a late freeze would be Oct. 26. In the warmest part, southeastern Iowa, a first freeze could be between Sept. 30 and Oct. 28.
For detailed information on frosts and freezes in your part of the state, click here.
Meanwhile, when the first light frost or freeze is predicted for your area, here are some things you can do:
- Cover plants, as much as is practical, with sheets, light blankets, or any other non-plastic cover. (Plastic tends to trap cold and is less good for protection.) Anchor ends with bricks or stones as needed to prevent blowing.
- Bring indoors any pots you can carry to enjoy a little longer. The types of flowers that do well outside in pots usually don’t do well in the limited light indoors. After a few weeks, they’ll start to get sickly, but for a while longer, you can enjoy their color and scent.
- A few types of plants in pots can indeed survive all winter indoors. Hibiscus, jasmine, rosemary, mandevilla, citrus trees, impatiens, begonias, and others will often survive the winter if given plenty of light and humidity.
- Pick all tomatoes. Bring them indoors, either green or red, to ripen on a window sill. (Don’t store in the fridge. It diminishes their flavor and prevents them from ripening any more.) There are a lot of good recipes on how to use green tomatoes (just Google them) but green enchilada sauce is an easy favorite that freezes well.
- Leave peppers, squash, and most other vegetables. They can take a light frost. In fact, some vegetables, such as brussels sprouts, actually taste better after a frost.
- Cut all basil. Pinch off any flowers and put the cut ends in a jar of water. Cover with a plastic bag and store in the fridge. Or make pesto. Or simply puree the basil leaves with olive oil and freeze in ice cube trays to use in soups and other dishes through the winter.
- Parsley and cilantro are fine with just a light frost. But they won’t last much longer after that. Cut them and store as you would with basil.
- If you have any houseplants still out, bring them in. Even if they’re in a covered area, most don’t like cool temperatures. Rinse them off with a gentle spray of water from the hose or the shower to make sure you don’t bring in any bugs.
- Consider taking cuttings. Plants like coleus, begonias, and many ivies root easily in water in a sunny window. Plant them in pots and then keep them in a sunny window until spring, when you can return them outdoors.