What To Do When Frost Threatens
Frost signals 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.
By Veronica Lorson Fowler
The Iowa Gardener
Frost occurs when temperatures dip below freezing. Frost kills tender annual plants, such as tomatoes, peppers, impatiens, marigolds, and more. (Perennials have tougher roots and tops, and survive the winter.)
When a light frost threatens—one in which the temperatures dip just below 32 degrees for a short period of time—you can take a few measures to protect your favorite plants for a while longer.
A hard frost, also called 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 annuals from a hard frost.
Know Your Average Frost Date
October 10 is the average first frost date for north central, northeast, and southwest Iowa. (Isn't that interesting how it's not simply northern Iowa? It's all about terrain.)
October 15 is the first average frost date for most of the rest of Iowa. And it's October 20 for southeast Iowa. (Click here for a listing of average frost dates by city.)
Take Steps to Keep Your Garden Going
Meanwhile, when the first light frost or two 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. Hibiscus, jasmine, rosemary, citrus trees, and others will often survive the winter if given plenty of light and humidity.
• Some people like to overwinter their annual geraniums to enjoy again in spring. Click here to see various methods Iowa State horticulture extension recommends.
• 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.)
• 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.
Also of interest:
• Check out our list of cool-season and warm-season annuals. Cool-season annuals will tolerate a little frost; warm-season annuals are killed by frost.
• We have a nifty Fall Clean-Up List to help you make sure you get it all done in time!
• Need tips on how to plant fall bulbs for spring bloom. Click here!
• Keep track of frost yourself! The National Weather Service has a handy-dandy web page of various weather alerts, including frost.