Whether a plant can handle frost depends on its native range. Frost won’t harm plants that grow naturally in a cool northern climate whereas it kills tropical plants.
If you garden in a climate where the temperatures drop below 32 degrees F, for anything that you plant in garden soil, it is best to select plants adapted to your local climate. That does not mean that you shouldn’t abstain from growing warm-climate or tropical plants, but they require considerably more vigilance and effort. You need to keep an eye on the weather and protect them from frost if necessary.
Why Plants Can’t Handle Frost
Frost causes the water in plant cells to freeze, which severely damages the cell walls. This gives a plant the typical wilted look after a frost.
Even hardy-fruit trees that can tolerate very cold winters, such as apple trees, can suffer frost damage. It usually occurs when unseasonably warm weather in late winter or early spring coaxes the trees into an early bud break and bloom. If frost occurs, it can injure the buds and blossoms and affect that year’s harvest.
When to Take Protective Steps
The two crucial times of the year when you need to be on frost alert is in the spring and fall. Find out your location’s last average frost date in the spring and the first average frost date in the fall and watch the weather forecast around that time. Have all the materials and equipment to protect plants ready so you can act promptly when there is a frost advisory.
Which Plants to Protect from Frost
Cold-sensitive, warm-climate plants are not the only plants that cannot handle frost. Here is an overview of the plants you should protect from frost.
For frost-sensitive plants that you have overwintered indoors, wait until the last spring frost in your location before moving them outdoors. If it’s an early warm spring, small containers, unlike large, heavy containers, can be moved outdoors before and brought back inside for the night when frost is in the forecast.
If there is an occasional frost in the early fall, and you don’t want to bring all your tender container plants in just yet, there are several things you can do to protect them. Place a large cardboard box over the plants, making sure not to bruise the foliage. If you have several plants, you can also place them under a patio table and cover the table with blankets and sheets to create a tent. For smaller plants, a chair turned upside down over a plant and covered with a sheet can do the job. For cover, use textile, fabric, or burlap, not plastic so the plants can breathe. Plastic, if not removed promptly the next morning after the frost, can lead to overheating and burn the leaves.
Small Fruit and Fruit Trees
Strawberries are very sensitive to spring frosts because they often bloom before the last frost date. If there is a frost warning, cover them with a thick layer of straw just for the night and remove the straw the next morning. Leaving it on the strawberries during the day will deprive them of light. You might have to do this a few times until all danger of frost is over but it’s worth it to save your strawberry harvest.
Small fruits such as blueberries and small fruit trees can be covered with a sheet or burlap. unfortunately, that method is not feasible for larger fruit trees so you can only hope that the frost will not damage the blossoms.
If you still have peppers, tomatoes, and eggplant growing in your garden and there is a single frost followed by a period of warm fall weather, you can protect the plants with bubble wrap that is held in place by stakes or tomato cages.
If there is a stretch of freezing nights coming, it is better to harvest all frost-tender crops such as peppers and tomatoes and ripen those vegetables indoors.
Other vegetables such as broccoli, cabbage, Swiss chard, lettuce, radishes, beets and leeks are cold-hardy and survive a light frost. And fall and winter crops such as kale, collard greens, as well as parsnips, Jerusalem artichokes, rutabagas and Brussels sprouts taste even better when harvested after the first frost.
Non-hardy Bulbs and Tubers
The first fall frosts kill the top growth of gladiolas, dahlias, and cannas but not the bulbs and tubers in the ground. You don’t need to dig those up right away but do dig them before a deep frost. For dahlias, it is even better to wait a couple of weeks because the longer the tubers stay in the ground, the more mature they will be before you store them for the winter.
In the spring, seedlings of cold-sensitive plants such as tomatoes, peppers, melons, cucumbers, etc. need to be hardened off slowly over a period of 7 to 14 days. Expose the seedlings to sunlight a little longer every day, and bring them back in the house, a heated garage, or a basement for the night until they can be planted in the garden after the last average frost date.
At what temperature should I cover my plants?
Temperatures between 36 and 32 degrees F warrant a frost advisory. A light freeze is when the temperature drops below 32 degrees to a low of 28 degrees F for five consecutive hours. In either a frost or a freeze, cold-sensitive plants need to be covered.
Will one frost kill plants?
It depends on your plants. If you have tender, warm-climate annuals such as impatiens, marigolds, or petunias, they will get killed by a light frost. Cold-hardy annuals such as pansies withstand frost.
How do I know plants have died from frost?
The plants look wilted first then turn black or brown and eventually dry out and become brittle because the roots don’t take up water any longer.