How to Protect Plants From Late-Season Frost
An early frost can end your garden season before you had planned, but it doesn’t have to be that way. There are ways to protect your plants from a late-season frost to give them a few more weeks of growing time. Here’s what you need to know.
Changing seasons can be a challenging time for gardeners and plant growers. As the weather transitions from summer to fall, temperatures may start to drop without notice.
If you’re not ready to say goodbye to tomato season, you need to learn how to protect plants from a late-season frost. Here’s what you can do to keep vulnerable plants safe to enjoy a longer growing season than your local conditions might otherwise allow.
Frosts occur when air temperatures fall below 32˚F (0˚C) at ground level. It’s considered a hard frost when temperatures drop below 28˚F (-2˚C) for four consecutive hours. While many annuals can survive brief frosts with minimal damage, a hard frost will kill most of them.
Frosts are most common on clear, calm nights when there’s minimal cloud cover to reflect warmth to the ground and no wind to disperse patches of warm air. When the air is still, heat rises, so cold air settles around the base of each plant, where it causes the most damage.
Plants that spend too much time at freezing temperatures will have ice crystals form on them. This disrupts the movement of fluids and deprives tissues of water, so the frost-damaged portions will turn black and shrivel once they thaw.
If you see freezing temperatures in the forecast, it’s time to take steps to protect your plants.
Late-season frosts can sneak up on you and prematurely end a successful garden season if you aren’t prepared. Here’s how to prep for chilly weather to give your plants the best chance of surviving without damage.
If frost in the fall is a concern, decide in the spring to plant cold-hardy plants. Many varieties can tolerate more prolonged exposure to frost than others, so choose to prioritize the species less likely to disappoint you.
Some frost-hardy vegetable garden favorites include broccoli, cabbage, carrots, lettuce, peas, radishes, spinach, and kale. For the flower garden, consider marigolds, pansies, dusty millers, sweet peas, and sunflowers.
Best of all, most of these vegetable varieties actually get sweeter once the weather gets colder because a frost causes them to concentrate their sugars to protect the leaves.
You can better protect frost-prone plants by putting them in movable pots. When the forecast looks questionable, you can move them indoors or even to an elevated location so they aren’t overwhelmed by ground-level cold air.
For plants you want to keep in place, consider planting them near south- or west-facing structures like benches, walls, shrubbery, or fences. All the better if they are dark-colored, as they will absorb heat during the day and radiate it out during the frost event to keep them somewhat toastier.
Make sure you avoid planting in frost pockets. These are low depressions in the ground that trap cold air. They can cause significant frost damage, even when there isn’t a frost risk for other areas in your garden.
When frost is in the forecast, it’s time to pull out the garden hose. Watering the night before the temperature drops ensures that the wet soil releases moisture into the air, keeping each plant a little more protected.
When you see frost in the forecast, it’s smart to cover at-risk plants before the evening. The earlier in the afternoon the better, as you want to trap stored heat around them for extra protection.
Make sure the cover goes down to ground level to insulate the plant against chilly evening air.
You have many coverage options, including:
The more layers you add, the more insulation for the plant.
Plastic or Fabric?
While gardeners use both kinds of materials for frost protection, your plants will do better with fabric. That’s because fabric better allows moisture to escape, so it doesn’t freeze on the leaves and stems.
If you choose to use plastic, take care that the material NEVER touches the plants. This will hold moisture against the plant tissue and is more likely to cause frost damage than not covering the plants at all.
Avoid this problem by using tall stakes or hoops to support the plastic in a tent, so the plant stays untouched. You can also cover plants with fabric and then add a layer of plastic on top for extra insulation on the chilliest nights.
Post-Frost Instructions
Remove the plant covers as soon as possible once temperatures rise above freezing in the morning — especially if you use plastic. This prevents heat from building up under the covers and stressing them further.
Cover in the early evening just before the sun goes down. Remove the covers in the morning, so the plants do not overheat.
Give vulnerable plants a little insurance against frost by filling plastic milk jugs with warm water to place next to each plant with a cover thrown over the top.
They will emit heat all night to keep plants slightly warmer than they would be otherwise. Alternatively, fill up the jugs early in the day and place them in the sun to absorb heat.
If your plants are small, you can cover each one with a “cloche,” which is a removable glass or plastic cover that goes over each plant. The same milk jugs can serve this purpose — cut out the bottom and place each one overtop any small plants. Flowerpots will also work.
The first frost of fall might feel like the end of the gardening season, but with some advance planning, you can protect your plants for a few more weeks of growth.
Once you see your flowers blooming and tomatoes ripening far into the fall, you’ll be sure to appreciate the effort.