These species are tolerant of a light frost often surviving down to 28°F or sometimes even 25°F with little damage to flowers or leaves. Most cool season annuals will see no damage during a late-season frost or freeze and those that do see light damage will recover over the next 7 to 14 days.
Annuals: Blooming annuals such as pansies and winter color like ornamental kale should come back after they're watered and have gotten some sunlight.
Reviving Plants After Frost Damage
Yes – severe frost bite could leave a portion of the plant dead, but the stem and roots might still be in good shape. Your first instinct is probably to prune the dead parts to allow for new growth, but we recommend waiting until you're sure you've seen the last of the spring frosts.
Here's a quick summary: Between 36 and 29 degrees, your most vulnerable flowers are your “tender annuals.” Tender annuals are your one-season flowers that like warm air and warm soil to grow. They like having “warm feet,” meaning they want their roots to be comfortably warm.
Watering is the only thing that can help your plants recover but make sure not to overwater. If the frost damage is on plants that you should have brought indoors for protection, move them right away to their overwintering location.
It is best to water in the afternoon or evening the day after a freeze so plants have had a chance to slowly raise their temperature. After a freeze, soft-stemmed, non-woody plants such as impatiens, cannas, elephant ears, agapanthus, amaryllis, begonias, philodendron and gingers may be pruned back to living tissue.
Second, know that a light frost—32°F and colder—kills all tender plants such as tomatoes. Hardier plants such as spinach and kale will survive until there's a hard freeze—28°F and colder.
Temperatures below 32 degrees can freeze the leaf tissue of cold-wimpy plants and turn them to mush after just a few hours. Some of the most tender annuals and veggies might not die but suffer cold-induced setbacks even when overnight lows dip below 40.
Overall even flowers can tolerate light freezes (low 30s to upper 20s °F) and even a little snow. Record cold temperatures (below mid to upper 20s) will damage or destroy many of the flowers of early blooming varieties. The foliage of fully emerged spring-flowering bulbs like tulips and daffodils can also be damaged.
Many “annuals” can be brought inside, even tender plants that need a winter dormancy period. These should ideally come indoors before nighttime temperatures dip below 45°F (7°C). As fall approaches and night temperatures reach about 50°F (10°C), start bringing the plants inside for the winter.
The easiest way to protect from a freeze is simply by covering plants with a sheet or a blanket. This acts like insulation, keeping warm air from the ground around the plant.
Once open, use cool water (or simply add some ice cubes to the vase) in order to keep the blooms lasting longer. Though water still absorbs into the stem, colder water slows any decay, bacteria, or mold from breaking down the organic matter.
-Don't make the mistake of thinking that a plant can remain covered for a few days when the weather calls for more than one night of frost. The protection needs to be taken off every morning when the sun comes out. Plants can't breathe under a heavy container, a layer of plastic or fabric.
The short answer is that annuals don't come back. Plants that flower and die in one season are annuals—although many will drop seeds that you can collect (or leave) to grow new plants in the spring. Annuals will also typically bloom all season until frost, so you get consistent color and showy blooms.
Cover Plants – Protect plants from all but the hardest freeze (28°F for five hours) by covering them with sheets, towels, blankets, cardboard or a tarp. You can also invert baskets, coolers or any container with a solid bottom over plants. Cover plants before dark to trap warmer air.
Fabric coverings will prevent the freezing air from coming into direct contact with the moisture on the plant while also capturing the heat that is radiating from the ground. Bed sheets or comforters work best for covering large plants and shrubs.
Some cool-season annuals can tolerate soil temperatures as low as 45 degrees Fahrenheit; however, most cool-season annuals will grow best in soil temperatures at 65 degrees Fahrenheit or warmer.
Tender plants, such as tropical houseplants and geraniums, are killed when the air temperature stays below 32˚F for a few hours. A freeze warning often signals the end of the growing season in fall because temperatures are low enough to kill off annuals and begin dormancy for hardy perennials, trees, and shrubs.
So what's the solution… Focus on frost hardy annuals. Some of these annuals that can withstand 20 degrees or so, including pansies, snapdragons, dianthus, alyssum, dusty miller, viola, and osteospermum. Keep in mind that flowers may be a bit ragged after such cold but the plants should make it through ok.
Generally speaking, the annual plants you want to protect or propagate should be brought indoors as the growing season begins to wane and before cold weather threatens to kill them.
To know when to cover your plants, you need to know when the plants you have are susceptible to frost damage. Some frost-tender plants need to be covered as soon as the temperature hits 32°F, while others can handle lower temperatures and/or longer freezing periods.
A covered porch usually provides protection from light frost, but the garage or sun room is better for freezing temperatures. A couple days in darkness won't hurt the plant. Or move them out during the day and back in at night, if cold temperatures persist.
Avoid watering when temperatures are below 40°F or when sustained freezing temperatures are expected within 24 hours as this water will freeze and not be available to the plants.