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.
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.
When Should You Cover Plants? Cover your plants at night and remove them during the day when the temperatures rise above 32 degrees F, so that the soil can warm up again.
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.
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.
Seedlings, with their tender new leaves, often give up the ghost when temperatures dip to 32-33°F. Tropical plants have differing low-temperature thresholds. Some keel over when temps fall to 40°F; others crumble at 35°F. Other plants are just hardy by nature and can withstand temperatures as low as 18-20°F.
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.
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.
Once temperatures drop below 40°F, you may want to start covering your plants with a frost blanket depending on what you're growing, which we'll get into in just a bit. Freeze – A freeze is when the air temperature is 32°F or below.
Light freeze - 29° to 32° Fahrenheit will kill tender plants. Moderate freeze - 25° to 28° Fahrenheit is widely destructive to most vegetation. Severe or hard freeze - 25° Fahrenheit and colder causes heavy damage to most plants.
As a whole, all tender plants should be brought indoors or protected with garden fabric to prevent frost when it is 36°F outside. For cold-hardy plants, this is entirely optional. But it's better to err on the side of caution when the temperature falls below 40°F.
Time It Right
While it might be tempting to ship indoor plants outside at the first sign of spring, it's best to wait until nighttime temperatures stay at or above 60°F, and after any threat of frost has passed. If you're planning ahead, use the average last frost date for your area as your guide.
As gardeners, our frost dates are based on 32 degrees Fahrenheit (0 degrees Celsius) to avoid the risk of any plant death. However, know that even if air temperatures are as high as 38°F, frosts may occur on the ground and on plants. It's better to protect plants just in case!
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.
If you're wondering at what temperature threshold you should be bringing your plants, there's a short answer: when nighttime temperatures reach 45 degrees (F), it's time to bring your plants indoors.
You can put them in a garage or shed that gets cold but doesn't freeze, ever. They will over-winter as live but semi-dormant plants. Keep the soil just moist, not letting the potting medium shrink and pull away from the pot edges.
For most flower types, optimal storage temperatures range between 33°F - 37°F while cold-sensitive blossoms and tropical flowers should be maintained at temperatures above 50°F.
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.
Petunias thrive when nighttime temperatures are in the 55 to 65°F range, and daytime temps are between 61 and 80°F. They will succumb to sub-freezing temperatures in a hurry, and anything below 40°F may kill them, though Wave® petunias can tolerate temperatures as low as 35°F.
Frost-hardy bedding plants include all perennials and many annuals. Those annuals that can withstand 20 degrees or so include pansies, snapdragons, dianthus, alyssum, dusty miller, viola, flowering cabbage and kale.
However, petunias usually tolerate temperatures as low as 39 degrees F. (4 C.) with no problem, but they are definitely not plants that will survive the winter in most climates. Petunias are damaged extensively at 32 degrees F.
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.
Signs Your Plants May Be Too Cold
The leaves brown and fall off. If your plant is too cold, it may begin to die, and its leaves will brown and fall off. The leaves turn yellow.
A: Geraniums come from tropical climes. You could leave them out for the winter if you lived in Los Angeles. But they blacken and die immediately when we get hard freezes, even though they can handle weather down into the 40s.