Most potted or hanging plants, like petunias, are fine as long temperatures don't dip below 39 degrees, and especially if the temperature doesn't dip to the freezing mark.
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.
To extend the beauty of your garden into autumn and put some color back into your landscape, consider planting some cold-tolerant annuals. Many of these can take temperatures to 20° F. As our seasons change, so can your garden by highlighting these beautiful tough plants in your beds, borders and containers.
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.
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 even though the weather forecaster isn't predicting a freeze, once temperatures start to drop below 40°F you need to be aware. 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.
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.
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.
How to Treat Frozen Plants. Annuals: Blooming annuals such as pansies and winter color like ornamental kale should come back after they're watered and have gotten some sunlight. If your annuals don't recover quickly, take it as an opportunity to re-invigorate pots with spring color.
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.
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.
Our recommendation would be to remove the cold protection covering once temperatures are above 32 degrees. If you leave the covering on when it gets warm and the sun is shining brightly, it may get too hot inside the cover and stress out the plants.
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.
WHEN TO BRING PLANTS INSIDE. As a general rule, tender plants should be brought in when nighttime temperatures are below 50 to 55 degrees F, even if they are hardy for your zone. A plant's roots are more exposed when planted in a container versus in the ground.
Remember to protect electrical connections from moisture. 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.
You can overwinter potted perennials in an unheated shed or garage. Allow them to naturally go dormant in the fall before moving them inside. Then put them back outside once the weather starts warming back up in late winter or very early spring.
Take Special Precautions for Potted Plants
To protect them, you can wrap the pots in an insulating material (think burlap, old blankets, or even bubble wrap), place them close to the foundation of your house, and arrange them close together. You can also put a layer of mulch over them for added protection.
Frost and Freeze Protection in the Spring
Vegetables, annuals, and tropical plants planted outdoors early in the season are the most important plants to protect during a late frost or freeze event in spring. Protecting emerging spring bulbs and perennials is not necessary.
Cutting off the dead and spent foliage a few inches above the ground in the fall will not harm the plant. Remove spent annuals and seasonal vegetables. Unlike perennials, annuals do not come back from season to season so there is no reason to leave these in the ground.
Get rid of the soil and store the pot indoors, away from the cold. But if you can't easily move them, wrap burlap around the planters and place mulch on the topsoil to retain as much heat as possible.
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. Some outdoor plants won't survive the harsh conditions of winter, bring them inside and use these tips for caring for them through winter.
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.
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.