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.
With that in mind, experts generally recommend flowers and other houseplants be brought inside or otherwise protected before the thermometer dips below 45 degrees Fahrenheit. For warmer-weather and tropical plants, that threshold is a bit higher, at around the 50-degree mark.
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.
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.
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.
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.
For best chance of survival, bring tender plants and annuals indoors before nighttime temperatures fall below 45 degrees. To be on the safe side, especially if you have tropical plants, bring them in when it's 60 degrees at night.
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.
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.
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.
I did some research and found out that houseplants need to be taken indoors before overnight temperatures dip below 45 degrees. Most tropical plants suffer harm from temperatures below 40 degrees. Well, we have temperatures forecast to dip into the 40s again on Thursday and lower 40s on Friday.
For many plants, temperatures below 50°F can cause problems. Adjust thermostats to cater to your comfort, but remember your plants need some consideration. Avoid placing plants near cold drafts or heat sources. Keep plants several inches away from exterior windows.
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.
Your flowers should also be kept out of the trunk and off the dashboard of your car, even if these might seem like the most logical places. Your flowers can last in a warm or cold car, as long as they're not receiving direct sun or being subjected to freezing temperatures.
It is generally not recommended to keep cut flowers in your car trunk overnight, as the temperature and humidity in the trunk may not be ideal for keeping the flowers fresh. Cut flowers typically need a cool, well-ventilated environment to stay fresh, and the trunk of a car may not provide these conditions.
Light freeze - 29° to 32° Fahrenheit will kill tender plants. Moderate freeze - 25° to 28° Fahrenheit is widely destructive to most vegetation.
Others need to be brought indoors when temperatures reach the mid-40s. Be sure you know this before you plant. Unless your plants are winter-proof, it's best to move them indoors or wrap them up when temperatures are near or below freezing for more than five hours.
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.
It's safe to move your plants outside when the outdoor temperatures stay consistently above 50°F. Pay attention to the weather report. If nighttime temperatures are set to fall below 50°F, bring your plants in for the night.
Your plants produce carbon dioxide during the night. CO2 creates warm air and by wrapping your plants with a protective cover, they will have an insulated blanket to keep them warm throughout cold and snowy nights. Plant wraps come in many varieties including burlap, garden blankets, and plant domes.
Use a frost blanket, cloche or row cover material – not plastic — to insulate the ground around your outdoor plants and keep them warm. Prop the cover with sticks or stakes to keep it from coming into direct contact with the new growth. And, make sure to remove the cover the next morning after the temperature warms up.
Cold weather, even if it doesn't drop below freezing, can harm cold-sensitive plants. Freezes are rare, but frosts can occur at above-freezing temperatures. Even temperatures at 45 degrees for more than several hours can harm some tropical orchids.
If a sudden cold snap shows up in the forecast after you've planted, you can always cover them overnight to be on the safe side. If you do cover plants - be it new or tender perennials or annual flowers or vegetables - cover only overnight. Remove your covering once the temperature goes above freezing the next day.
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.