Cornflowers, pansies, primrose and violets do well in cold but by 22 degrees, irreversible damage can occur. While this group is a little forgiving, there isn't much room for error as frost at 28 degrees can damage these flowers and a freeze of just 24 degrees can kill them.
The general rule of thumb is that most plants freeze when temperatures remain at 28°F for five hours. Of course, there are exceptions to this rule. 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.
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.
Winter time is very hard on flowers. They just do not like any temperature below freezing. Don't get me wrong flowers like it cold. Flowers are happiest in temperatures of 40-50 degrees.
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.
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.
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.
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.
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.
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!
Flower production is reduced or halted and plant growth may be stunted. Injury to flower buds or blossoms during the late winter or early spring season can be from frost or freeze injury. Late spring frost kills young buds and tender new growth in early spring resulting in fewer flowers and later leaf development.
An important factor in the chance of survival is how long the plants were exposed to cold temperatures. A few hours can do the job, depending on the plant. Generally, though, it takes 12 to 24 hours of exposure to cold temperatures to completely kill most tropical plant species. If you're in doubt, check the roots.
The amount of damage depends on the severity and duration of the freezing temperatures. If you have light freezes for short periods of time, plants can usually recover. After hard, long freezes, there's no guarantee. After the freeze, you should be patient.
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.
24 degrees (or colder)
This is also the freeze that kills most remaining annuals. There are a few exceptions. Pansies and violets, for example, are tenacious little flowers that can survive these freezing temperatures.
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.
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.
Some plants can survive outside even when temperatures hit freezing. Others need to be brought indoors when temperatures reach the mid-40s. Be sure you know this before you plant.
Most houseplants are tropicals and prefer temperatures between 65-75°F during the day and about 10 degrees cooler at night. For many plants, temperatures below 50°F can cause problems. Adjust thermostats to cater to your comfort, but remember your plants need some consideration.
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.
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.
Crops such as beets, carrots, cauliflower, parsley, parsnips, potatoes and Swiss chard are considered semi-hardy vegetables that grow with a minimum daytime temperature that is consistently between 40-50 degrees Fahrenheit.
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. Evergreen foliage can also be protected from excess water loss using anti-desiccant sprays. These sprays are sold at most garden centers.