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.
If temps dip into the 20s, crop death increases with a total loss at 20 degrees or colder. Flowers are equally susceptible to the cold but some do better than others. Cornflowers, pansies, primrose and violets do well in cold but by 22 degrees, irreversible damage can occur.
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.
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.
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.
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!
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.
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.
At or near the bloom stage, the critical temperature is the same for almost all fruits and flowers. Freezing temperatures of 28 degrees Fahrenheit will result in about a 10 percent loss and 24 F in a 90 percent loss.
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.
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.
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.
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.
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.
Semi-hardy cool-season crops (beets, carrots, Swiss chard, lettuce, cauliflower, potatoes, parsley) grow in minimum daytime temperatures of 40 degrees and higher. They cannot withstand hard frost without some kind of covering or protection.
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.
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.
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.
Most flowers are okay when there is light frost – which is from 29°F – 32°F. But moderate frost – which is 25°F to 28°F – is more problematic to flowers. Some of the most sensitive or tender spring flowers we see are Begonias and Amaryllis.
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.
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.
-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.