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.
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.
If it stays cold for more than one night, can I keep my plants covered, or should I remove the coverings at some point? A. Our recommendation would be to remove the cold protection covering once temperatures are above 32 degrees.
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!
Cover them during the cold hours of night. During the day, the soil absorbs the heat from the sun. By covering plants in the evening, the covering captures the heat the the soil re-radiates out into the night.
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.
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.
Flowers are happiest in temperatures of 40-50 degrees. Most of my flower coolers are about 36 degrees.
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.
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.
Hardy plants can be hardened off when the outside temperature is consistently above 40° F. Half-Hardy plants may be hardened off at 45° F.
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 plants prefer a temperature of no less than 60° F, though many can tolerate as low as 40° F. Along with this, while the improper temperature can certainly be an issue, fluctuations in temperature are the true killer. Learn the ideal temperatures for your green friends so they can be healthier and happier.
Between 28 and 25 degrees
It's going to be destructive to your annuals if you don't protect them. This is the temperature zone where you may start noticing damage to some of your perennials too. For example, you may notice their leaves turning a dark color or becoming a little gooey.
When weather forecasters issue a frost advisory in late spring and early fall, that's your heads-up to protect annuals and other vulnerable plants. You might think temperatures have to get to the freezing point (32˚F), but a frost can occur between 36˚F and 32˚F.
Plants may need covering if there's a long period of 25-degree weather, but they probably can survive a very short-lived cold snap during the night, Reeves said. Calm nights are actually harder on plants than nights with light winds.
Deciding the best temperature to store cut flowers depends on the species intended for preservation. 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.
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.
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.
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.
Most houseplants are tropical plants that can safely grow in a heated home during the winter. They will never survive outdoors in winter unless you live where the temperature stays at least above 50 degrees.
The easiest way to protect from a freeze is simply by covering plants with a sheet or a blanket. This acts like insulation, keeping warm air from the ground around the plant. The added warmth may be enough to keep a plant from freezing during a short cold snap.
-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.
Frost sensitive perennials include Hostas and Bleeding Hearts. They should be covered to protect their foliage and flowers. Cover any blooming or budded up perennials.