How Cold is too Cold for Perennials? A "hard frost" with temperatures below 28F (-2C) will kill frost tender plants from warmer climates and may kill off the top growth of hardy perennials. Leave this dead matter on the plant to protect it against more frost damage and remove it once all frost has passed.
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.
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. Cover plants before dark to trap warmer air.
Bulbs and perennials emerging from the ground do not need to be protected from sub-freezing temperatures. They are growing according to nature's schedule. You don't need to protect plants until after we've had an extended warm period.
Plants and perennials that require additional protection to survive your winter will need a deep layer of mulch added by mid November or later, when the ground is beginning to freeze. Do NOT add deep mulch too early or your plants will not be slowly exposed to colder temperatures, allowing them to acclimate for winter.
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.
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.
The best time to mulch perennials is after the top 1-2 inches of soil has frozen. The mulch provides insulation, keeping the soil consistently cool through winter. Loose organic mulch, such as shredded leaves, bark chips, pine needles, and straw, is a good choice for helping perennials survive winter.
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!
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.
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.
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.
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.
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.
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.
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.
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.
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.
Also, do not cut back hardy perennials like garden mums (Chrysanthemum spp.), anise hyssop (Agastache foeniculum), red-hot poker (Kniphofia uvaria), and Montauk daisy (Nipponanthemum nipponicum). Leave the foliage. It's important to protect the root crowns over winter.
1. In order to overwinter a perennial or shrub in a container outside, it needs to be two zones hardier than the zone you live in. 2. Perennials and shrubs that are in your zone or one colder can be overwintered in an unheated garage, buried in the ground, or transplanted.
Frost zaps many perennials back to the ground. Plants are busy storing sugars and other nutrients in their roots to sustain them through winter. To support plants through this season of transition, make sure they get adequate water until the ground freezes.