Evergreen trees and shrubs have waxy, needle-like leaves (pine, spruce, fir) or tough, broad waxy leaves (holly, magnolia) that are more resistant to the cold and moisture loss. Plants can also modify their life cycles, or rate of growth, to deal with the changing seasons and lack of moisture.
Regardless of which hardiness zone you live in, certain plants do tend to do better in cold weather than others. Flowers such as violas and crocuses are good options. So are snowdrops, hellebores, and heather. Even some types of roses can grow and bloom in 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.
Plants that must survive even lower temperatures, like the Lapland rhododendron, contain genes that encode cold-stress-responsive proteins. These proteins cause the plant to evacuate water from cellular protoplasm into intracellular spaces where ice crystals can form without damaging the plant.
Each plant has a different threshold when it comes to dealing with the cold. Some plants can survive outside even when temperatures hit freezing. Others need to be brought indoors when temperatures reach the mid-40s.
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.
As gardeners, our frost dates are based on 32 degrees Fahrenheit (0 degrees Celsius) to avoid the risk of any plant death. 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!
Some will start dying the second the temps dip, but others can regenerate from healthy roots below the soil even if the top part of the plant is completely frozen. 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.
Start by heavily mulching the roots of the plant with either wood mulch or hay. For added protection, you can nestle gallon jugs of warm water into the mulch each night. This will help drive off some of the cold that can kill the roots.
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. Do not go straight for the pruners and cut everything back.
Choose days when no snow is on the ground and the soil isn't frozen. In cold weather, water should be trickled slowly into the soil. Water only when air temperatures are above 40 degrees F.
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.
Cabbage, Kale, Broccoli, Cauliflower, Brussel Sprouts, Kohlrabi, Collard Greens, Celery, Spinach, Peas, Asparagus, Rhubarb, Radish, Beets, carrots, Lettuce, Arugula, Onions, Potatoes, Swiss Chard-all can handle light frost, but keep a cover handy for a sudden really cold dip (below 25 degrees F).
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.
A hard frost happens when temperatures hover around 32˚F for a few hours, enough to kill all above-ground parts. Below 32˚F is considered a freeze, which is more destructive than a frost. Tender plants, such as tropical houseplants and geraniums, are killed when the air temperature stays below 32˚F for a few hours.
To protect plants from frost, you will need to cover them to keep the moisture from freezing. While an unexpected frost can leave many gardeners scrambling to find anything to cover their tender plants; it is important to use the right materials.
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. In northern zones 4 and 5, plants need to survive months of below freezing weather every winter.
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.
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.
From that experience, I've found the best frost protection for your outdoor plants is either free or cheap. Cardboard boxes and brown grocery sacks make perfect frost cover and at the end of the season can be recycled. I keep various boxes on the patio and when frost is forecast simply put one over the plant.
When do I know it's safe to bring my plant outside? 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.