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.
For the best chance at success for keeping your garden alive through the winter, choose hardy native plants that are used to your area's climate. When the weather forecaster warns of an overnight freeze, cover your plants with burlap, an old sheet or blanket, or clear plastic.
According to Myers, the hardiest vegetables that can withstand heavy frost of air temperatures below 28 include spinach, Walla Walla sweet onion, garlic, leeks, rhubarb, rutabaga, broccoli, kohlrabi, kale, cabbage, chicory, Brussels sprouts, corn salad, arugula, fava beans, radish, mustard, Austrian winter pea and ...
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.
Know Your Plants' Cold Tolerance
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. You'll need to look up each of your plants to verify its cold tolerance.
Protect Plants from Late Spring Freeze.
Hardy plants such as daylilies, hosta, peonies, lily-of-the-valley will survive a late spring frost, but they will be out of commission until warmer weather returns. These are perfect times to replace them with frost-resistant plants.
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.
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.
Burlap. Wrapping shrubs and trees in burlap can protect them from harsh winter conditions and keep them from going through dreaded winter burn.
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.
Thoroughly Water plants if it's not going to rain before the freezing temperatures arrive. It may sound illogical. However, a moist ground stays warmer than dry soil. Watering the night before the freeze comes will insulate the root structure of the grass and plants and decreases the potential for cold injury.
In the event unusually low temperatures are forecast, simply wrap the container in a heavy blanket, bubble wrap, or some other material that will provide insulation during the severe cold snap. These protective coverings will help to trap heat and keep it at the root zone.
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.
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.
Nighttime Covering: Whatever route you go, if your potted Hostas are out in the elements for the winter, you will want to provide them with a nighttime covering. This can be burlap, frost cloth, sheets, cardboard boxes, or even inverted pots.
Second, know that a light frost—32°F and colder—kills all tender plants such as tomatoes. Hardier plants such as spinach and kale will survive until there's a hard freeze—28°F and colder.
Most roses can withstand a quick cold snap of temperatures down to 10 degrees F, but it is best to protect them if you expect an extended period of time when temperatures dip under 20 degrees F. The amount of protection your roses need depends on the climate in which you live.
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.
In fact, you can leave plants covered with blankets or sheets for several days without harming them if freezes are likely for several nights in a row. But eventually the covers must be removed so the plants can get light.
The easiest way to harden transplants is to place them outside in a shaded, protected spot on warm days, bringing them in at night. Each day, increase the amount of sunlight the transplants receive. Don't put tender seedlings outdoors on windy days or when temperatures are below 45° F.