Plastic – Plastic is definitely not the best winter covering for plants, as plastic, which doesn't breathe, can trap moisture that can kill the plant in a freeze. You can use plastic in a pinch, however (even a plastic garbage bag), but remove the covering first thing in the morning.
Plastic can be used to protect plants from frost, but it's not the best or most effective material. In fact, the horticultural experts here at Green Impressions actually recommend against it.
Covering Plants with Plastic Bags
Plants under plastic bags retain moisture and even capture what the plants produce by transpiration. Don't use plastic bags as a greenhouse for succulents, though, as they can definitely tolerate neglect, but won't tolerate this kind of moisture.
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.
Usually, covering plants with any plastic material isn't safe. However, you can cover your plants with garbage bags to protect them from pests and pest-carried diseases and sometimes excess moisture from too much rainfall.
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.
A fabric covering is best because it will allow moisture to escape while still protecting your plants from frost. Fabric coverings will prevent the freezing air from coming into direct contact with the moisture on the plant while also capturing the heat that is radiating from the ground.
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.
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.
Plastic bags are an essential tool for today's indoor gardeners. They are not only terrific plant sitters when you skip off for a winter weekend, they also help when your plant is not doing well. You can keep houseplants without water for several weeks in a polyethylene plastic bag.
-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.
Yes, that's right: plants provide all the “air” they need for their own survival. They're perfectly happy sealed in a plastic bag. How long can you keep your plants sealed up like this? Easily 6 months, quite possibly up to a year.
If freezing temperatures are planned to linger longer than one night, cover your plants with a floating row cover, burlap, an old sheet, or even plastic. Place the cover over your potted plant before dusk to capture heat. Also, make sure the cover is significant enough to reach the ground and secure it.
A frost blanket is a sheet of fabric, plastic, or other material used to cover plants in cold. There are different types of frost blanket, some designed to protect crops, others designed to shield plants in a home garden.
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. Well, we have temperatures forecast to dip into the 40s again on Thursday and lower 40s on Friday.
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.
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.
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.
Plants in the ground
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.
Before you cover the plants in late afternoon or early evening, water your plants lightly. Apply covers in early evening as winds die down, and remove the coverings when temperatures rise the next day (mid-morning) so that plants can get full exposure to the warming sunlight.
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.
How long should I leave my plants covered during cold temperatures? You should cover plants until temperatures are back into the upper 40's to 50's. If it looks like a couple of days of cold temps, leave the coverings on for a couple of days.
Freeze Warning - This is issued when there is at least an 80% chance that the temperature will hit 32 degrees Fahrenheit or lower. Light freeze - 29° to 32° Fahrenheit will kill tender plants. Moderate freeze - 25° to 28° Fahrenheit is widely destructive to most vegetation.
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.