Generally speaking, if your plant is hardy to two zones colder than the area that you are in, it will be a good candidate for being able to survive the winter in a container in your area. There are some exceptions to that rule, and certain plants are better able to survive in containers through the winter than others.
Depending on their hardiness, some potted plants will respond to the first frost by going dormant just like garden plants do. However, as the temperature continues to drop, their roots might die unless they are protected.
Wrap pots in burlap, bubble wrap, old blankets or geotextile blankets. It isn't necessary to wrap the entire plant because it's the roots that need shielding. These protective coverings will help to trap heat and keep it at the root zone.
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.
When frost threatens, it's time to move many of your outside plants indoors. Many tender bulbs, annuals, herbs, and tropical plants will only survive the winter inside. Here's advice on which plants to bring indoors this fall and how to winterize plants and pots.
Trim off any leggy branches or make any aesthetic cuts. Also, trim dead leaves and spent blooms as those are what diseases and insects use to find their way indoors. It's best not to bring insects inside your home. Be sure to check the top of the soil, the top of leaves and the underside of leaves.
Before bringing your plant inside, remove any insects by hosing your plant off with a strong stream of water. I recommend doing this a couple of times. Immediately before bringing them inside, give your plants' leaves, stems, and trunks thorough spraying of insecticidal soap.
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.
Remember to protect electrical connections from 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.
WHEN TO BRING PLANTS INSIDE. As a general rule, tender plants should be brought in when nighttime temperatures are below 50 to 55 degrees F, even if they are hardy for your zone. A plant's roots are more exposed when planted in a container versus in the ground.
You can overwinter potted perennials in an unheated shed or garage. Allow them to naturally go dormant in the fall before moving them inside. Then put them back outside once the weather starts warming back up in late winter or very early spring.
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.
Your plants won't need as much water during their dormancy as they do in the spring and summer, but be sure to water them deeply a few times a month.
Good drainage is important and fluffy soil also adds a measure of insulation. Best is to move them indoors wherever there is room. This could mean a garage, a basement or a greenhouse if you have one. An unheated garage will moderate outdoor temperatures and give your plants a good chance at survival.
When should I wrap my plants for winter? This Old House landscape contractor Roger Cook suggests to start wrapping your plants in November. If your plants are new, be sure to cover them for the first two years. The older your plants get, the sturdier they become in surviving winters.
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.
So even though the weather forecaster isn't predicting a freeze, once temperatures start to drop below 40°F you need to be aware. 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.
Use a frost blanket, cloche or row cover material – not plastic — to insulate the ground around your outdoor plants and keep them warm. Prop the cover with sticks or stakes to keep it from coming into direct contact with the new growth. And, make sure to remove the cover the next morning after the temperature warms up.
One way or another, when a warm spell during winter is forecast to be followed by a cold snap, make sure to protect not only the pot and the roots but also the top of the plant. Cover plants with plastic film, burlap, blankets or other cloth material at night.