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.
Luckily for gardeners in mild-winter regions (the warmer parts of Zone 8 and south), container-grown plants require little or no winterizing beyond moving pots to more sheltered locations and perhaps covering them with frost blankets when freezing temperatures are expected.
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.
Take Special Precautions for Potted Plants
To protect them, you can wrap the pots in an insulating material (think burlap, old blankets, or even bubble wrap), place them close to the foundation of your house, and arrange them close together. You can also put a layer of mulch over them for added protection.
Most houseplants are tropicals and prefer temperatures between 65-75°F during the day and about 10 degrees cooler at night. For many plants, temperatures below 50°F can cause problems. Adjust thermostats to cater to your comfort, but remember your plants need some consideration.
Most potted or hanging plants, like petunias, are fine as long temperatures don't dip below 39 degrees, and especially if the temperature doesn't dip to the freezing mark.
If it is not going to rain before the freezing temperatures arrive, you should water plants thoroughly. This is because moist ground stays warmer than dry ground. Watering the night before the freeze arrives will insulate the root structure of the grass and plants, decreasing the potential for cold injury.
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.
The key to success with garage storage is that this area is attached to the house but not heated. The garage never goes below 32 degrees but tends to hover in the 40's and low 50's all winter. This keeps the plant dormant but the plants never freeze. Watering is needed, but not too much or too often.
Bury the pots or bring them inside.
Simply dig a hole and plant the perennial pot and all. This way the soil in the pot will benefit from the temperature-stabilizing mass of the ground soil. Alternatively, bring the pot into an unheated, not-too-bright interior space, such as a garage or cold cellar.
Fiberglass, lead, iron, heavy plastic, and stone are the best weather-resistant containers to use; terra-cotta will eventually expand and crack with repeated freezing and thawing. Assemble your designs early enough that the plants have time to acclimate to their new pots before the hard freeze.
Whether it's hostas, daylilies, ornamental grasses, evergreens or everbearing strawberries, not only can perennials provide big interest and color in their pots, they also have the advantage of being able to grow again year after year. Well, that is, as long as you can protect them over winter!
One obvious sign of dead floras is mushy and fragile stems plus roots. Once a plant has reached this stage, no home remedies will save it.
Provide Additional Shelter. In addition to clustering your plants together, you can also cover the pots with a tarp, straw, or another insulating material. Doing so will moderate the temperature swings and increase the likelihood of your perennials surviving the winter.
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.
Avoid watering when temperatures are below 40°F or when sustained freezing temperatures are expected within 24 hours as this water will freeze and not be available to the plants.
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. Bed sheets or comforters work best for covering large plants and shrubs.
While it's tempting to carry annual pots indoors for winter, it's often better to take cuttings. Try this with coleus, sweet potato vine (Ipomoea batatas), Impatiens, Alternanthera, Tradescantia, either rhizomatous or cane or wax begonias, and ornamental sages, including Salvia leucantha.
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.
Moving houseplants outside is a big change in environmental conditions, so acclimating them is important. Never move houseplants from indoors into full sun. Leaf burn, discoloration, and drop can occur. On some species, especially succulents, the damage to the fleshy stems and leaves is permanent.
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.