Mulch Is Good. Adding a layer of straw, wood mulch or rotted leaves to the soil surface in containers will help to provide extra insulation from cold. Water Is Important. Though dormant ornamental plants don't drink much water, you want to keep an occasional eye on soil moisture, especially when there's been drought.
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.
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.
Because the soil will expand and contract greatly over a harsh winter, those who grow where the ground freezes hard should empty out plastic, ceramic and clay pots to protect them from cracking. Or you can just bring the whole schmageggie inside to a place that will remain above freezing.
Burying the plants – pots and all – is best for insulating the roots. Cover the dormant plants with a couple of inches of leaves or wood mulch for extra protection.
Protect your containers.
Place dormant perennials in the ground or in another, frost proof pot. They'll need a sheltered place to weather the season. Rinse pots inside and out, stack upside down and store out of the weather, preferably in an unheated garage or shed.
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.
Despite the fact your plants are dormant and brown, they should still be watered periodically. Plants that remain dehydrated in winter months often don't survive until spring. Not only does this create extra landscaping costs in warmer months, it can actually damage your plumbing.
Most plants prefer a temperature of no less than 60° F, though many can tolerate as low as 40° F. Along with this, while the improper temperature can certainly be an issue, fluctuations in temperature are the true killer.
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.
Just be sure not to leave the pot in the ground too long in the spring or the roots will start to grow out the drainage holes, anchoring the pot into the ground. You can overwinter them by moving the pots into a cold frame or unheated garage for the winter after the first hard frost.
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.
If you're still worried about leaving them out, you can cover them with a container cover or tarp. If they're small, turn them upside down and store them in place. A winter garden in a concrete container. Concrete pots can stay out all winter and look best if you plant them.
Suitable plants
Try box (Buxus sempervirens), bay (Laurus nobilis), skimmia, euonymus, ivy (Hedera) or Gaultheria mucronata varieties. Ornamental cabbage and kale provide additional foliage interest.
With reduced sunlight and dry air from indoor heating, winter isn't a time when your plants will thrive, but, with the right care, they can certainly survive. “Less sunlight typically sparks a plant's natural dormancy,” says Casey Godlove, creative director at PlantShed.
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.
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. Apply water at mid-day so there is time to soak in before possible freezing at night.
A second option is to sink your pots into the ground. By placing your pots in the ground, the surrounding soil can insulate the pot (and the plant roots), protecting from extremely cold temperatures and temperature fluctuations. Additionally, the pots and plants can be mulched to provide additional protection.
Move Pots Before Frost
Take them to the garage or a covered porch, then return them outdoors when temperatures rise again. This can gain you weeks of extra bloom-time late into fall. You may even find time to add miniature pumpkins, gourds, dried corn cobs and other fall garden container accessories.
The solution: Keep your houseplants in their plastic nursery pots for at least the first year. You can still use your pretty pot, Lawrence and Gutierrez say. Just slip the new plant, plastic pot and all, into the decorative pot and cover the top with Spanish moss or rocks to cover any gaps.