If you try to bring such a potted plant indoors, it will simply turn brown and die as it completes its lifecycle. Fortunately, there is another method of overwintering a true annual (which also works for some tender perennials): taking stem clippings and rooting them to create new plants.
You can put them in a garage or shed that gets cold but doesn't freeze, ever. They will over-winter as live but semi-dormant plants. Keep the soil just moist, not letting the potting medium shrink and pull away from the pot edges.
Many annual plants can be brought indoors over the winter as houseplants. Another method is to take cuttings of your favorites such as coleus or impatiens, rooting them, and then potting the cuttings to bring indoors for the winter. Many of these plants can root very simply by placing the cutting in a glass of water.
Annuals that do well indoors during the winter are limited to Coleus, Impatiens (including New Guinea), Nasturtium, Pansy Petunia, Verbena, Wax Begonia, Lantana, and Geraniums, with Coleus and Geraniums being the easiest. Gardening is about experimenting so try any annual you'd like.
The easiest way to overwinter a plant in a pot is to choose one that will be hardy in the pot. The rule of thumb for a plant to be winter hardy in a pot sitting on your patio is that it should be two zones hardier than the climate zone you live in.
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.
For pots that will stay outside all winter because plants are still growing in them, try to move the pots onto a concrete surface, or use bricks, planter “feet,” rocks or pieces of wood to raise them off the ground.
If you are blessed with a greenhouse or have ample window space, the easiest way to overwinter your annuals is to bring them indoors before they are killed by frost. If, like me, you only have a few windows you can take cuttings of these plants (except for Oxalis) and root them on your windowsills.
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.
Light freeze - 29° to 32° Fahrenheit will kill tender plants. Moderate freeze - 25° to 28° Fahrenheit is widely destructive to most vegetation.
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. A second option is what Shadrack calls “corrals.”
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.
Simply put, annual plants die in the winter season so you must replant them every year, while perennials come back every year so you only plant them once.
Planting Annuals Pots
Use containers with good drainage at the bottom and large enough not to be top heavy as the plants grow to their full size. A soil-based potting mix with added perlite will keep containers and hanging baskets draining well and lightweight.
Most annuals like soil slightly and evenly moist 2 or 3 inches down. When you water, if possible, water the soil, not the plants. Many annuals, especially petunias, object to wet leaves and petals. Set the hose on the ground on a drizzle or use soaker hoses.
Annual cleanup
Remove all of your summer annual flowers, including their seed heads, from your flower beds. (Throw these in your compost bin.) This does more than save you time next spring. Leaving annuals in your beds over the winter will invite pests and disease as the plants decompose.
Focus on frost hardy annuals. Some of these annuals that can withstand 20 degrees or so, including pansies, snapdragons, dianthus, alyssum, dusty miller, viola, and osteospermum. Keep in mind that flowers may be a bit ragged after such cold but the plants should make it through ok.
Be Sure to Harden Annuals Before Placing Them Outside
Any annual planted outdoors should be properly hardened to acclimate them to cooler temperatures. Those annuals that are hardened are more likely to survive a frost or light freeze with little to no damage than those that are not hardened.
Between 28 and 25 degrees
It's going to be destructive to your annuals if you don't protect them. This is the temperature zone where you may start noticing damage to some of your perennials too. For example, you may notice their leaves turning a dark color or becoming a little gooey.
When weather forecasters issue a frost advisory in late spring and early fall, that's your heads-up to protect annuals and other vulnerable plants. You might think temperatures have to get to the freezing point (32˚F), but a frost can occur between 36˚F and 32˚F.
Planters made from modern substances, like fibreglass, fiberstone, and non-porous plastic composites can safely remain outside over winter. Fiberstone planters are made from a mix of limestone and fibreglass, which forms a material that looks like stone but weighs a fraction of the natural material.
Store Your Perennials Indoors
An unheated garage, shed, or basement with a temperature range between 30 and 40 degrees can provide a perfect environment for overwintering perennials. Dormant plants should be brought inside and watered periodically whenever the temperature is above 40 degrees.