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.
Your annuals need lots of sunlight in order to survive the winter season, and so placing your plants in the brightest spot of your home will ensure they thrive. A windowsill that receives plenty of bright light will do!
True annuals and plants that we grow as annuals (considered tender perennials in southern regions) cannot survive cold winter temperatures. But there's no need to say farewell to these plants forever! Many “annuals” can be brought inside, even tender plants that need a winter dormancy period.
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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.
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.
Temperatures below 32 degrees can freeze the leaf tissue of cold-wimpy plants and turn them to mush after just a few hours. Some of the most tender annuals and veggies might not die but suffer cold-induced setbacks even when overnight lows dip below 40.
Light freeze - 29° to 32° Fahrenheit will kill tender plants. Moderate freeze - 25° to 28° Fahrenheit is widely destructive to most vegetation.
Steps To Care For Petunias in Winter
The plant will need to be brought inside in the winter in colder regions. Petunias can be allowed to go dormant, or you can try to keep the plant actively growing, but be aware that it can be hard to prevent dormancy.
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.
Some annuals have hissy fits when moved inside as a houseplant, and simply die. But others enthusiastically greet this winter greenhouse! Annual geraniums (Pelargonium) are commonly used for indoor décor, but a few others that typically do really well are coleus, impatiens (pictured), fuchsia and wax begonia.
Remove Most Annuals
In general, these plants are easy to spot because after the first hard frost, many of them, including impatiens, begonias, and coleus, have withered and turned brown. If the spent foliage and blossoms on these plants are free of mold and disease, we put them in the compost pile.
Continue to water annuals until freezing temperatures kill them. If your annuals are in containers, move them into a garage or other protected space when temperatures are forecast to dip into the 40s overnight. You can do this until daytime temperatures no longer rise above that threshold.
Cutting off the dead and spent foliage a few inches above the ground in the fall will not harm the plant. Remove spent annuals and seasonal vegetables. Unlike perennials, annuals do not come back from season to season so there is no reason to leave these in the ground.
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.
If they are in the ground, you can dig them up, re-pot them, (make sure you bring indoors before the frost), and place in a sunny window. You can bring in Geraniums and just cut them back; but for other annuals, such as Coleus, it's better if you begin by taking stem cuttings from the plant.
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.
It's not recommended to use dish detergent (like Dawn), laundry detergent, or hand soap (even the “natural” versions), since these soaps contain abrasive ingredients that could harm your plants. For DIY insecticide, organic pure castile liquid soap is the best solution since it's all natural and highly effective.
If you're bringing a plant in from outside, you may want to place it in a new pot to ensure the soil doesn't have pests. It's a good idea to isolate new plants for a few weeks when first bringing them into your home. Check them weekly to make sure they aren't infested.
Yet in reality, any plant can grow anywhere as long as it gets the right amount of light, water, and temperature. This means that many plants we currently think of only as “outdoor” plants, will do great inside our home, while others we think of as house-dwellers will live outside happily.