Many “annuals” can be brought inside, even tender plants that need a winter dormancy period. These should ideally come indoors before nighttime temperatures dip below 45°F (7°C). As fall approaches and night temperatures reach about 50°F (10°C), start bringing the plants inside for the winter.
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.
Generally speaking, the annual plants you want to protect or propagate should be brought indoors as the growing season begins to wane and before cold weather threatens to kill them.
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.
It's important to pay attention to the temperature dropping when the sun goes down. Once temperatures are consistently between 60 and 45 degrees Fahrenheit, it's a good idea to start bringing your plants inside at night. However, you can keep them out during the daylight hours if the temperature stays above 45 degrees.
Some cool-season annuals can tolerate soil temperatures as low as 45 degrees Fahrenheit; however, most cool-season annuals will grow best in soil temperatures at 65 degrees Fahrenheit or warmer.
So what's the solution… 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.
Winter time is very hard on flowers. They just do not like any temperature below freezing. Don't get me wrong flowers like it cold. Flowers are happiest in temperatures of 40-50 degrees.
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 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.
Although most people keep their hanging baskets up during spring, summer and early fall, bringing them back inside when the weather gets cold, there are hanging basket plants that thrive in the cold.
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.
During winter, geraniums grow best with night temperatures of 50° to 60°F (10° to 16°C) but will survive if they drop to 32°F (0°C) and/or rise above 80°F (27°C), as long as they are kept relatively dry. When new growth appears in the spring, cut off all the old leaves.
With that in mind, experts generally recommend flowers and other houseplants be brought inside or otherwise protected before the thermometer dips below 45 degrees Fahrenheit. For warmer-weather and tropical plants, that threshold is a bit higher, at around the 50-degree mark.
If you're wondering at what temperature threshold you should be bringing your plants, there's a short answer: when nighttime temperatures reach 45 degrees (F), it's time to bring your plants indoors.
However, know that even if air temperatures are as high as 38°F, frosts may occur on the ground and on plants. It's better to protect plants just in case!
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.
Petunias thrive when nighttime temperatures are in the 55 to 65°F range, and daytime temps are between 61 and 80°F. They will succumb to sub-freezing temperatures in a hurry, and anything below 40°F may kill them, though Wave® petunias can tolerate temperatures as low as 35°F.
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!
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.
Tender plants, such as tropical houseplants and geraniums, are killed when the air temperature stays below 32˚F for a few hours. A freeze warning often signals the end of the growing season in fall because temperatures are low enough to kill off annuals and begin dormancy for hardy perennials, trees, and shrubs.