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.
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.
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.
It's important to keep an eye on the weather – specifically the night ranges – and ensure that your green friends are never outdoors in degrees lower than 45 degrees. If you care for tropical plants, anything lower than 40 degrees will cause severe harm.
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.
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.
I did some research and found out that houseplants need to be taken indoors before overnight temperatures dip below 45 degrees. Most tropical plants suffer harm from temperatures below 40 degrees.
When Should You Cover Plants? Cover your plants at night and remove them during the day when the temperatures rise above 32 degrees F, so that the soil can warm up again. Some outdoor plants won't survive the harsh conditions of winter, bring them inside and use these tips for caring for them through 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.
Some plants can survive outside even when temperatures hit freezing. Others need to be brought indoors when temperatures reach the mid-40s. Be sure you know this before you plant.
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.
If low temperatures loom, cover plants with cloth, burlap or plastic at night. If you use plastic, be sure to remove the covering during the day since temperatures can heat up, causing premature bud growth. Also, when covering, avoid damaging the top part of the plants.
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.
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.
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.
As fall approaches and night temperatures reach about 50°F (10°C), start bringing the plants inside for the winter. Most tropical plants will suffer damage at temperatures below 40°F (4°C), a few even below 50°. You will need to act well in advance of any actual frost or freeze to acclimate them.
Before bringing your plant inside, remove any insects by hosing your plant off with a strong stream of water. I recommend doing this a couple of times. Immediately before bringing them inside, give your plants' leaves, stems, and trunks thorough spraying of insecticidal soap.
It's also a good idea to check the soil; if it's damp, you can wait a few more days before watering, but if the soil feels dry, add water until it drains out the bottom of the pot (just don't let the extra moisture sit in a saucer).
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.
Light freeze - 29° to 32° Fahrenheit will kill tender plants. Moderate freeze - 25° to 28° Fahrenheit is widely destructive to most vegetation. Severe or hard freeze - 25° Fahrenheit and colder causes heavy damage to most plants.
While it might be tempting to ship indoor plants outside at the first sign of spring, it's best to wait until nighttime temperatures stay at or above 60°F, and after any threat of frost has passed. If you're planning ahead, use the average last frost date for your area as your guide.
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.
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.
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.