Frost Advisory - This is when the temperature is expected to fall to 36 degrees to 32 degrees Fahrenheit. Freeze Warning - This is issued when there is at least an 80% chance that the temperature will hit 32 degrees Fahrenheit or lower. Light freeze - 29° to 32° Fahrenheit will kill tender plants.
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 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.
Seedlings, with their tender new leaves, often give up the ghost when temperatures dip to 32-33°F. Tropical plants have differing low-temperature thresholds. Some keel over when temps fall to 40°F; others crumble at 35°F. Other plants are just hardy by nature and can withstand temperatures as low as 18-20°F.
It's safe to move your plants outside when the outdoor temperatures stay consistently above 50°F. Pay attention to the weather report. If nighttime temperatures are set to fall below 50°F, bring your plants in for the night.
So even though the weather forecaster isn't predicting a freeze, once temperatures start to drop below 40°F you need to be aware. Once temperatures drop below 40°F, you may want to start covering your plants with a frost blanket depending on what you're growing, which we'll get into in just a bit.
Signs Your Plants May Be Too Cold
The leaves brown and fall off. If your plant is too cold, it may begin to die, and its leaves will brown and fall off. The leaves turn yellow.
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.
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.
For many plants, temperatures below 50°F can cause problems. Adjust thermostats to cater to your comfort, but remember your plants need some consideration. Avoid placing plants near cold drafts or heat sources. Keep plants several inches away from exterior windows.
While some plants can survive light frosts, others will die as soon as their cells freeze. 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.
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 depends on the temperature, and specifically the nighttime low temperatures. You'll want to move your houseplants indoors before night temperatures get below 45°F. Any colder, and you're likely to see damage, especially on tender new leaves and stem tips.
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.
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.
Hardy cool-season crops (cabbage, broccoli, onions, radish, spinach, turnips, peas, and radish) grow in 40 degree daytime temperatures and can handle some frosty nights without protection.
Fabric coverings will prevent the freezing air from coming into direct contact with the moisture on the plant while also capturing the heat that is radiating from the ground. Bed sheets or comforters work best for covering large plants and shrubs.
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.
Most houseplants are tropical plants that can safely grow in a heated home during the winter. They will never survive outdoors in winter unless you live where the temperature stays at least above 50 degrees. In northern zones 4 and 5, plants need to survive months of below freezing weather every winter.
You have to remember that most common houseplants are tropical, and a lot of them are extremely sensitive to temperatures under 50 degrees Fahrenheit. Some will start dying the second the temps dip, but others can regenerate from healthy roots below the soil even if the top part of the plant is completely frozen.
But most houseplants are adaptable. Although they may not be happy at 55 degrees F., they'll survive. Others suffer damage when they are forced to shiver. But other indoor plants -- cyclamen and florist azaleas, for example, both popular at holiday time -- actually need to be in a chilly room (45 to 55 degrees F.)
Second, know that a light frost—32°F and colder—kills all tender plants such as tomatoes. Hardier plants such as spinach and kale will survive until there's a hard freeze—28°F and colder.
Thoroughly Water plants if it's not going to rain before the freezing temperatures arrive. It may sound illogical. However, a moist ground stays warmer than dry soil. Watering the night before the freeze comes will insulate the root structure of the grass and plants and decreases the potential for cold injury.
Leaves and tender shoots subjected to freezing temperatures or chilling damage appear water-soaked and wilted. These tissues will usually turn black within a few hours or days. The tips of narrow-leaved evergreens, such as junipers, may turn uniformly brown.