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.
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.
An important factor in the chance of survival is how long the plants were exposed to cold temperatures. A few hours can do the job, depending on the plant. Generally, though, it takes 12 to 24 hours of exposure to cold temperatures to completely kill most tropical plant species. If you're in doubt, check the roots.
A hard frost happens when temperatures hover around 32˚F for a few hours, enough to kill all above-ground parts. Below 32˚F is considered a freeze, which is more destructive than a frost. Tender plants, such as tropical houseplants and geraniums, are killed when the air temperature stays below 32˚F for a few hours.
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.
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.
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.
If it stays cold for more than one night, can I keep my plants covered, or should I remove the coverings at some point? A. Our recommendation would be to remove the cold protection covering once temperatures are above 32 degrees.
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.
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.
Avoid watering when temperatures are below 40°F or when sustained freezing temperatures are expected within 24 hours as this water will freeze and not be available to the plants.
It is best to water in the afternoon or evening the day after a freeze so plants have had a chance to slowly raise their temperature. After a freeze, soft-stemmed, non-woody plants such as impatiens, cannas, elephant ears, agapanthus, amaryllis, begonias, philodendron and gingers may be pruned back to living tissue.
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.
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.
Leaves Wilt or Droop — As the cell damage occurs, the leaves will lose their rigidity and start to droop or even curl in on themselves. If you have been watering as usual, it is time to consider other causes for your plants' wilting and drooping appearance including cold shock.
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.
While the damage to the leaves is permanent, plants are pretty resilient. If the leaves are severely damaged, they will die and fall off. New leaves should take their place. It may take several weeks or months to see full recovery, but given warmth, proper light and water, most plants bounce right back.
Add a Thick Layer of Mulch
Just like slipping on a sweater when it's chilly, adding a layer of mulch to your garden beds will help protect the soil from sudden swings in temperature. Use straw, wood chips, leaf mold, or even just a heap of leaves to provide crucial insulation for the plants' root systems below ground.
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.
Time It Right
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.
Indeed, a plant that is drought stressed will often show more damage after a light freeze. So if they are dry, always be sure to water your potted plants before a freeze to help them better handle the cold.
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 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.
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.