Most houseplants are native to tropical areas and will not tolerate freezing temperatures. Many indoor plants will see damage when temperatures drop to the mid to lower 40s°F and nearly all will die if exposed to below freezing temperatures.
Below 40°F (4°C): Most houseplants are at risk of damage. Frost-sensitive plants can start to experience leaf damage or die if exposed to these temperatures. Below freezing (32°F or 0°C): This is typically lethal for most houseplants.
Each plant has a different threshold when it comes to dealing with the cold. 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.
That all depends on what plants you're growing indoors. Some like cooler temperatures and others prefer more warmth, but since most houseplants are tropical, a good rule of thumb is to keep them somewhere between 60 and 75 degrees F.
Cover Plants
Once temperatures drop below about 28°F, covering is typically no longer effective at preventing cold damage. Occasionally even with covering you may still see damage at temperatures above 28°F, especially when the below freezing temperatures are prolonged (5 hours or more).
First, it's good to know which plants are sensitive to temps around 35 or 40 degrees F. These are common tender plants that will need some protection. Hanging baskets: begonias, calibrachoa, gerbera, impatiens, vinca, euphorbia, and scoparia.
Grown under the marginal conditions of most homes and apartments, houseplants can become tired-looking during the winter months. You can renew your plants by moving them outdoors during the summer. Put your plants out only after night temperatures remain above 60°F (usually mid-May or early June).
So, how hot IS too hot for plants? The general answer is around 90 degrees F, with some exceptions to the rule. This means that when temps rise above 90 and remain there for a lengthy spell: Leaves wilt.
Spray with Insecticidal Soap: Before bringing your plants indoors, thoroughly spray their leaves, stems, and trunks with insecticidal soap to eliminate any remaining pests. Allow the spray to dry thoroughly before bringing the plants inside.
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.
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.
One of the first signs plants are too cold is the wilting or drooping of leaves. Normally resilient and perky, leaves that begin to droop or wilt can indicate that the plant is unable to properly regulate water within its tissues.
Some houseplants 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. An important factor in the chance of survival is how long the plants were exposed to cold temperatures.
Depending on the species, temperatures dipping below 45-50°F can cause damage. Here are some things to consider before you bring your houseplants back indoors.
Saturate the soil with 1/2 cup hydrogen peroxide 3% per 1 litre water. Put wet soil in a watertight container and leave overnight before planting. This kills pathogens such as fungi and bacteria including insect eggs and nematodes (roundworms).
A Basic Recipe for using soap as a pesticide:
Use a mild dishwashing liquid or Castille Soap. Natural Hand soap can be used too. Add 1 tsp of dishwashing liquid soap to 1 Litre of water. Place in a spray bottle and gently spits your houseplant, covering the top and bottom of the leaves.
There are, of course, many variables that can affect winter hardiness in a plant, but in general, my rule of thumb is as long as the mercury stays in the 20's at night and rises above freezing during the day, our plants should be just fine.
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.
If you're looking for tips for protecting your plants from frost, avoid using garbage bags as plant covers. Trash bags are not the best protection against frost damage, primarily due to their material. Most garbage bags are made out of low-density polyethylene plastic.
A few more tips on containers. Early in spring when your plants are smaller and the temperatures are lower you may only have to water every 3 or 4 days. As the plants get larger and the mercury creeps higher be prepared to water every day, with small pots or water “pigs” you might even have to water twice a day.
Cacti and Succulents
It shouldn't come as a surprise that succulents and cacti will thrive outdoors. These plants prefer bright light and to dry out in between waterings, but just because they are desert plants, don't just move them into the brightest spot outside one day and assume they'll fend for themselves.
Tropical plants should remain in their winter quarters until temperatures no longer fall below 10°C. Balcony beauties such as geraniums, fuchsias, or angel's trumpet should remain protected, as well, until mid-May.