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.
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.
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. Adjust thermostats to cater to your comfort, but remember your plants need some consideration.
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.
When temperatures dip below 50-55, I bring my plants in. This is my preference, but most houseplants will tolerate temps down to 40-ish. I'd say anywhere between 45-55 degrees is a good time to bring them in. This is when most pests and bugs have either died off or gone south for the winter.
Plants native to the subtropics are in danger below 40 degrees F and pretty certainly going to be killed at 32 degrees F. Plants native to the tropics are often damaged below 50 degrees F and perish at about 40 degrees F.
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.
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).
This is usually about four to six weeks before the average first frost in your area. A couple of weeks before the temperatures dip to 50 degrees or below, put your potted plants underneath a tree or onto a covered porch so they can start adjusting to the reduced light they'll get in your home.
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.
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.
On a cellular level, this means that as the water turns to ice, it expands, rupturing the cell wall. This is permanent damage, but it doesn't mean the plant will die. If the damage is isolated to a portion of the plant, it can be revived and recover.
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.
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.
H2 Rating - 34 to 41° F (1 to 5 C)
Overall, these annuals are the most delicate of all the hardy annuals - one might call them half-hardy since they are really only hardy into the few temperatures just above freezing. And it's important to NOT let them freeze.
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.
Most houseplants are native to tropical areas and will not tolerate freezing temperatures. Several species of indoor plants will see damage when temperatures drop below 45°F, and nearly all will die if exposed to below-freezing temperatures.
For most grows, the ideal temperature range is between 65-80°F (18-26°C). Keeping temperatures within this range helps maximize plant growth and development and contributes to large and healthy buds. There are many types of temperature controllers growers can use to control their environment.
Many tropical plants cannot survive temperatures below 40 degrees. If you cannot move your pots (with annual or perennial plants), or you have tender plants in the ground, cover your plants. The idea behind covering plants is to trap the heat from the soil and keep the cold air out.
A local study done on frost formation relating temperature to dew point has these guidelines for frost: temperatures from 38 to 42 F can lead to patchy frost, 33 to 37 areas of frost, and 32 and below widespread frost/freeze.
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.
Normally, it is best to keep tropical houseplants above 50 degrees F, but 60 F is even better. My basement is generally in that range, but with the severe cold spells we have had this winter, the temperatures have dropped into the upper 40s. The other important thing to remember is that heat rises.
So will peppers survive 40 degrees? To sum it up, most peppers won't necessarily DIE from 40˚ F temperatures, but their growth can be stunted, and they may be slow to recover so it's best to keep them warm if possible.