When nighttime temperatures start to dip consistently to around 50°F, it's time to bring plants back indoors.
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.
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.
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.
Plan to bring in your houseplants when temperatures are forecast below 45 degrees F to protect them from stress and cold damage. Tender perennial herbs like Rosemary or Lemon Verbena can be brought indoors as well for winter cooking.
A freeze can happen when the surface air temperature falls to 32 degrees F or below; frost may or may not form. A light freeze (between 32 and 29 degrees F) can kill tender plants. A moderate freeze/hard freeze (between 28 and 25 degrees F), can cause wide destruction 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.
The general rule of thumb is that most plants freeze when temperatures remain at 28°F for five hours. Of course, there are exceptions to this rule. 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.
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.
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.
Most plants tolerate normal temperature fluctuations. In general, foliage plants grow best between 70 degrees and 80 degrees F. during the day and between 60 degrees to 68 degrees F. at night.
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.
Morning watering is actually preferable to evening watering as the plant has time to dry before the sun goes down. At night, water tends to rest in the soil, around the roots, and on the foliage, which encourages rot, fungal growth, and insects.
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.
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 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.
A moderate frost, with temperatures ranging from about 28°F to 32°F, necessitates a bit more precaution. In addition to tucking your plants in well with some frost cloth or cover, it's also a good idea to water your plants the evening before frost is expected to hit.
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.
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.
Plants need to be transitioned indoors before the outside temperature starts dropping below 45 degrees Fahrenheit (7 degrees Celsius) at 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.
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.
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.
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.
Of course the main benefit of soaking houseplants in soapy water before bringing them back indoors is killing all the bugs, but there are a few other benefits too. This method for debugging and cleaning potted plants is great because now your houseplants will get a good watering before you bring them back indoors!