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.
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 do I know it's safe to bring my plant outside? 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.
Another temperature to keep in mind: tomato plants will not survive in temperatures below 35 degrees Fahrenheit, and will not thrive at temperatures below 50 degrees. Gardeners can use a frost date calendar to note their region's average dates of last frost in spring and first frost in fall.
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 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.
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.
Even prolonged exposure to temperatures below 55 degrees F. (13 C.) can cause decay, pitting, and water-soaked areas on fruit. Sudden cold snaps can cause cucumber plant damage on leaves, stems, and fruit or even kill the plants.
It's sunny again… wait… ), but some vegetables won't be happy (and grow into incredibly prolific tomato producers - yes, I'm talking to you, tomato fanatics) if you don't protect them until night temperatures warm up consistently to 50 degrees and above.
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.
There is no universal temperature for all plants that is too cold, so that is why it is important to do your research. However, it is safe to say if living in an environment with harsh winters and temperatures below 20°F, it is always wise to bring plants indoors during this time, if possible.
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.
Low Temperatures
Wait until the temperature is consistently above 65 degrees Fahrenheit (18 degrees Celsius) before moving houseplants outside. If temperatures dip below, bring your plants back inside.
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. A severe freeze/killing freeze (24 degrees F and below), causes heavy damage to most plants.
Even if you're antsy and can't wait to see the pop of color outside the porch, DO NOT put the basket outside until temperatures are consistently above 50F (minimum!). Let the basket stay toasty warm here, or keep it in a bright sunny space in your home away from sudden drafts or temperature fluctuations.
And while all or partly red tomatoes can tolerate these same low temperatures, they really do best at temperatures above 50º F. After a few days at or below 40º F, flavor, firmness, and shelf life start to diminish.
For the most tender plants, temperatures in the 40's can do them in. Others can make it through a frost or two but take the sign of this colder weather as their signal for dormancy. In either case, don't expect them to look very good once Jack Frost has paid a visit.
Cucumbers are on of the best garden vegetables. If you start them too early in the garden, the frost will kill them. Timing is critical to your cucumber success.
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.
Cover Plants – Protect plants from all but the hardest freeze (28°F for five hours) by covering them with sheets, towels, blankets, cardboard or a tarp. You can also invert baskets, coolers or any container with a solid bottom over plants. Cover plants before dark to trap warmer air.
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.
Plants need to be transitioned indoors before the outside temperature starts dropping below 45 degrees Fahrenheit (7 degrees Celsius) at night.
Night Frost Survivors — You likely will have plants that should be able to survive a night of frost. These include brassicas (cabbage, broccoli, cauliflower), peas and spinach. These should be okay, but it won't hurt to cover with a bed sheet or a frost cloth.
So the answer is yes, cardboard boxes will protect plants from frost. They offer a simple, cost-effective and readily available way to insulate your outdoor plants. We love this idea and are stockpiling our Amazon boxes in readiness so we can help our plants deal with whatever winter weather is heading our way.