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.
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.
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.
Most houseplants are tropical plants that can safely grow in a heated home during the winter. They will never survive outdoors in winter unless you live where the temperature stays at least above 50 degrees. In northern zones 4 and 5, plants need to survive months of below freezing weather every winter.
Covering plant material at temperatures between 39-30 with freeze cloth or covering can be effective. Prolonged exposure at temperatures of 28 degrees and below will notice protection results diminish with each degree dropped.
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.
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 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.
Protecting Cucumbers from Cold
(18-23 C.). Even prolonged exposure to temperatures below 55 degrees F. (13 C.) can cause decay, pitting, and water-soaked areas on fruit.
Basically, if you start to see new sprouts in a few months, your plant is set to survive. Unfortunately, if you don't see any new growth, the plant most likely won't recover from freezing. “It most likely will be fine with some proper rejuvenation if not left out for the whole winter,” says Angelov.
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.
If the temperatures are below freezing all winter, it is unlikely you will need to water your plants. In freezing temperatures your plants would not be able to benefit from watering because the ground would be frozen and any water would just run off or freeze rather than being absorbed by the plant's roots.
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.
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.
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.
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.
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.
So when temperatures start dipping below 50º F, leave the tomatoes on the vine if warmer days and nights are forecast. But if temperatures have started on a steady downward slide, go ahead and pick the mature green tomatoes and bring them indoors.
Cucumbers are very tender, warm-season plants that grow best in temperatures from 65° to 75°F (18.3° to 23.9°C) with a minimum temperature of 60°F (15.6°C) and a maximum of 90°F (32.2°C).
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.
Will a covered porch protect plants from frost? Yes, it will provide partial protection from light ice and chilly winds. Uproot tender perennials — Tender perennials are plants that cannot withstand deep freeze (roots, bulbs, tubers, and corms).
So, yes, it's been cold at night around Southern California, but folks, 45 degrees is a still a long way from the freezing temps that can burn our tender tropicals and other plants, like a multiday cold snap in 2007 that severely damaged crops in Central and Southern California.
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.
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
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).