Some plants tolerate frost and cold temperatures better than others. Plants always killed by frost include summer annual flowers and summer vegetables like impatiens, marigolds, coleus, tomatoes and peppers. Subtropical and Tropical plants are the most sensitive to frost (citrus, hibiscus and bougainvillea).
Perennial Flowers: Most perennial plants do not suffer frost damage and do not need to be covered. Frost sensitive perennials include Hostas and Bleeding Hearts. They should be covered to protect their foliage and flowers. Cover any blooming or budded up perennials.
Plants that have a relatively high base temperature (46° F or higher) are labeled as “cold sensitive” and those with a low base tem- perature (39° F or less) are “cold tolerant”. Those that are in between this range can be called “cold intermediate”.
However, there are many tender bulb flowers that cannot take freezing temperatures no matter where they are planted outdoors. Dahlias, begonias, and freesias will be destroyed if the bulbs are left outside. These bulbs can be dug up and stored inside until the next planting season.
Cabbage, Kale, Broccoli, Cauliflower, Brussel Sprouts, Kohlrabi, Collard Greens, Celery, Spinach, Peas, Asparagus, Rhubarb, Radish, Beets, carrots, Lettuce, Arugula, Onions, Potatoes, Swiss Chard-all can handle light frost, but keep a cover handy for a sudden really cold dip (below 25 degrees F).
With that in mind, experts generally recommend flowers and other houseplants be brought inside or otherwise protected before the thermometer dips below 45 degrees Fahrenheit. For warmer-weather and tropical plants, that threshold is a bit higher, at around the 50-degree mark.
Petunias thrive when nighttime temperatures are in the 55 to 65°F range, and daytime temps are between 61 and 80°F. They will succumb to sub-freezing temperatures in a hurry, and anything below 40°F may kill them, though Wave® petunias can tolerate temperatures as low as 35°F.
Geraniums are robust and easy to care for, and they flower abundantly until autumn. They can also cope well if they are left in a dry place for a few days. But there is one thing that geraniums just can't stand: frost and cold. Geraniums react to temperatures below five degrees Celsius with pauses in their growth.
Flower production is reduced or halted and plant growth may be stunted. Injury to flower buds or blossoms during the late winter or early spring season can be from frost or freeze injury. Late spring frost kills young buds and tender new growth in early spring resulting in fewer flowers and later leaf development.
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.
Once temperatures drop below 40°F, you may want to start covering your plants with a frost blanket depending on what you're growing, which we'll get into in just a bit. Freeze – A freeze is when the air temperature is 32°F or below.
To know when to cover your plants, you need to know when the plants you have are susceptible to frost damage. Some frost-tender plants need to be covered as soon as the temperature hits 32°F, while others can handle lower temperatures and/or longer freezing periods.
A covered porch usually provides protection from light frost, but the garage or sun room is better for freezing temperatures. A couple days in darkness won't hurt the plant. Or move them out during the day and back in at night, if cold temperatures persist.
A mature, cold hardy plant can withstand actual freezing temperatures (32F/0C) for a period of time and even temperatures below freezing. Cold hardy plants can easily handle light to moderate frosts without being harmed.
A light frost may cause minimal damage while a severe frost may kill plants. Young, vulnerable plants are much more susceptible to a light freeze, which occurs when temperatures are 29 to 32 degrees Fahrenheit, while mature plants may only suffer from short-term effects.
SNOWDROPS (Galanthus nivalis)
True to their name, these early-blooming bulbs often emerge while snow is still on the ground to provide a breath of fresh air for winter-weary gardeners. In mild winters, snowdrops can flower as early as late January, but February or March is more common.
It's important to note that zinnias don't tolerate frost, so make sure to wait until all chance of frost has passed before planting them outdoors. In areas with a long growing season, you may also consider planting a second crop of zinnias in mid-summer for an extended bloom period.
Zinnias are sensitive to frost, so do not seed until the last frost has passed in your area. See your local frost dates. Zinnias will grow in a minimum daytime temperature of about 60°F (16°C), though a range of 74–84°F (23–28°C) is preferred.
Calendula (pot marigold or English Marigold) are cold tolerant and can be planted before the last spring frost. Coreopsis, annual are cold tolerant and can be planted 2-3 weeks before the last spring frost. Dracaena (spike) are cold tolerant and can be planted before the last spring frost.