Annuals able to tolerate a light frost include varieties of petunia, nicotiana, calendula, Victoria sage, and verbena canadensis. There are half-hardy annuals which tolerate cold temperatures but not direct frost. Select from the upright verbenas, asters, ageratum, gazania, geraniums, and lobelia.
So what's the solution… Focus on frost hardy annuals. Some of these annuals that can withstand 20 degrees or so, including pansies, snapdragons, dianthus, alyssum, dusty miller, viola, and osteospermum. Keep in mind that flowers may be a bit ragged after such cold but the plants should make it through ok.
24 degrees (or colder)
(It can depend on how protected your flowers are in your yard.) This is also the freeze that kills most remaining annuals. There are a few exceptions. Pansies and violets, for example, are tenacious little flowers that can survive these freezing temperatures.
Some cool-season annuals can tolerate soil temperatures as low as 45 degrees Fahrenheit; however, most cool-season annuals will grow best in soil temperatures at 65 degrees Fahrenheit or warmer.
Overall even flowers can tolerate light freezes (low 30s to upper 20s °F) and even a little snow. Record cold temperatures (below mid to upper 20s) will damage or destroy many of the flowers of early blooming varieties. The foliage of fully emerged spring-flowering bulbs like tulips and daffodils can also be damaged.
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.
True annual plants have a life cycle of one year—germinating from seed, blooming, producing seed, and dying all in a single growing season. Some examples of true annuals are cosmos, marigolds, and sunflowers.
Pansies. Remarkable as a winter plant, Pansies can survive freezing coldness and can thrive vigorously when summer returns. Its “smiling face” flowers are vibrant in color and fragrance that can fill up your space with so much joy.
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.
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).
Bright reds, soft pinks, sunny yellows –– these colors can still pepper your lawn, even when there's snow. You just need winter flowers that bloom in the cold. Viola, winter jasmine, calendulas, and many plants love cooler temperatures, and they don't hesitate to take advantage of winter's pollinators.
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.
Examples of half-hardy annuals: baby's breath, bells of Ireland, blue sage, forget-me-knots, and strawflower. Tender annuals are native to tropical regions, are very sensitive to cold soil temperatures, and are easily damaged by frost.
Half-hardy plants cannot withstand winter wet and cold, and will be killed by the frosts. However you don't need to wait until the warmer weather to get going – sow your seeds or grow your seedlings in a light, frost-free place, protecting them under cover until the frosts are over.
Pansies are a remarkable winter annual capable of surviving temperatures down to the single digits, freezing solid, then bouncing back with vigor when warm weather returns.
During winter, geraniums grow best with night temperatures of 50° to 60°F (10° to 16°C) but will survive if they drop to 32°F (0°C) and/or rise above 80°F (27°C), as long as they are kept relatively dry. When new growth appears in the spring, cut off all the old leaves.
Many newer cultivars are able to withstand hotter temperatures, but no petunias thrive in chilly temperatures. When spring temperatures dip below 40 degrees F, either cover your outdoor petunias with frost cloth or bring them indoors. Temperatures below freezing will kill them in a jiffy.
Temperature And Humidity
Impatiens are quite tolerant of humidity and high temperatures but do not like to dry out. Expect to water more frequently when temperatures climb into the 90s or when the weather is hot and dry. Impatiens don't tolerate cold weather and will wilt and die once frost arrives.