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.
Leafy greens and herbs
Leafy greens such as spinach, lettuce, arugula, and Swiss chard also grow very well in cooler months, as do leafy herbs such as parsley and cilantro. In fact, many of these vegetables grow more abundantly in cooler weather and do not bolt to seed as easily as they do during hot summer days.
Flowers are happiest in temperatures of 40-50 degrees. Most of my flower coolers are about 36 degrees.
Marigold cannot tolerate cold temperatures.
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.
Winter pansies are cold weather all-stars. These cheery winter blooming flowers can freeze solid and emerge from that state ready to grow and flower. Other bedding plant flowers that grow in winter include nemesia, snapdragon, sweet alyssum and flowering stock.
Christmas Rose. This evergreen perennial blooms from winter to early spring depending on the selection and where you live (mostly early winter in warmer regions, and as late as February or March where it's very cold).
Name: Helleborus sp. commonly known as winter rose or Hellebore. Belongs: to the buttercup family Ranunculaceae. Origins: Europe to western China. Flowering: Winter to early spring.
Most evergreen conifers, however, that have a strong central leader, such as: Japanese red-cedar (Cryptomeria); fir (Abies); spruce (Picea); and cedar (Cedrus) can be covered in snow without it causing a serious effect.
When should you plant marigolds? Plant your marigolds in the spring, after the last frost. If you choose to start from seed indoors, you can begin the process about 2 months before the last expected frost. Seeds will germinate anywhere from 4 to 14 days in warm soil that has an average temperature of 70°F – 75°F.
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.
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.
Our recommendation would be to remove the cold protection covering once temperatures are above 32 degrees. If you leave the covering on when it gets warm and the sun is shining brightly, it may get too hot inside the cover and stress out the plants.
A: Geraniums come from tropical climes. You could leave them out for the winter if you lived in Los Angeles. But they blacken and die immediately when we get hard freezes, even though they can handle weather down into the 40s.
As a general rule of thumb, a perennial plant should be rated for two cold hardiness zones colder than your climate to be dependably hardy in a container through winter. For example, a gardener in USDA cold hardiness zone 5 can expect perennials rated for zone 3 or colder to survive the winter in containers.
Fiberglass, lead, iron, heavy plastic, and stone are the best weather-resistant containers to use; terra-cotta will eventually expand and crack with repeated freezing and thawing.