The newly emerged growth of most perennials can tolerate temperatures in the low 30s and upper 20s Fahrenheit. Freeze damage may occur if temperatures drop into the lower 20s or teens. Most well-established perennials that have been damaged by freezing temperatures should survive.
So, keep an eye out for your local weather forecast and get your plant coverings ready when temperatures are forecast to dip to 32 degrees F and below.
I did some research and found out that houseplants need to be taken indoors before overnight temperatures dip below 45 degrees. Most tropical plants suffer harm from temperatures below 40 degrees.
Bulbs and perennials emerging from the ground do not need to be protected from sub-freezing temperatures. They are growing according to nature's schedule. You don't need to protect plants until after we've had an extended warm period.
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.
So even though the weather forecaster isn't predicting a freeze, once temperatures start to drop below 40°F you need to be aware. 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.
Remember to protect electrical connections from moisture. 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.
2. Perennials and shrubs that are in your zone or one colder can be overwintered in an unheated garage, buried in the ground, or transplanted. 3. Perennials and shrubs in containers will need water through the winter but should not be kept wet.
It's common to think that everything should be chopped down to the ground in the fall, but some perennials actually need their foliage to protect new shoots through the winter. Other varieties offer up important habitat for local wildlife and some perennials provide height and interest through the winter months.
Some perennials, like hostas, peonies and daylilies, need to be pruned in fall to avoid winter damage. Plants like these should be pruned after the first few frosts in late fall or early winter. Other perennials like mums and coneflowers are better off being pruned in spring just before new growth comes in.
Signs Your Plants May Be Too Cold
The leaves brown and fall off. If your plant is too cold, it may begin to die, and its leaves will brown and fall off. The leaves turn yellow.
If you're wondering at what temperature threshold you should be bringing your plants, there's a short answer: when nighttime temperatures reach 45 degrees (F), it's time to bring your plants indoors.
Hardy plants can be hardened off when the outside temperature is consistently above 40° F. Half-Hardy plants may be hardened off at 45° F.
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.
Also, do not cut back hardy perennials like garden mums (Chrysanthemum spp.), anise hyssop (Agastache foeniculum), red-hot poker (Kniphofia uvaria), and Montauk daisy (Nipponanthemum nipponicum). Leave the foliage. It's important to protect the root crowns over winter.
Many gardeners cut back their perennials much too soon. There are nuances specific to certain perennials that dictate whether it is better to cut them back in fall or spring, but the general rule I like to follow is to only cut back perennials that start looking bad in fall.
At any time during the warm season it is okay to cut back dead or ugly foliage. When winter has arrived, and your perennials have either died back or stopped growing, you can remove all dead foliage. Then apply an inch or two of compost or mulch around plants. Leaves work great for mulch as well.
The key to success with garage storage is that this area is attached to the house but not heated. The garage never goes below 32 degrees but tends to hover in the 40's and low 50's all winter. This keeps the plant dormant but the plants never freeze. Watering is needed, but not too much or too often.
Know Your Plants' Cold Tolerance
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. You'll need to look up each of your plants to verify its cold tolerance.
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.
From that experience, I've found the best frost protection for your outdoor plants is either free or cheap. Cardboard boxes and brown grocery sacks make perfect frost cover and at the end of the season can be recycled. I keep various boxes on the patio and when frost is forecast simply put one over the plant.