So a potted perennial must be bone-hardy in your area and colder to survive in a container with no special treatment. Offer protection. Potted perennials rated to your hardiness zone or just one zone colder can be left outdoors, but they need some protection.
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.
Overwintering Techniques
The general consensus seems to be that the best way to overwinter containerized perennials is to take the entire pot and bury it in the ground. This way, the roots are protected like they would be if the plants were actually planted in the garden.
Plants and perennials that require additional protection to survive your winter will need a deep layer of mulch added by mid November or later, when the ground is beginning to freeze. Do NOT add deep mulch too early or your plants will not be slowly exposed to colder temperatures, allowing them to acclimate for winter.
Perennials are a wonderful addition to any landscape, and while they are relatively easy and low-key, not all perennials are as hardy for winter as they may seem. Fortunately, it is easy to prepare perennials so they can survive the cold and will come back in spring larger and lovelier than ever.
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.
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.
In dry-winter areas that don't freeze or have little snow, water perennials once a month to keep them alive and healthy. In all other areas, cut back on watering to help plants harden off in preparation for winter. On perennials that have finished for the season, cut back stems to 6 to 8 inches from the ground.
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.
Hostas, daylilies, black-eyed Susan and many other common perennials need little more than a trim back and a bit of protection from a layer of mulch to survive.
Perennials, on the other hand, live for three or more growing seasons. They are less work than annuals because they grow back each year from roots that go dormant in the winter. They will be around from year to year, so they put their energy into growing strong roots instead of growing lots of flowers like annuals do.
You can overwinter potted perennials in an unheated shed or garage. Allow them to naturally go dormant in the fall before moving them inside. Then put them back outside once the weather starts warming back up in late winter or very early spring.
Perennials NOT to Cut Back
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.
Do not fertilize perennials in the fall, but adding compost is a good thing for the soil. Keep removing spent flowers as well as dead and dying foliage. Keep the base of the plants free of dead leaves and debris. Unless it is a plant that needs dry soil conditions, keep watering until the ground freezes.
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.
Store Your Perennials Indoors
An unheated garage, shed, or basement with a temperature range between 30 and 40 degrees can provide a perfect environment for overwintering perennials.
Cutting back of dead or dying top growth is the only maintenance that is needed for winter preparation. Cut the stems down to the ground, or to the basal growth if you are gardening in the warmer hardiness zones. Alternatively, the flower stems can be left to cut back in the spring.
To have success with overwintering, it's important to be familiar with which plants can successfully make the transition from outdoors to indoors. Many of the tender perennials we now consider annuals originally found their place in horticulture as houseplants and are excellent candidates for bringing inside.
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.
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. Cover plants before dark to trap warmer air.
When Should You Cover Plants? Cover your plants at night and remove them during the day when the temperatures rise above 32 degrees F, so that the soil can warm up again. Some outdoor plants won't survive the harsh conditions of winter, bring them inside and use these tips for caring for them through winter.
Yes – if you secure the plant properly. Garbage bags work to cover plants and protect from frost, but they must not be allowed to touch the plant's surface. Use stakes and supports to create a tent-like structure over the plant, which will retain warm air. Make sure the trash bag goes all the way to the ground.
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.