Some Hardy Perennials Need Winter Protection in the North and Midwest, and Tender Perennials Require Special Protection Methods. Many perennials hardy for your zone make it through winter just fine with no special attention.
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.
1. In order to overwinter a perennial or shrub in a container outside, it needs to be two zones hardier than the zone you live in. 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.
The best time to mulch perennials is after the top 1-2 inches of soil has frozen. The mulch provides insulation, keeping the soil consistently cool through winter. Loose organic mulch, such as shredded leaves, bark chips, pine needles, and straw, is a good choice for helping perennials survive winter.
Winterizing plants like roses or newly-planted perennials requires protecting plant crowns with a deeper layer of mulch or soil after the ground freezes. You may want to create a framework around plants using chicken wire, hardware cloth or tomato cages that you stuff with leaves or straw to protect plant crowns.
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.
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.
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.
A layer of mulch about 4 to 6 inches deep over the plants works well. Most perennials will pop through the mulch in spring. If the plants are young, small, or newly planted in fall, you may need to clear some of the mulch in spring so that the plants can emerge without difficulty through the winter protection.
Leaving the winter blanket in place is likely to smother them or encourage the growth of harmful mold—removing the mulch will allow your perennials to grow and thrive. Taking the insulating blanket of winter mulch off your perennials will allow them to breathe and grow.
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.
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.
A hard freeze in May is not uncommon for some areas and can kill tender perennials. Hardy plants such as daylilies, hosta, peonies, lily-of-the-valley will survive a late spring frost, but they will be out of commission until warmer weather returns.
Bed sheets or comforters work best for covering large plants and shrubs. Newspaper can be used on low-growing foliage, but it can often be difficult to get it to stay in place. I have used old pillow cases, sheets, towels and even cardboard boxes.
Fall mulching saves time and energy in the spring because it cuts down on the need to weed. It also saves you the labor of spreading new soil and mulch after winter snows. You may also save money too. Mulched perennials are likely to last the winter, reducing your outlay on new blooms.
If you smother a perennial in mulch, you upset the plant's biology and run the very real risk of keeping the crown wet during winter dormancy, and it will rot. I avoid organic mulches on perennials that need free drainage and good air circulation, such as bearded irises, lavenders, rosemary and sages.
Cedar and eucalyptus mulches are particularly well-suited for water retention and for keeping pests away from your perennials.
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.
Heated Greenhouses with Roll-up Sides
This method requires minimal fuel and results in very few plant losses. To use this method, perennial houses are heated to 50°F until all of the plants are well rooted, then the temperature is slowly decreased to 35°F so the plants will vernalize and flower in the spring.
Most perennial plants go into a state of dormancy, or winter rest, as a result of the cold temperatures and shorter daylight hours of winter. These sleeping plants lose their stems and leaves and are dormant, not dead! They will re-grow from their roots with the arrival of spring.
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.