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.
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.
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.
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.
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.
If it stays cold for more than one night, can I keep my plants covered, or should I remove the coverings at some point? A. Our recommendation would be to remove the cold protection covering once temperatures are above 32 degrees.
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.
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.
Moving plants indoors is the best protection. 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.
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.
A fabric covering is best because it will allow moisture to escape while still protecting your plants from frost. Fabric coverings will prevent the freezing air from coming into direct contact with the moisture on the plant while also capturing the heat that is radiating from the ground.
A hard frost happens when temperatures hover around 32˚F for a few hours, enough to kill all above-ground parts. Below 32˚F is considered a freeze, which is more destructive than a frost. Tender plants, such as tropical houseplants and geraniums, are killed when the air temperature stays below 32˚F for a few hours.
It is best to water in the afternoon or evening the day after a freeze so plants have had a chance to slowly raise their temperature. After a freeze, soft-stemmed, non-woody plants such as impatiens, cannas, elephant ears, agapanthus, amaryllis, begonias, philodendron and gingers may be pruned back to living tissue.
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.
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.
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.
Even an unheated garage or shed is typically enough to keep tender plants in an above-freezing environment. Potted tropicals and houseplants should move inside even if temperatures don't drop below freezing. Many of these suffer damage even at 40 degrees.
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.
Plastic can be used to protect plants from frost, but it's not the best or most effective material. In fact, the horticultural experts here at Green Impressions actually recommend against it. Plastic materials such as vinyl and traditional camping tarps aren't breathable, causing moisture to get trapped inside.
When should I wrap my plants for winter? This Old House landscape contractor Roger Cook suggests to start wrapping your plants in November. If your plants are new, be sure to cover them for the first two years. The older your plants get, the sturdier they become in surviving winters.