Winterize both garden-grown and container-grown hostas in the fall to prevent root damage. They will reward this extra winter care with lush new foliage in the spring and showy flowers in the summer. There's only one way to winterize hostas growing in the ground, but a few options exist for container hostas.
Hostas and Winter Damage
Hostas (Hosta spp.) are hardy in U.S. Department of Agriculture plant hardiness zones 3 to 9. They can survive winter temperatures as low as -40 degrees Fahrenheit when planted in the ground. Damage occurs to the root system when temperatures fluctuate. This is a bigger problem for hostas planted in the fall because they have not had a chance to grow their roots deep into the ground, but damage can occur in established hostas, too. As the ground goes through thaws and freezes, it pushes or heaves the hostas up, exposing their roots to the elements.
Some hosta varieties will not survive in USDA zones 9. They require a two-month winter rest with temperatures below 40 degrees F.
Hostas in the Ground
Cover hostas with 2 to 3 inches of shredded bark mulch in the fall after the ground freezes to a depth of 3 inches. The mulch will help insulate the soil, reducing the severity of temperature fluctuations, and protect the crowns of the plants. Remove the mulch in the spring when the ground thaws and there is no longer any danger that it will freeze again.
The tops of the hostas must be uncovered right away if new shoots begin to poke through the mulch. It can be left covering the crowns of the plants, though, if there is a chance temperatures may plummet again. Remove mulch from the crowns of the plants and pull it back a few inches away from stems after the danger of freezing temperatures has passed.
Hostas are toxic to cats, dogs and horses. Keep them out of the reach of pets.
Hostas in Containers
Protect container-grown hostas from extreme cold, temperature fluctuations and moisture. Hostas in containers will be subjected to wind-chill temperatures far below what they would experience in the ground, and fluctuations will be more extreme. In addition, moisture will gather in the pots on top of the frozen roots when the sun warms the container, which will cause root rot.
- Move hostas into an unheated garage or shed when temperatures begin to drop below freezing. Water them if temperatures warm to above freezing and the soil thaws only if it is completely dry. Set them back outdoors in the spring when temperatures warm to above freezing. If new shoots appear before freezing temperatures are over for the year, move the hostas outdoors to get some sunshine during the day while it is warm then back into the enclosure at night.
- Dig holes in the ground large enough to hold the containers and sink the hostas, containers and all, into the ground in the fall before it freezes. Cover them with 2 to 3 inches of shredded bark mulch after the ground freezes. Pile the mulch over the top of the soil within the containers beneath the leaves and add more until plants are covered. This will prevent water pooling on the soil in the containers. Remove mulch in the spring and move the containers back to their usual positions when the danger of freezing weather has passed.
- Bunch the potted hostas close together in an area protected from the wind such as right up against the north side of a building or next to the foundation of the house underneath a deck. Pile shredded bark mulch or shredded leaves over the soil in the containers and then over all of the plants to a depth of 2 to 3 inches. Pack it along the outer edge of the group of containers to a thickness of 3 to 4 inches. Uncover them and move them back to their usual locations in the spring when temperatures are likely to stay above freezing.