Most varieties of day lilies are extremely hardy perennials and will survive cold winter temperatures easily. You should research the type of day lily you have to learn about its specific needs and hardiness, as some varieties are more frost-tender than others. Place your day lilies in a protected area, in a shed or in a garage, if you live in USDA zones 1 through 3 and experience winter temperatures below -35 degrees Fahrenheit. If your day lilies are planted in the ground, you can take some simple measures to help protect them during the cold winter months.
Water your day lilies well between the end of its blooming season and the beginning of winter. Water the lilies daily to supplement rainfall, as this will help to strengthen the day lilies for the winter. Stop watering after the first frost.
Remove all dead and fallen leaves in the autumn to prevent diseases. Clear out the area around the day lilies, removing all debris.
Mulch around your day lilies in absence of consistent snowfall. Spread a 2- or 3-inch layer of organic mulch on the ground around your day lilies to insulate them from harsh winter conditions, as well as protect the roots and crown if frost heaves occur.
Remove all the mulch and any dead leaves from the ground around the day lilies in early spring, as soon as the danger of hard frosts or deep freezes has passed. Leaving the dead foliage and mulch on the ground around the day lilies can attract harmful insects and fungus.
Give special care to day lilies during their first winter after planting them. When the first winter frost occurs, lay three bricks on the ground around the day lily's crown, forming a triangle around the plant. The bricks will absorb heat from the sun and release it into the ground and protect the newly planted day lily from frost heave. Remove the bricks in the spring, when the danger of frost has passed.