Sometimes perennial flowers don’t need extra protection. Many types like peonies, iris, daylilies and hosta are so rock-hardy that they’ll usually do just fine with little or no extra cover. Other types of perennials, like some of the lilies, should have extra care. Unfortunately our winter-weather crystal ball can’t predict if we’ll have an extreme winter. A few years ago, winter conditions killed even long-established tough perennials like iris, hosta and peonies. Many gardeners wished they had fall-protected perennials with special value or meaning.
There are two winter conditions that kill perennials: extremely cold temperatures when the ground isn’t covered with insulating snow, and the freezing/thawing that happens during winter warmups when the ground isn’t covered with snow.
A key element of these two winter-kill situations is “when the ground isn’t covered.” Deep, consistent snow cover beginning in late November through early March is a perennial’s best friend. Snow is a great insulator. Unfortunately it isn’t predictable. Sometimes in early winter we get extreme cold on bare ground before snow comes. And sometimes the snow disappears in midwinter, leaving the ground exposed.
The unpredictability of winter snow is the reason for mulching perennials in fall to aid in winter survival. Mulching keeps the ground covered, to reduce the problems of bare ground winterkill.
Here’s what to do:
- Protect perennials in November, after the soil surface has begun to freeze, and you can no longer insert a shovel into the ground. Don’t cover perennials too early, as it can keep stems and ground too warm, promoting rots and problems. But don’t wait too late, either. Protect before temperatures begin dipping consistently into the single-digits.
- The goal of winter protection is to keep perennials comfortably and consistently frozen during winter. Just enough cold to keep them frozen and dormant, but safe from extremely low temps and fluctuating winter warmups.
- After some cold has entered the ground, and perennial tops are nicely dormant, cover with 12 to 24 inches of straw, leaves, wood products or compost. This can be done even if there’s a light sprinkling of early snow.