The question of whether ferns are cold hardy has no single answer, as their ability to survive winter depends entirely on the species. Ferns are non-flowering vascular plants that reproduce via spores, with species thriving in tropical rainforests and frigid, northern climates. A gardener must correctly identify their specific fern to understand its tolerance for freezing temperatures. Cold hardiness refers to a fern’s ability to maintain a viable root system through sustained cold, not necessarily its ability to keep its fronds green. Understanding the hardiness rating system is the first step toward successful winter protection for outdoor ferns.
Understanding Fern Hardiness Ratings
Gardeners rely on a standardized system to determine a plant’s likelihood of surviving the coldest part of the year. The most widely used guide in the United States is the Department of Agriculture (USDA) Plant Hardiness Zone Map, which divides the country into 13 distinct zones. This map is based on the average annual extreme minimum winter temperature over a 30-year period.
Each zone represents a 10-degree Fahrenheit range, with lower numbers indicating colder regions. Each zone is split into two 5-degree subzones, labeled ‘a’ and ‘b,’ allowing for a more precise assessment. A plant rated “hardy to Zone 5” is expected to survive the average minimum temperatures of that zone and any warmer zones. While useful, the map only assesses temperature and does not account for microclimates, elevation, or soil conditions, which can influence a fern’s actual winter survival.
Deciduous Versus Evergreen Hardy Ferns
Ferns are broadly categorized by their foliage retention during the cold season, which dictates their cold tolerance and appearance in winter.
Deciduous ferns, such as the Ostrich Fern (Matteuccia struthiopteris) or the Japanese Painted Fern (Athyrium niponicum), handle deep freezes well. The fronds of these species brown and die back completely to the ground in late autumn, signaling that the plant has entered a state of winter dormancy. Their survival mechanism lies in their robust, underground root structures, or rhizomes, which are protected from the cold by the soil.
Evergreen ferns maintain their green foliage through the winter. Examples include the Christmas Fern (Polystichum acrostichoides) and the Holly Fern (Cyrtomium falcatum). While the fronds remain attached, they may look weathered by late season, especially in harsh conditions. The foliage helps protect the crown and root system from temperature fluctuations and drying winds, though new growth will emerge in spring to replace the old.
Conversely, many popular ferns, particularly those used in containers, are considered tender and lack substantial cold tolerance. Tropical species, such as the Boston Fern (Nephrolepis exaltata), are native to warm climates and cannot survive sustained freezing temperatures. These ferns are generally hardy only in USDA Zones 9 or higher and must be brought indoors or stored in a protected, unheated space to survive the winter.
Essential Winter Care for Outdoor Ferns
For established, cold-hardy ferns planted directly in the ground, minimal intervention is needed to ensure survival. The most effective step is applying a thick layer of organic mulch, such as shredded leaves or wood chips, over the root zone in late fall. This layer, ideally three to four inches deep, insulates the soil, stabilizing the temperature and preventing damaging freeze-thaw cycles that can heave a plant out of the ground. Keep the mulch from directly covering the fern’s crown to prevent rot.
Watering requirements decrease as the plant enters dormancy, but dry winter conditions can be detrimental. While ground-planted ferns in regions with regular winter precipitation require no extra watering, those experiencing drought should be watered sparingly on warmer days. This moisture is important for evergreen varieties, which continue to lose water through their fronds.
For containerized ferns, especially those borderline hardy for the local zone, a different approach is necessary. These plants are more susceptible to cold because their roots are exposed to freezing air temperatures. They should be moved to a sheltered location, such as an unheated garage or porch, once nighttime temperatures consistently drop below 50 degrees Fahrenheit. During storage, the soil should be kept barely moist to allow the fern to remain dormant without drying out.