Ferns represent one of the oldest plant groups on Earth, having thrived for hundreds of millions of years across diverse climates. These ancient plants reproduce via spores and have evolved an incredible range of tolerance to environmental conditions. How cold a fern can survive depends entirely on its species origin, as tolerance varies dramatically between tropical and temperate types. Understanding this variation is important for any gardener seeking to protect their collection through the winter months.
Defining Fern Hardiness
Plant hardiness provides the foundational context for measuring a fern’s ability to survive cold. The standard metric is the United States Department of Agriculture (USDA) Plant Hardiness Zone System. This system maps North America into numbered zones based on the average annual minimum winter temperature. Each zone represents the lowest temperature a plant can endure and still survive to grow the following spring. Selecting a fern whose hardiness zone aligns with or is lower than your local zone is the primary way to ensure its long-term survival outdoors.
Types of Ferns and Their Specific Limits
Fern species can be broadly categorized into two groups based on their cold tolerance: hardy and tender varieties. Hardy or temperate ferns have adapted to surviving sub-freezing conditions by shedding their foliage or employing wintergreen strategies. Ostrich Fern (Matteuccia struthiopteris), for instance, can thrive in USDA Zones 3 through 7, tolerating average minimum temperatures as low as -40°F. The Christmas Fern (Polystichum acrostichoides) is a resilient example, remaining evergreen and surviving in Zones 3 through 9, often enduring deep snow and ice.
Conversely, tender or tropical ferns are highly susceptible to cold and frost damage, making them popular houseplants or seasonal patio plants in cooler climates. Species like the Boston Fern (Nephrolepis exaltata) and Maidenhair Fern (Adiantum spp.) are generally suited for Zones 9 through 11. These tropical varieties begin to experience chilling injury when temperatures drop below 55°F and will not survive brief exposure to freezing temperatures, typically requiring protection when temperatures fall below 50°F.
How Cold Temperatures Damage Fern Tissue
Cold weather damages fern tissue through both freezing and non-freezing mechanisms, directly impacting cellular structure and function. The most common form of damage is caused by the formation of ice crystals within the plant cells during a freeze. As the water inside the cells turns to ice, it expands, physically rupturing the cell walls and membranes, which results in the leakage of internal fluids and the death of the tissue. This cellular destruction is what causes the foliage to turn black and wilt after a hard frost.
Even temperatures above freezing can harm tropical ferns, a phenomenon known as chilling injury, which typically occurs between 32°F and 55°F. At these temperatures, the cold disrupts the function of cell membranes, impairing metabolic processes and nutrient transport. This often manifests as discoloration in the fronds, where they may turn yellow, red, or purple, and can lead to wilting even though no ice has formed. Evergreens can also suffer from desiccation, losing water through their leaves faster than their frozen roots can absorb it, leading to a scorched or burned appearance on the foliage.
Techniques for Winterizing and Protection
Protecting ferns from cold depends on their hardiness classification, requiring different approaches for in-ground and container plants.
Protecting Hardy Ferns Outdoors
For hardy ferns planted in the garden, the primary goal is to insulate the crown and roots, which are the plant’s survival centers. Gardeners should apply a thick, loose layer of organic mulch, such as straw or shredded leaves, over the root zone after the ground freezes to moderate soil temperature fluctuations. Many deciduous hardy ferns benefit from their own spent fronds being left in place over winter, providing a natural layer of protection against the elements.
Protecting Tender Ferns Indoors
Tender, tropical ferns grown in containers must be moved indoors well before the first frost, typically when nighttime temperatures consistently fall below 55°F. Once inside, they need a location with bright, indirect light, ideally near a south- or east-facing window. Managing the indoor environment is important, as heated homes often lack the humidity tropical ferns require. Placing the pot on a tray filled with pebbles and water or using a room humidifier will help maintain the necessary moisture levels around the fronds. During their indoor dormancy period, reduce watering significantly, only providing moisture when the top inch of soil feels dry to prevent root rot.