Ferns exhibit a wide range of adaptations to survive cold temperatures. Whether a fern lasts through winter depends entirely on its specific species and the climate of its location. For gardeners, the answer rests on whether the plant is a cold-hardy perennial intended for outdoor landscapes or a frost-intolerant tropical variety that must be protected. Understanding the difference between these two categories is the first step in ensuring the plant’s survival through the coldest months.
Understanding Fern Winter Strategies
Ferns employ distinct biological mechanisms to tolerate or avoid freezing temperatures, categorizing them into two main survival groups. Deciduous ferns respond to cold by shedding all their above-ground fronds after the first hard frost. This process is a form of dormancy, where the plant’s energy is withdrawn into underground rhizomes or crowns to wait out the winter. The stored carbohydrates and nutrients within these underground structures allow the fern to regenerate entirely new fronds, often called fiddleheads, in the spring.
The second group is the evergreen fern, which retains its green fronds throughout the winter, even under snow and ice. These species utilize physiological adaptations, such as concentrating certain compounds in their frond cells to lower the freezing point, much like a natural antifreeze. This mechanism allows them to continue photosynthesizing on warmer winter days, offering a slight advantage in energy collection. Evergreen ferns will typically only shed their old fronds in the spring, just as the new growth begins to emerge, maintaining a continuous display of foliage.
Distinguishing Hardy Outdoor Ferns from Tropical Varieties
Determining a fern’s winter fate requires understanding its natural hardiness, often defined by the USDA Plant Hardiness Zone system. Hardy outdoor ferns are those species whose rhizomes can withstand the typical minimum winter temperatures of a specific hardiness zone. For example, the Christmas Fern (Polystichum acrostichoides) is a common evergreen species known for its resilience in zones 3 through 9, providing year-round foliage.
Deciduous hardy ferns, such as the Ostrich Fern (Matteuccia struthiopteris), are equally cold-tolerant, surviving even colder zones by dying completely back to the ground. In contrast, tropical varieties, which are commonly sold as patio plants, lack the ability to survive freezing temperatures. The Boston Fern (Nephrolepis exaltata) is a non-hardy fern that is only reliably perennial in zones 8 or warmer, and it will perish if left outside in a colder winter. Identifying the specific species and its corresponding hardiness zone is the most reliable method for predicting winter survival.
Practical Steps for Winterizing Ferns
For hardy ferns planted in the garden, a few simple actions can boost their chances of successful overwintering. Applying a thick layer of organic mulch, such as shredded leaves or wood chips, around the base of the plant is beneficial. This material acts as an insulator, protecting the underground crown and rhizomes from the damaging cycle of freezing and thawing, which can cause the plant to heave out of the soil. A layer of mulch two to four inches deep is sufficient to moderate soil temperatures.
The timing of pruning is an important consideration, differing between the two main types of ferns. Deciduous ferns should not have their brown, spent fronds removed until late winter or early spring. The decaying fronds provide a natural layer of insulation and protection for the crown throughout the coldest months. Evergreen ferns, conversely, should only have their oldest or damaged fronds trimmed back in late winter before new fiddleheads appear, which cleans up their appearance.
While ferns in winter require less moisture than in the growing season, they should not be allowed to dry out completely. In periods of prolonged dry winter weather, an occasional deep watering is beneficial, particularly for evergreen species that retain their foliage and continue to transpire. For tropical ferns, winterizing involves bringing them indoors before the first hard frost, pruning back long fronds for manageability, and placing them in a cool, bright location where temperatures stay above 55 degrees Fahrenheit.