Ferns are an ancient and diverse group of plants that typically thrive in moist, shaded environments. Their survival through the winter depends on recognizing the specific needs of the species, which range from tropical rainforests to temperate woodlands. Whether a fern is kept indoors or remains in the garden, the change in temperature and light requires proactive care. Winterizing ferns involves managing dormancy, light, and humidity to allow the plant to rest or continue slow growth until spring.
Identifying Fern Hardiness and Type
The first step in winter care is determining a fern’s hardiness, separating species into two categories: tender and hardy. Tender or tropical ferns, such as the Boston fern (Nephrolepis exaltata) and the Bird’s Nest fern (Asplenium nidus), cannot survive freezing temperatures. They must be moved indoors or treated as annuals, as they require active management to maintain growth or induce controlled dormancy.
Hardy ferns are perennial species adapted to withstand the cold temperatures of temperate zones, often surviving in USDA Hardiness Zones as low as 3 or 4. These ferns, including the Christmas fern (Polystichum acrostichoides) and the Autumn fern (Dryopteris erythrosora), are best left outdoors to enter their natural dormancy period. The care regimen for an outdoor dormant plant is fundamentally different from one actively growing indoors.
Managing Environmental Conditions for Indoor Ferns
Indoor winter environments challenge tropical ferns due to the low humidity caused by central heating systems. Ferns require a relative humidity level between 50% and 70%, but heated homes often drop below 30%, causing browning frond tips and leaf drop. To counteract dry air, use a humidifier near the plants to maintain consistent moisture levels. Grouping several potted ferns together also creates a beneficial microclimate, as collective transpiration increases localized humidity.
Another method for increasing humidity is placing the fern’s pot on a pebble tray filled with water, ensuring the pot bottom does not touch the water to prevent root rot. Misting provides only a temporary boost and should be done in the morning to allow foliage to dry completely, reducing the risk of fungal disease. The temperature should be stable, ideally kept between 60 and 75 degrees Fahrenheit, and away from cold drafts or direct heat vents.
Light requirements shift during winter’s shorter, cloudier days, but ferns still need bright, indirect light, such as from an east or west-facing window. Too little light causes weak, leggy growth, while harsh, direct sun can scorch the fronds. If natural light is insufficient, supplemental artificial grow lights can be used.
Watering frequency must be adjusted because the plant’s growth slows and less water is lost in cooler, lower-light conditions. Overwatering is a common cause of winter loss, so water only when the top inch of soil feels dry, keeping the mix consistently moist but never saturated. Stop fertilizing completely during the winter, as the plant is not actively growing and excess nutrients can cause root damage.
Dry indoor conditions make ferns susceptible to common houseplant pests like spider mites and scale insects, which thrive in low-humidity air. Before bringing any fern indoors, thoroughly inspect and hose it down to remove pests. Regular inspection throughout the winter allows for early treatment with insecticidal soap or neem oil before infestations become widespread.
Preparing Outdoor and Dormant Fern Varieties
Hardy ferns remaining in the garden require an approach focused on insulation and protection from cold and wetness. Avoid pruning in the late fall, as the old fronds provide natural insulation and protection for the crown and emerging fiddleheads. The spent foliage shields the plant from harsh winter winds and sudden temperature fluctuations.
Apply a thick layer of organic mulch, such as shredded leaves or straw, around the base of the fern after the ground begins to freeze. This protective layer regulates soil temperature, preventing the freezing and thawing cycle that can heave the plant out of the ground and damage the root system. The mulch should be several inches deep but must not completely cover the fern’s central crown, which needs to remain uncovered to prevent rot.
In areas with prolonged winter drought or sandy soil, a deep watering may be necessary just before the ground freezes. Excessive winter moisture is detrimental, so ensure the planting location has excellent drainage to prevent dormant roots from sitting in soggy, cold soil and developing root rot. Marginally hardy ferns, those at the cold limit of their hardiness zone, may benefit from temporary protection like a cloche or evergreen boughs during the coldest parts of the season.
Transitioning Ferns Back to Spring Conditions
The transition back to active growth in spring requires careful timing to avoid shocking the plants. For hardy outdoor ferns, remove or rake back the protective layer of winter mulch and old fronds once the threat of severe frost has passed. This allows the soil to warm up and new fiddleheads to emerge without being smothered. Any tattered or brown fronds remaining from the previous year can be cut back to the ground just as the new growth begins to unfurl.
Indoor ferns that have been overwintered need to be gradually acclimated to outdoor conditions. Before moving them outside permanently, introduce them to brighter light and fluctuating temperatures by taking them out for a few hours each day over one to two weeks. This slow acclimatization prevents sun-scald on the fronds and minimizes environmental shock. Once outside, gradually increase the watering regimen, and introduce a diluted, balanced fertilizer to support vigorous growth.