Ferns are popular choices for both indoor and outdoor spaces. Maintaining their health depends on understanding their environmental needs, with temperature being a particularly important factor for their survival and growth.
How Cold Affects Ferns
Cold temperatures impact ferns at a cellular level, disrupting physiological processes. When temperatures drop, water within plant cells can freeze, forming ice crystals that expand and rupture cell walls, leading to irreversible tissue damage. This is known as freezing injury, causing significant damage to the plant’s internal structure.
Even temperatures above freezing can be detrimental, a condition called chilling injury. This occurs when metabolic functions slow down, impairing the plant’s ability to absorb water and nutrients. Photosynthesis also becomes inefficient, leading to a decline in overall plant health. These disruptions weaken the fern, making it susceptible to disease and potentially leading to its demise if cold exposure persists.
Identifying “Too Cold” Temperatures
The threshold for “too cold” temperatures varies considerably among fern species, depending on their native habitats. Most ferns generally thrive in temperatures between 60°F and 75°F (16°C to 24°C). However, their minimum tolerance differs significantly. The duration of cold exposure and the plant’s overall health also influence its cold tolerance.
Tropical Ferns
Tropical ferns, such as Boston ferns (Nephrolepis exaltata) and bird’s nest ferns (Asplenium nidus), are sensitive to cold due to their origins in warm, humid climates. These varieties begin to experience stress when temperatures fall below 50-60°F (10-15°C) and can suffer severe, often fatal, damage if exposed to temperatures below 40°F (4°C).
Temperate Ferns
Temperate ferns, commonly found in woodland environments, exhibit greater tolerance to colder conditions. Many can withstand temperatures down to freezing (32°F / 0°C) without immediate damage. However, prolonged periods at or below freezing can still cause harm, especially if the ground freezes deeply or consistently.
Hardy or Deciduous Ferns
Hardy or deciduous ferns, which often go dormant during winter, are the most resilient to cold. Species like some Dryopteris genus ferns have adapted to thrive in colder climates and can survive well below freezing temperatures. These ferns are suited for outdoor planting in specific hardiness zones, where they endure winter cold by shedding their fronds and relying on their underground rhizomes for survival.
Signs of Cold Stress and Damage
When a fern is subjected to temperatures that are too low, it displays visual symptoms of stress or damage. Common signs include wilting or drooping fronds, often followed by a change in color.
Browning or blackening of leaf tips, or even entire fronds, indicates cold injury. Affected areas may also develop a crispy or brittle texture. In severe cases of prolonged cold exposure, the fern’s growth can become stunted, with new fronds failing to emerge or appearing noticeably smaller than usual. Extensive damage can lead to significant leaf drop.
Protecting Ferns from Low Temperatures
Protecting ferns from damaging low temperatures is important for their long-term health. For indoor ferns, strategic placement is important, keeping them away from drafty windows and doors to maintain a stable environment. Avoid proximity to air conditioning vents, which create localized cold spots and dry air. Maintain consistent room temperature within their preferred range and utilize humidifiers.
Outdoor ferns require different preventative measures, especially in regions prone to cold snaps. Apply a thick layer of mulch around the base of outdoor ferns to insulate their roots from freezing temperatures. For tender varieties or during unexpected frosts, cover ferns with frost cloths or burlap. Potted outdoor ferns can be moved indoors to a sheltered location, like a garage or porch, when temperatures are predicted to drop significantly. Select fern species appropriate for your USDA hardiness zone to ensure they can handle local winter conditions.
Helping Cold-Damaged Ferns Recover
If a fern has sustained cold damage, immediate action can aid in its recovery. First, move the affected plant to a warmer, more stable environment. Once in a suitable location, assess the extent of the damage.
Prune away any fronds that are dead, brown, or black, as these will not recover and can prevent fungal issues. This pruning encourages new growth.
During recovery, avoid overwatering, as damaged roots may not absorb water efficiently, leading to root rot. Allow the topsoil to dry slightly between waterings. Provide indirect light and maintain consistent humidity. Recovery can be a slow process, and while some ferns will recover, severe damage may be irreversible.