Ferns are relatively easy to care for and popular in part for their ability to grow indoors under conditions that other plants can't tolerate; however, not all ferns have the same environmental requirements. It's important to understand the needs of your particular fern in order to prevent health issues.
Variation Among Species
The yellowing and death of some fronds is not abnormal in ferns. This does not necessarily signal a larger problem; however, you should be aware that delicate fern species and cultivars are more prone to damage than others. Maidenhair ferns (Adiantum tenerum) are one example. Autumn ferns (Dryopteris erythrosora) undergo a natural change of colors with the seasons and should turn a yellowish brown in fall.
Dry indoor conditions can lead to yellowing of ferns. Humidity needs vary among ferns. Boston ferns (Nephrolepis exaltata), staghorn ferns (Platycerium bifurcatum) and maidenhair ferns require high humidity and may need daily misting with distilled water. Maidenhair ferns require 50 percent humidity. You can increase humidity in the air by placing planters in or near trays of water and selecting a cool location for your ferns, away from hot air drafts.
Poor drainage will cause fern fronds to yellow. Plants that sit in standing water are also at risk of developing root rot. You should research the watering needs of your ferns, as the same care recommendations do not apply to all ferns. Water your plants well, as frequently as your species requires. Ensure that their pots provide good drainage, and use a soil mix high in humus.
Light affects fern foliage color. Ferns such as the staghorn fern need filtered or indirect light. Indoor climbing ferns (Lygodium japonicum) need bright light. The foliage of asparagus ferns (Asparagus densiflorus "Sprengeri") turn yellow when they are grown in full sun. These drought-tolerant ferns are adapted to growth in full sun, but they show better color and performance when provided with partial shade.
Ferns require no fertilization during winter and only light fertilization from spring to fall. You are more likely to scorch or burn a fern from overfeeding than damage it from lack of nutrients, but plants do sometimes show nutrient deficiencies. Nitrogen deficiency may appear as slow growth and a light green or yellow foliage. Phosphorus deficiency causes stunted plants and yellowish-green leaves. Magnesium, iron and manganese deficiencies can also lead to yellowed foliage. The Oregon State University Extension Service suggests to increase the feeding of your ferns from three to four times per year to four to five times per year if their fronds yellow.