Leatherleaf fern (Rumohra adiantiformis) produces sturdy, glossy dark green fronds that work well when incorporated into flower arrangements. The evergreen fern is equally beautiful when grown in a garden or inside, where its lush fronds can add some much-needed texture and greenery in shady or low-light sites. This attractive fern requires minimal care to look its best.
Choose a Shady Site
Leatherleaf is a frost-tender fern hardy in U.S. Department of Agriculture plant hardiness zones 9B to 11, where it can handle temperatures as low as 25 degrees Fahrenheit. The fern requires a site with partial, shifting or deep shade, and it will suffer in direct sunlight. If planting multiple ferns, provide 24 to 36 inches of space between plants to avoid overcrowding.
Organic Soil is Best
Leatherleaf fern tolerates a range of soil types, though like other ferns it grows best in soil enhanced with organic matter. Mixing 2 inches of composted pine bark into the soil can improve both heavy clay soils and sandy soils.
Water regularly during the growing season in the garden, allowing the soil to slightly dry out between waterings. Mulch in spring and fall with 2 to 3 inches of organic matter such as pine straw or leaves to maintain moisture in the soil and suppress competitive weeds.
Grow Indoors Where Not Hardy
Leatherleaf fern may be grown indoors in indirect sunlight, such as a north-facing windowsill, in colder climates. Avoid placing the fern near heating vents or south- or west-facing windows as these conditions may scorch the fronds. Choose a container with a hole in the bottom for drainage and soil-less potting mix that is 50 percent peat moss.
Water occasionally, allowing potting media to slightly dry out between waterings. Leatheleaf fern does not have high humidity requirements and does not need to be misted. Fertilize every four to six weeks during the growing season with 3 1/2 to 7 drops of a 10-10-10 fertilizer mixed into 1 quart of water. Repot every few years in the spring, replanting in fresh potting media. Divide large plants by carefully splitting the rhizomes.
Check for Pests
Leatherleaf fern may attract common pests such as scale, hard or soft-shelled insects that form dense colonies on fronds; mealybugs, cottony white insects; and mites, tiny, spider-like insects. Outdoor plants may attract slugs and snails. Handpick insects as they appear or wash fronds with a direct stream of water. Cut off fronds infested with scale. Drown slugs and snails in soapy water.
Avoid pesticides, which may burn the sensitive fronds. Monitor the fern every week or so to see if pests re-emerge. House plants that are heavily infested may need to be discarded before the pests spread to healthy plants.
Treat for Blights
Several fungal blights affect leatherleaf ferns. Anthracnose or Colletotrichum Blight affects new growth on the fern, with older fronds not easily infected by the disease. New growth becomes black and further development ceases. Rhizoctonia Aerial Blight occurs most often during summer, with the fronds covered in gray or brown spots that eventually travel up the center where moisture levels are highest. Control the problem by keeping old fronds pruned to increase the circulation of air. Always use sterilized pruning tools when pruning the fern, by wiping the blades off with alcohol and allowing to dry before use.
Treating both fungal diseases with a product containing Daconil helps control outbreaks. Mix 2 1/2 tsp. of Daconil into a sprayer containing one gallon of water. Thoroughly cover all sides of the foliage until dripping. So the fern's foliage isn't damaged, do not spray when temperatures are above 90 degrees Fahrenheit or during sunny conditions. Wear a long-sleeved shirt, gloves and safety glasses when using and wash hands immediately after use and keep children and pets out of the area until the product has dried.
Control Root Rot and Leaf Spots
Pythium root rot can be problematic for leatherleaf ferns grown in areas with improper drainage. Infected ferns become limp and turn a green-gray in color and the roots stop growing and turn to brown mush. The problem is most likely to occur during winters where rains are excessive. To control the problem, grow the fern in soils that are not soggy and drain well.
Cylindrocladium leaf spot is most likely to affect leatherleaf ferns during summer or when winters are warm. Red or brown-gray spots up to an inch in size cover most of the frond and can have a water-soaked appearance. Control the disease naturally by watering early enough in the day so the fronds dry by nighttime. If the problem is serious, treat the fern with the same Daconil treatment as you would with blights.