The idea of a fern that grows into a tree may seem rather strange to North American gardeners. Most familiar ferns have the crown of the plant -- or the part where the fronds and roots connect -- low to the ground. In warmer parts of the world, some ferns grow a trunk as new fronds emerge, forming a tree 30 feet tall or more. About 800 species of tree ferns live in tropical and subtropical areas around the world. Australia has around 15 native tree ferns, with Australian tree fern (Cyathea australis) the most commonly available.
Australian tree fern is often marketed under different scientific names, including Alsophila australis, Cyathea or Alsophila cooperi or Sphaeropteris cooperi. Sometimes it's called rough tree fern because of the little spikes along the base of the fronds. It's also sometimes confused with other similar tree fern species native to Australia, such as Tasmanian tree fern (Dicksonia antarctica, USDA zones 9 through 11).
Australian tree ferns propagate from spores, which are produced on leaf undersides of sexually mature plants. Young plants have small fronds but quickly grow a large head, capable of spreading from 1 to 6 feet wide in one year. After a wide clump forms, upward growth begins. Under favorable conditions, plants can grow several feet a year. Australian tree fern can reach 20 feet tall and 12 feet across.
Soil and Light Requirements
Growing in forest understories, tree ferns need partial sun or filtered sunlight in areas with hot summer temperatures. In coastal areas or if the roots are kept moist, they can take full sun. When grown indoors, give Australian tree fern bright filtered light. Provide an organic-rich, well-drained soil.
Native to tropical areas, Australian tree fern is hardy in U.S. Department of Agriculture plant hardiness zones 10b through 11. If freezing weather is predicted, protect the tree fern by packing straw around the crown of the tree -- the central area on top of the trunk where the fronds emerge -- and fold the fronds together over the top of the plant to protect the vital growing point. Hold fronds in place if necessary with twine tied around the frond bases. If your garden regularly experiences freezing weather, grow the plant in a container and move it indoors before the first frost.
Regular watering is needed to keep the soil moist and the humidity high. Pay particular attention during hot weather, when Australian tree ferns may need watering more often than once a week. Occasional misting during hot weather for both indoor and outdoor plants is beneficial, spraying the trunk and soil. Don't water the crown area of the tree fern.
In tropical areas, Australian tree fern can grow year-round. In other areas, it grows during warmer spring and summer seasons. Apply fertilizer once a month from mid-spring to mid-summer, using a water-soluble fertilizer such as 10-10-10 at the rate of 1 tablespoon for every gallon of water used for outdoor plants. For indoor plants, mix 1/2 teaspoon into every 2 quarts of water used. Soak the root area. Don't apply fertilizer past mid-summer to avoid encouraging new growth as colder fall and winter temperatures approach.
The only pruning needed is to remove old fronds as they die. Wear long sleeves and gloves, since the hairs on fronds can irritate skin. Dip pruners in rubbing alcohol before making pruning cuts.
Australian tree ferns don't make offsets, so if the terminal growing point on the crown of the plant is killed or if it is cut off, the tree fern dies. Don't attempt to start an Australian tree fern from bare pieces of trunk; this only works for Tasmanian tree fern.
Diseases and Problems
Australian tree fern is generally disease-resistant and mostly pest-free.
If the tree fern's crown is kept wet through incorrect watering techniques, a fungus disease called tip blight can occur, disfiguring new fronds.
Sometimes pests occur on plants kept indoors or in greenhouses. Red spider mites or mealy bugs suck juices from plant tissues. Spider mites are small and almost invisible, but you may notice their webbing. Wash them off with a strong spray of water. Mealy bugs are oval, whitish or pinkish, flattened insects that attach to stems or leaves. Rub them off with a cotton swab soaked in soapy water.
Transplanting Container Plants
Container-grown Australian tree ferns can be transplanted into larger sized pots as needed, usually when the roots fill the pot and are coming out the drainage holes. Take care to support the trunk during transplanting and use containers with multiple drainage holes.
Moving Tree Ferns
In-ground plants can be moved if necessary -- though transplanting large plants is usually a multiple-person job. Move plants during cool, moist weather, digging out the plant with as many roots and accompanying soil as possible. Dig the new hole wider than the root ball and fill in around it, firming soil to hold the tree fern erect.
Some tree ferns, such as Tasmanian tree fern, can be transplanted by cutting the trunk off above the ground and re-rooting the base. Such techniques kill an Australian tree fern.
Try to avoid transplanting by choosing a good location to begin with, free of obstructions and in a sheltered place protected from wind.