Hawaiian tree ferns (Cibotium glaucum) bring delicate, bright green texture to their native Hawaii. You can also grow them elsewhere in U.S. Department of Agriculture plant hardiness zones 10a through 11 or grow them indoors. While they can reach a mature height of 15 to 20 feet in their native habitat, container-grown tree ferns usually stay much smaller. They also grow slowly, adding only 3 1/2 inches per year when young and only 1 inch per year when they get older. Meeting their cultural requirements takes some effort, but it can be done.
Whether you plant Hawaiian tree ferns outdoors or in containers, the soil must drain quickly and have a pH below 7. Hawaiian tree ferns also grow best in soil that is rich in organic matter. Mix a 3- to 6- inch layer of compost, leaf mold, composted bark mulch or sphagnum peat moss into the soil before planting. For container growing, use a peat-based potting mix that contains pine bark and course sand for improved drainage.
Hawaiian tree ferns prefer partially shady areas where they are shaded from the hot afternoon sun. The planting site must also be protected from strong, drying winds. Set indoor tree ferns in bright but indirect light. They can be placed next to an east-, west- or south-facing window or right in front of a south-facing window with a sheer curtain hanging between the fern and the window.
Hawaiian tree ferns thrive in moist, humid conditions. Water outdoor tree ferns two to four times each week or as often as necessary to keep the soil consistently damp. Give it 3 gallons of water then check the soil 24 hours later. If it is dry, water the tree fern again. If it is moist, check it again in a few days. Organic mulch can be spread over the soil around the tree fern to help reduce moisture loss and control weeds. Container-grown Hawaiian tree ferns should be watered when the top of the soil begins to dry. Pour water over the soil until it runs out of the drainage holes in the bottom of the container. The amount of water needed will vary depending on the size of the plant. Set indoor tree ferns above a humidity tray or run a humidifier if the indoor air is dry.
Slow-release fertilizer should be sprinkled around the Hawaiian tree fern every three to six months, depending on how long the fertilizer is formulated to last. Give houseplant fertilizer to container-grown tree ferns and garden fertilizer to landscape tree ferns. Use a balanced fertilizer such as 10-10-10. The rate of application is generally 1/2 teaspoon per 1 square foot of soil, but this varies depending on the fertilizer formulation.