The exotic, edible starfruit (Averrhoa carambola) earned its name with its unusual shape, which resembles a five-pointed star when sliced. Starfruit trees seldom appear in commercial nurseries, but they can be grown from fresh seed taken from a ripe fruit. Seed-grown starfruit trees differ from grafted types because they take longer to bear fruit and their fruit may not be of the same quality. Still, the ornamental value of their rosy flowers and evergreen foliage makes them worthwhile to grow.
Ripe starfruit contain up to twelve oval-shaped, 1/4- to 1/2-inch-long seeds. Only healthy starfruit seeds with a dark brown, glossy seed coat should be used for propagation. The seeds lose viability quickly, so plant them immediately after removing them from the fruit to increase your chances of success. No pretreatment or stratification is needed to sprout starfruit seeds, but two- to three-day-old seeds benefit from a 24-hour soak in water to revive them
Starfruit hails from the tropics and is available in markets throughout the year. The seeds will germinate year-round, although they take only one week in summer and two to three weeks in winter. No matter the time of year, start the seeds indoors so their germination conditions can be more easily monitored and controlled.
Sowing and Germination
Prepare a 2-inch pot with multiple drainage holes for each starfruit seed. Wash the pot in hot, soapy water, rinse it and wipe it dry.
Fill each pot with a mixture of 2 parts vermiculite or perlite and 1 part sterile potting soil. Do not compact the growing medium too much because it won't drain properly.
Sow one starfruit seed in each pot. Place the seed on the surface of the soil. Spread a very thin layer of medium over the seed, then tamp it down lightly to increase contact. Do not cover starfruit seeds with a thick layer of medium; the seed should be visible beneath the medium, but still covered.
Set the pot on a propagation mat or on top of the refrigerator -- anywhere where temperatures stay above 70 degrees Fahrenheit. Drape a sheet of plastic wrap loosely over the pot to hold in humidity. Do not seal the edges, and do not place the pot in direct sunlight.
Lift the plastic wrap every day and check the moisture level in the growing medium. Water with a spray bottle if it feels almost dry on the surface, then put the plastic wrap back in place. Avoid overwatering and do not pour water into the pots because a strong stream can dislodge the starfruit seeds.
Watch for sprouts in one to two weeks, depending on the season. If seedlings don't emerge after three weeks, the seeds are probably dead and should be discarded.
Remove the plastic wrap. Move the starfruit seedlings to a room with south-, west- or east-facing windows where temperatures stay above 55 F.
Starfruit seedlings grow rapidly and should be transplanted into gallon containers filled with sandy, loam-based potting soil as soon as they produce two or more sets of leaves. Provide them with:
- Regular water. Water deeply when the soil surface looks dry. Avoid letting them wilt.
- Fertilizer. Starfruit trees are moderately heavy feeders. Water weekly with 1/2 teaspoon of 15-15-15 fertilizer diluted in a gallon of water.
Grow them indoors near a south-facing window, or move them outdoors to a bright, sheltered location. Starfruit trees will only grow outdoors year-round in U.S. Department of Agriculture plant hardiness zones 10 to 12, but they can be grown outdoors during the summer months outside their range and moved indoors during the winter. Grow them in their container for at least two full seasons, then plant in the ground in spring after the soil has warmed above 70 F.