With mature heights varying from 3 to 30 feet, birds of paradise plants (Strelitzia spp.) have flowers that resemble birds' heads. Those blooms are orange and blue on common bird of paradise (Strelitzia reginae) and white and blue on giant bird of paradise (Strelitzia nicolai), also called the bird of paradise tree. Both plants are hardy in U.S. Department of Agriculture plant hardiness zones 9 through 11. After their seeds are sown, the seedlings produced from them take three to 10 years to bloom.
In their native South Africa, bird of paradise plants are pollinated by birds. Although the common yellowthroat warbler (Geothlypis trichas) occasionally performs that function for the common bird of paradise in the United States, pollinating your bird of paradise's flowers by hand may be necessary if you wish the flowers to produce seeds. Seeds also can be purchased.
Begin the hand-pollination process by selecting one flower and parting its two blue petals that fuse together over structures called anthers. Gather white pollen from those anthers, using an artist's brush, and "paint" the pollen onto the stigma of another flower. The stigma is the pointy white tip protruding from the blue petals.
If the pollination is successful, then a seed capsule will form, which requires five months to mature. When the capsule finally turns brown and breaks open, it should contain about 60 to 80 seeds.
Pretreat Seed Shells
Plant the seeds in March or April, but first remove their orange, hairy tufts -- called arils, and refrigerate the seeds at 40 to 45 degrees Fahrenheit for two weeks inside a plastic bag with one handful of damp seed-starting mix. If possible, sow fresh seeds immediately, before their black outer shells harden. If you will plant purchased seeds or harvested seeds that have hard shells, then pretreat them to speed their sprouting, or germination.
After removing seeds with hard shells from the refrigerator, immerse them for 24 to 48 hours in lukewarm water, softening them. Then scratch the side of each seed with a file or the tip of a utility knife until you break through the black shell, or coat, to reveal a lighter color.
If you don't want to risk scratching your seeds, then use another method that reportedly helps speed their germination. The smoke treatment, for example, requires immersing a smoke seed primer disc in about 1/5 cup of water along with the seeds for 24 hours. If you prefer to soften the seeds with saltpeter, steep them for 24 to 48 hours in 1 cup of water to which you added 1/8 teaspoon of saltpeter. If you do not have access to either smoke discs or saltpeter, try soaking the seeds in buttermilk -- or warm water laced with a little liquid soap -- for one to two days instead, and rinse the seeds well with clean water before you plant them.
When you are ready to plant the seeds, fill a plant pot -- which has bottom drainage holes -- to within 1 inch of its rim with a damp mixture that is 1 part seed-starting mix and 1 part sand. Space the seeds 1 inch apart on the surface of the mix, and cover them with an additional 1/2- to 1-inch layer of the mix.
Enclose the pot in a zipper-type plastic bag to maintain the mix's moisture, and place it in a location with a temperature near 77 degrees Fahrenheit. About one-half of the seeds should germinate within one to two months and an additional one-third within one year.
Once the seeds begin to sprout, remove their pot from the bag, and place the pot under a grow light or on a windowsill where it receives sunlight for only part of the day. During the seedlings' first month, feed them once every two weeks in place of a regular watering with a plant food such as an organic 2-4-2 fertilizer that you dilute to one-half strength by mixing 2 tablespoons of it with 1 gallon of water. After that first month, raise the amount of fertilizer to 4 tablespoons per 1 gallon of water, and apply it at the same intervals.
When the seedlings each have four to five leaves, transplant them into individual seedling pots filled with a mixture that is 1 part organic potting soil, 1 part peat moss and 1 part sand. If you plan to grow the plants in partial sunlight outdoors, then you can transplant them into the ground outside when they are at least 2 to 3 months old. If, however, you intend to place them in a full-sun site, then allow them to remain under a grow light or on a partially shaded windowsill until they are 6 months old because the young plants burn easily.
When moving the seedlings outdoors, place them in shade first, moving them by stages to a position with more sunlight. Once they are fully adapted to outdoor conditions, plant them in rich, well-drained soil in a site exposed to full or partial sun, spacing them about 6 feet apart from each other and watering their soil well. If you apply any mulch, keep it off the plants' centers and at least 2 to 3 inches from their outer stems.
Continue to fertilize the bird of paradise seedlings once every two weeks from spring through autumn, but only once each month during winter. Keep their soil damp during the growing season, and allow it to dry out somewhat between waterings in the winter months.