Growing from 10 to 25 feet tall with ferny leaflets and 3- to 6-inch clusters of fragrant yellow flowers, the Mexican bird of paradise (Caesalpinia mexicana) remains evergreen in frost-free areas. Perennial in U.S. Department of Agriculture plant hardiness zones 9 through 10, the shrub or small tree may drop its leaves during winter in the colder areas of its range. It can also survive in parts of USDA zone 8, but often dies back to the ground there over winter, before growing up to 6 feet tall during summer. Because this "bird's" seeds are hard-shelled, they require pretreatment.
After the Mexican bird of paradise tree has finished blooming in autumn, look for 3- to 6-inch seedpods on its twigs. Once those pods turn tan and brittle, they twist and spring open to hurl their contents away, so try to harvest the seeds just before that happens.
After you crack open the pods, spread the seeds on paper towels until they are thoroughly dry before storing them in a lidded glass jar in your refrigerator. Because these seeds are poisonous, make sure that they are clearly labeled.
In the spring, about six weeks before the last frost date for your area, remove the seeds from the refrigerator and scratch or nick each one to enable it to soak up water. To scratch a seed, rub its rounded edge with a file or emery board until a white spot shows through the brown shell. To nick it instead, use needle-nosed pliers to hold the seed still on top of a cutting board, and chip a tiny piece out of its rounded edge with the tip of a utility knife. Always scratch or nick a seed on the side opposite its hilum, the mark or indentation indicating where the seed was formerly attached to its seed stalk inside the pod.
Afterwards, drop the seeds into a container of warm water for 24 hours. They should sink to the bottom and swell somewhat in size. You may need to discard any that didn't sink, as they aren't likely to be viable. For those that didn't swell, try scratching or nicking them deeper until they do.
Fill a container that has drainage holes to about 1 inch short of its top with a damp combination of 1 part seed-sowing mix and 1 part sand or perlite. If you prefer, blend 1 part cactus potting mix and 1 part perlite instead. Space the seeds about 2 inches apart on the surface of that mix and cover them with 1/4 to 1/2 inch of it. After topping the container with plastic wrap, place it on a seedling heat mat, if necessary, to raise the soil temperature to about 77 degrees Fahrenheit. Ensure mix stays damp. The seeds should sprout within one to four weeks.
Once seeds have germinated, remove the plastic wrap and place the container on a sunny windowsill or under a grow light that runs for 14 to 16 hours per day. Keep the seedlings' mix lightly damp.
When seedlings have at least four leaflets in addition to their two seed (original) leaves, transplant them into individual 4-inch pots with drainage holes, that are filled with cactus potting soil. Feed them once every two weeks with a plant food at half strength. For a 0-0-1 organic seaweed emulsion, for example, mix 1 tablespoon of the emulsion into 1 gallon of water. When the seedlings are one month old, increase the amount to the full 2 tablespoons of emulsion per 1 gallon of water.
After the last spring frost, adapt the seedlings to outdoor conditions before setting them in the ground. Place their pots in the shade first and edge them gradually into a sunnier position. Choose a site with well-drained soil in full or partial sun, and space them at least 6 feet apart from each other or from other plants.