Bottle tree is the common name of several species of tree within the genus Brachychiton. All are native to Australia, where they are referred to as kurrajong trees. Approximately 30 species exist within the genus, including several that are commonly cultivated as ornamental trees. The size and growth habit ranges from shrubby 12-foot trees to graceful 90-foot tall specimens; however, all bear a dazzling summertime display of reddish flowers. Seed growing is the easiest and most successful means of propagating bottle trees, particularly for gardeners within U.S. Department of Agriculture Plant Hardiness Zones 9 to 11.
Harvest the seed pods from a healthy bottle tree in late summer. Ripe seed pods will crack open and reveal a cluster of pale yellowish-brown seeds. Clip the attachment stem with a pair of bypass pruners and put the pods in a bag.
Put on a pair of gloves before removing the seeds from the pods. Crack and pull back the pod husk with a pair of pliers. Remove the seeds with tweezers. Do not handle bottle tree seeds with your bare hands because they are covered in fine, irritating hairs that are difficult to remove from skin.
Combine equal parts peat moss and perlite to create sprouting mix suitable for bottle tree seeds. Fill one 4-inch plastic pot with the mixture for each seed you wish to germinate. Moisten the soil to a 4-inch depth using a garden hose with an adjustable nozzle set to mist. Allow the soil to drain for 20 minutes before sowing the seeds.
Press one seed into the center of each pot of soil. Push the seed in until the top is flush with the surface of the soil. Sprinkle a small amount of soil on top but do not completely cover it.
Stretch plastic wrap over the pot. Poke two or three small holes in the plastic wrap to allow for evaporation and increased air flow.
Place the pots where they will receive bright light and temperatures above 80 degrees Fahrenheit during germination. Place the pots on a propagation heat mat to maintain constantly warm temperatures for at least 15 days. Apply 1 inch of water to the seeds after seven days.
Repot the bottle tree seedlings into 2-gallon plastic pots once they reach 3 inches in height. Fill the new pots to half their capacity with a mixture of 3 parts potting soil, 1 part horticultural sand and 1 part perlite.
Remove the bottle tree seedlings from their original pots. Set them atop the soil inside the new pot and fill in around the sides with the potting soil mixture until the soil level is flush with the base of the stems. Firm gently and apply 4 inches of water.
Place the pots in a warm greenhouse for the duration of winter. Plant them in a permanent bed when outdoor temperatures reach 70 degrees Fahrenheit. Choose a planting site with full sun and good drainage.