Woody perennial plants such as bougainvillea can be trained as standards which are a tree like form. Because of the bougainvillea rapid growth rate, they can be made into a 3- or 4-foot "tree" in as little as three years, depending on the length of the growing season and the vigor of the plant.
Buy a variety of bougainvillea that will do well in your climate and that will grow to the size wanted. Some varieties known for their use as standards include: Barbara Karst, San Diego, Superstition Gold, Torch Glow, Jamaica Red, Bagen Beauty, Violet, and Double Orange Pink, and there are many more. Don't try to use one of the miniature or dwarf varieties known for their compact growth habits.
Start with a small 6- to 8-inch plant for potting or transplanting into the yard. In USDA climate zones 9 or higher bougainvilleas can be grown outside, but in areas where repeated freezes are a danger they are best grown in pots that can be moved in during the winter.
Cultivate fertile, acidic, humus filled soil that drains well. Their tender roots cannot stand wet soggy soils. Prepare a soil by using equal parts: peat moss, rotted leaves or pine bark, garden loam, and sand, or buy a good tropical plant soil mix. Thoroughly mix up the soil mixture and work it into the ground where the plant will be grown or fill a good sturdy 2-gallon pot with it. By using a larger pot, it won't be necessary to transplant again for several years and the bougainvillea will actually do better as its roots get more crowded.
Training the Stems
Cut off an established plant near the ground to force new flexible growth to occur. It is possible to train one main bougainvillea stem or allow several to grow and twine them together to form a braided trunk. Bougainvilleas can be pruned back at any time of the year in warm winter areas or if grown in pots, as they will respond with vigorous new growth as long as they are well watered and fed. If planted outside in areas with cold enough winters that it needs protection, don't cut the bougainvillea plant back too late in the season. It should not have young tender growth just as the weather starts turning cold. Instead start the pruning process in the spring to give the bougainvillea plant a whole season to grow.
Place a sturdy stake of some type well into the soil right next to the bougainvillea plant or push the stake into the pot till it touches the bottom and plant the bougainvillea next to it. Depending on how tall the bougainvillea standard tree form is to be, it may take a 4- to 8-foot long stake to allow for enough of it to be sunk into the ground to support the top. Don't use treated poles as the chemicals will affect the plant.
Once the stem or stems to be trained start growing out, tie them to the stake using soft cloth ties or old nylon stockings. Don't use wires or hard plastic ties as the bougainvillea plant will chafe against them and be cut by them. Braid multiple stem together while they are flexible and tie them to the stake to force them to grow together and straight. Cut off any side stems that try to grow out and cut off any thorns on the main trunk to make it easier to handle. Once the stem has reached the height wanted, start pinching back the main terminal bud to get it to branch out and make a bushy top. After the first two or three years there should be a beautiful bougainvillea tree to adorn the property.