Among the many flowering shrubs, bougainvilleas stand out because of their repeated and prolonged blooming periods and because of how easy they are to grow in hot dry climates. Bougainvilleas are tropical plants and as such they cannot take repeated freezes or prolonged periods of temperatures below the 40's without the plant being damaged. Bougies like hot weather with lows in the 60's and highs in the 90's. This means in North America bougainvilleas can be grown outside only in zone 9 and further south, but they can also be grown as potted plants in more northern climes. One way to train bougainvilleas is to make them into standards having a tree like form with a cental main trunk. Because of how fast they grow in warm climates, it may only take 2 to 3 years to have a beautiful blooming bougainvillea tree in the landscape or as a potted plant.
As a vining plant, bougainvilleas are usually planted where they will have a wall or trellis to grow over and give them support. By training a bougainvillea vine to a standard or tree form it frees you to plant them anywhere in the landscape where they will get the full sun, at least 6 hours a day, they need. They will need staking while being trained until the trunk is strong enough to support the flowering top of the vine. The stake needs to be of a length to go into the soil far enough to support the plant and to reach high enough so the vine can be tied to it as high as you want the "tree" to be when full grown. Don't use a thin piece of wood or a weak bamboo stake that will only break apart after one or two seasons. A good strudy stake that will last for several years is a must.
Besides picking an area with lots of full sun and having a good stake for support, the bougainvillea tree will also need the right type of soil and drainage. The soil for bougainvilleas needs to be rich in organic matter, acidic, and well draining. You can buy soil for azaleas/camellias or make your own by mixing equal parts of rotted compost, leaf mold, peat moss, garden soil, and coarse sand. If azaleas or camellias already grow well where you live then the soil is already acidic and the peat moss will not be needed. The roots of bougainvilleas are tender so don't add any chemical fertilizers to the soil when planting them as it may burn the roots. While bougainvilleas need lots of water when blooming, they cannot stand water logged soils and need to dry out between waterings. It is best to plant them on a mound so any water will drain away from the plant and if grown in a pot keep the drainage holes open and don't use saucers under the pot.
Making a bougainvillea standard can mean either starting with a new young plant or training an established older plant. For regions where the bougainvillea will be planted outside, it is best to plant them out in the spring so the roots have plenty of time to get established before cooler weather sets in. Selecting the site, preparing the soil, and setting the stake in the ground, can all be done the fall before planting the bougainvillea plant. In fact that would be the best way to go so it all gets settled in and the soil mellows before the actual planting. Now you need to pick out the right variety of bougainvillea for making into a standard. Don't use one of the dwarf or low growing types that are for hanging baskets or ground covers. You want a plant that will grow long vining stems that can be trained. Some good choices include: Torch Glow, San Diego, Barbara Karst, Bagen Beauty, Superstition Gold, Violet, Double Orange Pink, and Jamaica Red. If you are going to train a bougainvillea that is already established in the yard and has shown that it likes where it is growing by blooming heavily, then the first step would be to cut it back all the way to the ground to get new vining stems to grow starting in the spring.
Once the vine has put out new stems that are around 6 inches long, start training them by selecting one strong stem for a single trunked tree or three stems for braiding together to make a braided main trunk. You braid by crossing first the right and then the left side stem over the middle one so that each time there is a new center stem to pass the side stem over. Tie the stem to the stake using soft plant ties or old nylon stockings every 6 inches of so as they grow. Pinch off any additional stems that try to grow out so all the plant's strength goes to the trunk stems. Remove any spines from the stem or stems so they are easy to handle. When the main trunk gets as tall as you want it let the stems grow out and hang down. When they get about 10 inches of growth on them pinch them back to encourage the top of the plant to get bushy with more side stems. Bougainvilleas bloom on new growth so the more new growth you can encourage the better they will bloom.
Depending on your climate and soil, it may take two seasons to form a full sized tree that will only get larger with more blooms every year. To keep its tree-like shape the bougainvillea standard will have to be pruned each year in the spring and after each blooming period. Simply remove any unwanted lower stems that try to grow out and cut back the top stems by 1/2 to encourage more branching. The more you prune the more new blooms you will get in as little as 4 weeks. Once the bougainvillea is established it should be fertilized during the blooming season using a commericial bougainvillea fertilizer that is high in potassium, low in nitrogen and that has trace minerals such as 6-6-10 with iron and magnesium. Too much nitrogen will encourage leaf growth instead of flowers. Water the plant deeply at least once a week if the weather turns dry.