To develop a tree shape, remove all the limbs growing from ground level except three to five limbs. Then cut off all the side branches halfway up the height of the plant. As the plant grows taller, lower branches can be removed each year. You may also need to remove any future growth that occurs at ground level in order to keep the desired tree shape. 

Crape myrtle tip

For strong growth and abundant flowering, crape myrtle should be planted in full sun. Heavy shade will reduce growth and flowering and will increase disease problems, such as sooty mold on the foliage and powdery mildew on the flower buds and young growth. 

Large shade trees will also compete with the crape myrtle for moisture, and the result is poor growth and flowering. Lack of sunlight and moisture are the common causes of poor growth and flowering.