Crape Myrtle - Lagerstromia Indica
Pruning may take place throughout the growing season. Pruning late Spring is important to control shape and allow the flowers to bloom in Summer. If flowering isn't desired then pruning during Summer is not a worry. Cutting a Crape Myrtle back hard before Spring is not a particular worry since they back bud profusely.
Pruning cuts heal really well, so you can prune larger cuts without too much worry. Also because of the way they heal you don't need to make concave cuts, you can make them flush with the bark.
If you desire flowers within the shape of your bonsai tree use previous years to prune and stop at a certain date. Then take note when the flowers come. If the flowers come and the stems that grow out are 8 inches long and you need them shorter, then the next year you can continue pruning until a later date because the flowers will come near the same date as they did the previous year. - Ted Matson
Crepe Myrtle trees enjoy full sun, which is beneficial to flowering. Keeping a Crepe Myrtle in partial shade will decrease flowering. Full sun even means full sun for us in the High Desert.
The Crape Myrtle is hardy down to 5° F keeping in mind that smaller branches will be less hardy than larger branches. There is a possibility of branches being damaged in our cold Winters.
Feed the plant every two weeks during the growing season. Different balances will affect flowering, so pay attention to the nutrient balance.
The tree may be shaped by pruning or by wiring. If wiring is to be undertaken, it is best to do so during the growing season. However, the bark should be handled in a careful manner.
Water the tree frequently during periods of growth to ensure moist conditions. Limit watering during dormancy.
Re-pot every 1-3 years when needed. Be sure to re-pot early Spring before buds break. Since the roots are fine feeder roots you can be pretty aggressive with how many roots you take off.
Scale and aphids are common problems easily dealt with using a solution of 1 tsp. dish soap and 1 quart warm water sprayed over the plant until runoff is achieved. Do not spray blossoms or they will brown. After spraying plant well, rinse with clean water and repeat as needed. Though Crepe Myrtles are susceptible to powdery mildew, you can make efforts to prevent this by ensuring that you do not train your bonsai to be too compact in its branching.
You can propagate your bonsai from seed in Spring, or by softwood and semi-hardwood cuttings taken in spring or summer and given a treatment of rooting hormone to promote root growth.
Crepe Myrtle trees may be enhanced by deadwood characteristics. This can be achieved through carving, or by encouraging new growth on die-back wood.
Defoliation of a collected Crepe Myrtle: https://www.youtube.com/watch?v=lSggrk4NKaQ
To learn about specific species and their care head to my Bonsai Information page