This isn't something we grow here in Britain, but if you google "crepe myrtle" and select the"plant care" option, there's loads of info there.

Where in the U. S. are you trying to grow it? If you're trying to grow it as a large shrub or tree, it needs to be in one of the Sunbelt states, at low elevations. It can grow a state or two farther north, or at higher elevations, if a dieback shrub is acceptable. Otherwise, it likes good drainage, neutral or slightly acid soil, plenty of sun, and a certain amount of heat in the summer. In it's best range, it is best planted in the fall, but it may be difficult to find then, since it loses all its leaves in the winter. Look for hybrids named after Native American nations: they're hardier and more mildew resistant than most others.