Q.When to fertilize crepe myrtles?
What kind of fertilizer to use on the crepe myrtle? And when to apply it?

It is not usually necessary to fertilize crepe myrtle trees, unless blooms are in short supply. Crepe myrtles oftentimes are not in full bloom until the second year after planting. A soil test can be administered to see if the soil pH falls into the ideal range for Crepe myrtles, which is 5.0 to 6.5, at which point you would know which amendments needed to be made to the soil.
For more information on crepe myrtle trees, please visit the following link:
https://www.gardeningknowhow.com/ornamental/trees/crepe-myrtle/crepe-myrtle-care.htm