You can use just about any fertilizer that promotes blooms on any flowering plant - with a high middle number. Or if you like organic fertilizers, a seaweed/fish emulsion would probably be good or some bonemeal would work too.

Nitrogen -- crape myrtles bloom on new growth, which this promotes.

Benign neglect is the first order for crepe myrtles. Do not fertilize or overwater them. Simple organic lawn fertilizer applied to your lawn and the crepe is all that is needed. Over fertilized crepes will put out lots of canes and leaves but not many flowers.

I think I will agree with texasredhead because we have a large (about 20ft) lavender crepe myrtle that is doing great even with all the rain and I don't fertilize (much).

I was afraid of mildew which we have had problems with on occasion in the past. It is growing in a kind of low area of clay near the street so drainage is poor. I added a little compost year before last and a small amount of used coffee grounds (from Starbucks) this year and that may (or may not, LOL) be the reason I'm not seeing a problem with powdery mildew.

Bottom line - I have not fertilzed in over 10 years and this tree is doing GREAT! If you do fertilize, it is my experience that you should keep it VERY organic. (I pruned the first year or two but quit doing that when I learned about "crepe murder".)
It is SOOOOO beautiful, blooming very very well. The tree is over 60 years old and still doing so well, amazing.

I've read in several places that epsom salts are good for helping crepemyrtles bloom. I've sprinkled some the last couple of years and mine are blooming very nicely. Don't know if it's because of the epsom salts or not....just like elphaba and her compost and coffee grounds (LOL)!

Anyone know if epsom salts are considered organic?

I just read an article on the Crape Myrtle Trails of McKinney, Tx. The section on fertilizing said that CMs bloom on new wood and require nitrogen to make the new growth. They suggested lawn fertilizer........the kind without weed-kill in it. Dont see what epsom salts could hurt - I use that on my roses.
Judith