This season I've been using Fox Farm's "Marine Cuisine" and some various types of guano depending on what I'm fertilizing. This way I have a well balanced base to which I can add extra N, P, or K in the form of bat or sea bird guano. I think it's best to get some fertilizer that you can use on more than one plant and the Crepes are not that demanding or fickle. They can have a splash of whatever you are using on your veggies or they'd be happy with just a thick top-dressing of fresh organic compost and a restored mulch layer. As with many plants grown primarily for their flowers, too much Nitrogen can be a detriment so don't fertilize too heavily and at all costs avoid the "lawn" type fertilizers which are full of bioavailable nitrates that will just serve to make your Myrtle a big bushy bloomless monster (- nitrates which will also rapidly leech into the nearest creek or river as a major pollutant).

If you really want to baby something, I like the OMRI-listed Earth Juice line of products. They have seperate formulations for "Grow" (high N) and "Bloom" (high P & K, no N), as well as a micronutrient solution and one that's purely for boosting the Potassium levels (called Meta-K). This can get quite expensive when used for large applications and I mostly use it to keep container plants as happy as possible, but if all you are growing is Crepe Myrtles and you want to go all out "organically" then get a soil test and spoil your babies with a precisely formulated Earth Juice regimen.