Rather than the 13-13-13, I would use tree spike fertilzer or Miracle Gro Shake and Feed. The MG S & F is a balance fertilizer that is time released, unlike the 13-13-13 that is water soluble. The instructions should be on the bag or container, small container of the MG would cost about 6-7 and be more than enough.

Are you trees in full sun?

And do you cut them back in the spring?

The variety you have planted may be as much a problem as anything else. Your soil, too, may be problematic.

The Crapes with Indian names seem to be the most Mildew-resistant. Muskogee and Natchez are the largest and most vigorous. I was aching to try Catawba, a smaller tree, with seriously, glowingly purple blooms. There were quite a few in Madison, and they were covered in bloom...really exciting to see.

But I've also grown some horrendously wretched no-name white Crape Myrtles, which were home to Aphids, and whose leaves stayed crusted-over with black mildew. As with Roses, only specific Crape Myrtle cultivars are viable: the rest are invalids not worth your time, or space in your garden. Choose wisely.

My bad Crapes were at a rental property, where it was impractical for me to spray them often (thank heavens we've dumped all our apartment buildings). But 'Safe Soap' insecticide worked just fine, for killing the Aphids, the one summer I bothered with it. The trees still underperformed, even when aphid/mold-free. Truth is, I had made a bad investment in an inferior cultivar.

As for soil, Crape Myrtles LOVE really good dirt. Dirt is not critical for some plants. But with Crapes, the Cost/Benefit ratio is definitely on the side of planting them in excellent soil. A mixture of Loam and Compost, in a hole at least three feet in diameter, will really pay off. The usual caveats about slight mounding of the soil...not creating 'water-holding pots'...apply.

Frankly, I think you might do better scrapping those pitiful Crapes, and planting a really good variety, in really good soil. Google Image Search should be combined with observation of which varieties do really, REALLY well.

Crape Myrtlettes (or just plain 'Dwarf Lagerstroemia') are the appropriate cultivars for people who want small shrubs. So, please resist suggestions to lop your trees off, White-Trash-Style.

Yearly Tip Pruning (six to eight inches..basically just enough to remove the previous year's bloom heads) is fine. And selectively thinning out/limbing-up is fine. But the hideous and scummy practice of simply lopping the trunks off is unattractive in the extreme. The primary beauty of Crape Myrtles is derived from their gorgeous growth habit...the wonderful texture and shape of their trunks and branches.

And while grotesquely maiming Crape Myrtles may produce a very few large bloom heads (like you're going to the State Fair with your blossoms, to win a ribbon, or something), the total amount of bloom is obviously greatly reduced.

The beauty of Crape Myrtles is in the plant as a whole. The blooms are lagniappe.

Oh, and as with so many plants, Crapes appreciate underplantings which will shade their roots. Like most plants, they will love a layer of semi-decomposed mulch, applied yearly, over the old layer. And my Crapes which seemed happiest were sheltered from too much wind (they're a Jungle tree, after all, and are not meant to occupy windy hilltops and barren, sun-scorched lawns).