How do I make a plant food for crepe myrtle and hibiscus plants?

My crepe myrtles thrive with one watering of diluted Miracle Gro every two weeks...nothing else.

If your soil needs to be more acidic, then you should buy Miracle Grow’s pink box, called Miracid. It helps a lot.

You can mix one tablespoon of Epson Salts in a gallon of water. Gently pour it on the soil at the base of your plants. This put enzymes back into the soil. Plants thrive wish this solution. Do this every two weeks. Used coffee grounds will create an acidic soil. These two methods are very effective and inexpensive.

This may help: https://thegrownetwork.com/15-simple-and-inexpensive-homemade-fertilizers/

We used the suggestions from this site and were very happy with the results: https://homeguides.sfgate.com/fertilizers-crape-myrtles-31519.html