Here Glyphosate is a grass killer and you need to use a broad leaf killer such 2,4-D (Dimethylamine Salt of 2,4-Dichlorophenoxyacetic Acid). It should have been applied at the stump when the Crepe Myrtle was first cut so it would got absorbed by the root system. Or better yet injected into the plant a few weeks before cutting it down.
As with all chemicals you got the read and follow the directions and even be extra careful with the application as to not affect nearby plants or have runoff problems.
Whatever you do don't use the ester version of the 2,4-D as has a low vaporization point and will hang low to ground and destroy desirable plants. Here it can't be use above 75F without causing problems. The salt version is more stable but still should be use below 80F if openly sprayed just as a precaution.
