Need Some advise On Trimming/Pruning Crepe Myrtles - Pics

[gsrsol84mm]

Have 6 crepe myrtles that need attention. They have grown so tall that it's hard to see the blooms and their cover has been reduced. Three are about 18-20 feet tall and the other three about 12-15 feet tall. I wanted to cut them all flat at 6 feet but would appreciate any advise or feedback you could offer. Most people around here just chop them off. When is the best time to prune them ?

Thank You

 

[B Harrison]

Early, just before they put out leaves, if your picture is recent, I am guessing there may be other issues, they should have more foliage than that in my opinion. Do you fertilize that grass with anything, and do they get much water? Maybe its just the lighting but they look weak.

 

[gsrsol84mm]

Early, just before they put out leaves, if your picture is recent, I am guessing there may be other issues, they should have more foliage than that in my opinion. Do you fertilize that grass with anything, and do they get much water? Maybe its just the lighting but they look weak.

It may be partly the lighting. The leaves are turning already and dropping. We had a ton of rain this year. We do fertilize the grass.
I have other crepe mytles that are about 3 years old that are half the height but do look a lot fuller. It's probably 8 years since these 6 crepe mytles have been prunned.

Would you just cut them straight at 6 feet high ?

Now that I look at them compared to other crepe myrtles, they do look weak. What would you do to get them built back up ?