Experts and BHG readers answer.
Salvia does not grow upright
Thanks for writing. There are several reasons that salvia may flop; they include the plant being kept to moist, the plant being fertilized too much or grown in soil that's too rich, or growing it in partial shade.
Have you deadheaded it (cut off the faded flower spikes)? If not, that will help it stay upright, and encourage new blooms to boot!
---Justin, Senior Garden Editor, BHG.com