You need to have cut it back when small to give it good base branching.

You also need a ring support that it can grow thru' to help support this tallish, floppy salvia.

My May Night has never grown tall enough to flop. When the first flush of flowers fade I cut all those stems back to the ground even those with leaves. Keeps it neat and I get a few more shorter bloom stalks throughout the rest of the season.