Verbena bonariensis cutting back

I’ve grown vebena bonariensis from cuttings from last yr. I haven’t cut back during winter but wondering what I should do with them now in spring (March)? Do I cut back to ground level, or leave as they are? I don’t want them flopping over in the summer.

 

With Verbena bonariensis I tend to remove any obviously dead bits, maybe shorten the live bits by two thirds and then have another look at it when it starts to grow properly. As for falling over I think that's just in it's nature mine reach 5 ft + and like to lean on the surroundings. So tie back to a fence or a cane.
You can get dwarf varieties if the falling over is important.