Yes .... put a brick on it .

I think I'd wonder why they're flopping on the ground. Are they planted where they get all-day full sun? My P. 'David' only gets half-day sun in healthy, organic sandy loam and it's totally upright, as are all my other Phlox paniculata cultivars this rainy season. I've nearly a dozen cultivars planted in various beds and all are performing beautifully this year--none are flopping altho' they have in previous seasons.

It's only a guess but I don't think they'll root if you staple them down but since I've never tried it, I can't speak from experience.

Best of luck!

My Delta Snow has been floppy this year, growing in full sun and sandy loam soil not amended. But the flower heads on it are huge this year compared to previous years (in a previous garden in a different state with different soil and sun exposure), so I think the weight of the flowers plus dew and rain weighing them down, and wind, has caused them to flop and the stems on some to even break.

Karen