Giant floral wreath/swing!

I think this is going to be a pretty big undertaking. Not just to make but, to transport as well.

If you want a hoop behind you could you purchase a large hula hoop and wrap that with greenery/flowers? It will make for a pretty backdrop but, may not be large enough for what you want.

ETA: You could hang it with clear fishing line so that it remains secure but, the wire is not visible.

There are a number of ways"

You can buy the circular frame

http://www.orientaltrading.com/circle-arch-frame-a2-3_3620.fltr?keyword=circular+arch

This you tube video shows how to make a swing. You could adapt it for an arch and use any pipe that would be lighter and less expensive than what she had to use to make it sturdy enough for a swing.

https://www.youtube.com/watch?v=7p1NnR7Jrmk

I would go to an architectural salvage place and construction supply places and see what you can find. Be open to anything round or half round (you can put them together)

You could also have someone cut two half circles from plywood and attach them to a supportive frame.

There is a product that is already kerfed called Kerfcore that a building supply place may be able to bring in for you.

We had a large wreath as part of our band backdrop. We used an extra large hula hoop and covered it with garland and berries. I think if you want it larger you may want to check some sort of flexible tubing that you could probably find at Home Depot or someplace like that.