Well I don't know about the wreath part but I can tell you it reseeded all over my garden the one year I had it. It was EVERYWHERE!!! So for me it was an aggressive reseeder!
Joann

I grew sweet annie a couple of years ago, and the wreaths I made from it (mixed with statice) are fine--nothing dropping, but they do go completely brown. They look very nice though. I have had some reseeding, in my garden, but nothing that is out of control, and unmanageable. It is a good filler for wreaths, I found, as the plants are huge and productive , over 6'.
Maplea

Carin a better plant if you are looking for something for wreaths is Artemsia silver king. I quit growing sweet annie and only grow silver king for wreaths and dried bouquets. As a perennial it is hard to beat and is easy to harvest, also is great habitat for praying mantis, so becareful when harvesting.

Bryan