David, I grow zinnias in Texas. I do not think it will work. Zinnias whole purpose is to flower and the plants energy goes towards flowering not rooting. Zinnias are also used as cut flowers and I have not had one grow roots while in water in a vase. There is always a first time for everything, so try one that isn't your favorite. Your best bet is collect their seeds, each flower if pollinated produces lots of seeds. Zinnias are great flowers. Have a joyful day!
