Purely anecdotal, but last year we had super muggy weather (lots of rain, high humidity, etc) and my marigolds formed air roots. So I cut one off & planted it, mostly just as an experiment. It rooted, bloomed, and came back this year bigger & bushier. I don't know if the air roots were the cause, or how to force a plant to grow them, but it is possible to propagate marigolds from cuttings (especially if you have no idea what you're doing).
