This is a tough project for sure. I agree with the other anon, you're going to need a hoop skirt for structure and I also think that a petticoat isn't the right direction for this. A petticoat is zero structure, all fluff. And while that sounds appealing to give the skirt body, it means that you won't have a stable foundation to build the "petals" upon. Furthermore, it would be near impossible to create those rigid upright shapes with fabric and thread alone -- especially the types of silky, light, and soft fabrics that you would be incline to use for a flower costume because they also provide no structure and are finnicky/slippery to work with.
In other words, I think you should take this in a new direction.
If I were making this, I would use a short hoop skirt and then sew. On top of that would be a skirt made with two layers: the under layer would a sturdy and non-stretch fabric that would be able to handle the weight of the petal materials and glue and the second, top layer would be a fabric or material that matches the color of the petals. Then, to create the petals I would use the methods used in one of these two tutorials by Kinpatsu cosplay: https://www.youtube.com/watch?v=V-8L2n5pveU
. Then attach the petals to the two-layer skirt with a hefty glue like contact cement. It would be tough with the way the top is designed, but it would be ideal to make a shoulder harness to distribute the weight off of the waistband. The top I would make into a dress separate from the skirt for ease of wear and so that the bodice isn't pulled on the weight of the skirt.
Anyway, that's what I would do. I hope that helps.