You can put them on top and let them wilt there. The worms will come up from the bottom when they become "Just Right", and if the flowers cause any heating up, the worms can back away from the heat until things cool down.

Not if you have sprayed them with chemicals!

no chem's. thanks guys

If the flowers are organic, you're good to go. Plastic? We need to talk.

I have not seen it happen though I have read the flowering stems of comfrey can take root so it is best to wilt them before use where they might begin anew in a place where you dont want them.

Have no clue as to the tenacity of Hibiscus but laying just about any plant in the sun for a few hours will generally tame any tendencies to take hold once in contact with moist fertile soil.

I have been leaving comfrey leaves in a large sealed clear bag in the sun for a week or more, it makes for a disgusting sludgy mess and the worms are sure to love converting all of those great nutrients into tiny capsules of garden goodness.