Before you cut a flower ...

... Always do a quick search to determine how to treat it. Tithonia (Mexican sunflower), below, has hollow stems that must be seared with a flame or boiling water--or it will immediately wilt.
Luckily, I only cut one blossom because I could not bear to take it away from the butterflies. I did experiment to see if the heat treatment would reverse the wilting several hours after cutting, and did, completely.