How can I make my flowers glow in the dark?
I lack both the chemistry and botany knowledge to address you question directly.  @ChrisH's comment suggests that your phosphorus pigments won't wick up through the stems like the compounds in the referenced article do.

Have you considered applying the pigment directly to the flower petals using either a paintbrush or airbrush?

Since you are planning to encapsulate the flower in clear resin, you don't have to worry about the paint rubbing off of the petals.  How it gets to where you want it is less important that how it looks once it is in place.  To that end, I would recommend a very light (high-air) spray of a very dilute pigment using an airbrush with a high-number needle and nozzle (3 or above).  Mist the flower with the glowing paint and test the results in darkness after each coat.

Skipping that idea, there is an immersion technique at the end of the referenced article which might work even if the stem-based approach fails with the pigment you choose.