Flower Essence is nothing more than a flower tea using water. A couple of drops into their food or on the cat itself will relieve stress. The flower water gives off an energy unique to the flower. Essence is made by putting flowers in a bowl of water and having it sit in the sun for 2-3 hours or pouring warm water on the flowers and let sit 2-3 hours. After use, discard the essence. It is better when made fresh and not stored.
It is not so much as the properties of the flowers itself that does the healing, but the energies that the flowers give off, the essence. Chamomile for example is a calming flower. The following link has a list of flowers they use in their Essence.
You can purchase Flower Essence here: They use brandy to preserve their Essence. They give instructions to de-alcohol their Essence too.http://www.catfaeries.com/essences.html
I have never used Flower Essence, some have. When Mittens was sick, I put a couple drops of Chamomile tea in her food. Catnip tea is also good.